[Replacing HVAC - considering heat pump]

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Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

mervinj7 wrote: Tue Dec 07, 2021 6:16 pm
finite_difference wrote: Tue Dec 07, 2021 6:01 pm I do agree that radiant heat is the most comfortable one though. So we need a heat pump that heats up water and circulates it through the house? Rather than slightly warm air blowing on your face.
A heat pump water heater for radiant floor heating is totally possible! A more ideal one would use a geothermal heat pump. I've only ever seen it in one house, so they are not common as far as I know.

https://www.arcticheatpumps.com/radiant ... -pump.html
They certainly exist in Canada.

It's a common technology in Europe: Germany, Sweden (particularly), Switzerland (I believe). I think in Japan they tend to be more ASHP. It gives a nice even warmth - very comfortable. (Because of the thermal mass of the floor & the physics of HP efficiency, run "low and slow" with no setback - so the thermal enveloped of the house needs to be fairly tight).

As per an earlier discussion I had with Talzara, the European building approach is to lay a concrete floor screed with the pipes in it - very simple and straightforward. It is more complicated for North American floors.
Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

adamthesmythe wrote: Tue Dec 07, 2021 6:27 pm
illumination wrote: Tue Dec 07, 2021 10:40 am
jharkin wrote: Sat Dec 04, 2021 4:14 pm It’s funny that these heat pump threads keep popping up me every time the same people insist they don’t work below 40.

That is 1980s thinking that is simply NOT true anymore. Mitsubishi, Fujitsu, Daikin and others all make cold climate heat pumps that provide full rated BTUs down to -15F and partial heat even lower.
If your contractor tells you you need to switch to backup heat at 40, or even 30, you need to fire them and find a contractor that actually knows what they are talking about.

I installed a 2.5 T Mitsubishi hyper heat ducted unit to supplement my propane hot water baseboard. It’s been down to 24F already and it’s not even running full time, house feels more comfortable than with the baseboard and my propane usage seems to be cut in half. Electric has gone up for sure but won’t know how much until next bill, thoug I don’t think it will be even as bad as the the 30 year old AC it replaced was in summer.

But an "average" central air heat pump in most homes needs "help" when it gets below 40 degrees outside. Most all electric ones have an electric resistance coil that turns on for "Aux" heat. It's incredibly inefficient when running that way. Even modern heat pump manufacturers have this as a disclaimer.

A heat pump with a gas furnace (dual fuel) makes a lot of sense if you already have nat gas. I can't imagine someone not going in that direction if you have any sort of cold winter.

Mini splits are a different set up, most people don't want to convert over from central air with ductwork to a home full of minisplits in every room. But they can do a better job at lower temps and are more efficient.
I was unpersuaded by the postings about newer heat pump designs which contained claims rather than explanation. The same thing was true about links from heat pump manufacturers. However I HAVE managed to turn up one link with real technical content that provides a justification for believing that improved designs, capable of useful operation at lower temperatures, are practical.

(https://www.energy.gov/sites/default/fi ... 042314.pdf)
The manufacturers publish their spec sheets. I don't know what the industry Quality Assurance is on those, but I presume they have to be somewhat accurate - because the installers rely on them in specifying systems.
Briefly, it appears that higher operating pressures, obtained either with multiple compressors or oversized compressors, will push the temperature at which COP is usefully high to lower outside air temperatures. This means that the outside temperature at which supplementary heat is required will be reduced (and also the electric power consumption for a particular outside air temperature will be reduced).

The price of this improvement would be presumably higher cost (larger or more compressors) and increased complexity. Possibly also increased repair cost if higher pressure is associated with more frequent leaks.

Now this does not change the fact that there is some outside temperature at which it becomes more economic to use supplementary natural gas heat or an even lower temperature where it becomes economic to use supplementary electric heat. It just reduces the range where supplementary heat will be needed.
It's always the case that when the COP is 1.0 (large enough gap between internal and external temperatures) that electric bar heating will be no more expensive. COP of a gas furnace is typically around 0.9-0.95 (energy efficient one). But gas is usually much cheaper (less so in New England & NYS perhaps?) per unit energy, so the breakeven point is above that.

If the homeowner does not have natural gas as an option, then a good Air Source Heat Pump (ie capable at lower temperatures) is always an option worth looking at. The house does need to be as insulated & airtight as it can be, so remodelling is often a time to look at that. Whilst an oil or propane (or wood) furnace is an option - it may not be absolutely necessary because the range of temperatures at which that would be cheaper than just running electric bar is so low (below c 10 F, perhaps?).

(A caveat on burning wood. The problem of air pollution from wood-burning appliances is much, much worse than we realised say 20-25 years ago. In particular PM 2.5 and below (micro and nano-particles). Studies are coming back with some horrifying numbers in terms of shortening of life spans, coronary & vascular diseases, dementia etc -- due to particulate pollution. It's been discovered that 30-40% of winter urban air pollution (in London) may be from wood burners - being entirely gentrified & renovated homes (wood heat is very expensive, no one in urban Britain would use it as a default). I gather in Aspen CO that it is impossible to get permission for a new wood-burning fireplace?).

If there is a desire to add solar PV then that can work well with an ASHP (one should also look closely at battery storage, in that case) - -heating all day for nearly free is possible.
Last edited by Valuethinker on Wed Dec 08, 2021 4:55 am, edited 1 time in total.
Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

illumination wrote: Tue Dec 07, 2021 4:08 pm
aquaman wrote: Tue Dec 07, 2021 3:53 pm
In other words, energy efficiency is obviously important, but so is comfort. Yet, an overwhelming majority of the posts in this thread focus solely on the energy efficiency without mentioning anything about the comfort drawbacks that people would have to accept from heat pumps.
Exactly.

I have two houses, one with an all electric heat pump and one with a natural gas furnace combo. The gas furnace is a MUCH better experience for comfort in the winter. Warms the place up much faster and I also find I don't need to keep the temp on as high a setting for everyone to be comfortable.

But I would also bet a natural gas furnace is cheaper to run in the winter for most people than an "all electric" central heat pump, just like it's usually cheaper to run natural gas for a water heater. But some states/utilities handle things differently.
Re HP. What you need is one you can turn on remotely? So, say 4-8 hours before you get to your 2nd home, you turn the heat up? The mantra remains "HPs are best run "low and slow"" and gas furnaces "hard (high) and fast". As long as the HP's output temp is above the current air temp in the home, it will be heating the home.

A HP, unless run on electric bar (resistance) heating, won't heat up a space as fast as a gas furnace. It needs to be run differently.

Typical breakevens on HP's are COP of around 3.0 (for heating) ie electricity 3x price of gas**. I agree that at US domestic natural gas prices, it's often cheaper to run an NG furnace than an electric HP.

(There is such a thing as a HP run by NG (just as they used to sell fridges run by NG). I've not seen much about the technology though).

** local factors. Like what is the fixed charge for the network, vs the variable per unit charge, that you are paying? Also NG prices vary a lot depending on pipelines & how the retail market is structured.
mgensler
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Re: [Replacing HVAC - considering heat pump]

Post by mgensler »

OP - We had the setup that you described for 12 years in our home. It is typically known as dual fuel. We would run the heat pumps down to 25F or so and then the 92% natural gas furnace below that temp. We would also use the gas furnace to recover after setting back the heat at night. We used the Ecobee thermostats to handle this setup and they worked well. Furnaces and heat pumps were a mix of Carrier and Trane. We're in climate zone 4a which is mostly heating-dominated.

This year we did a major kitchen remodel and decided to air seal and insulate to code as much of the house as we reasonably could. As part of upgrading the kitchen appliances we decided to replace the gas ovens with electric and the gas cooktop with induction. We also decided to replace the HVAC systems with Asian cold-climate mini-splits from Fujitsu. This would mostly eliminate our use of natural gas. The mini-splits are ducted so they appear similar to our old HVAC setup. So far this winter, cost is running about the same as our old setup. We're in an area that has both cheap natural gas and cheap electricity. In the spring we will be installing a solar array to hopefully offset most of our power use and get us close to net zero on a monthly basis.
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illumination
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Re: [Replacing HVAC - considering heat pump]

Post by illumination »

mgensler wrote: Wed Dec 08, 2021 8:14 am OP - We had the setup that you described for 12 years in our home. It is typically known as dual fuel. We would run the heat pumps down to 25F or so and then the 92% natural gas furnace below that temp. We would also use the gas furnace to recover after setting back the heat at night. We used the Ecobee thermostats to handle this setup and they worked well. Furnaces and heat pumps were a mix of Carrier and Trane. We're in climate zone 4a which is mostly heating-dominated.

This year we did a major kitchen remodel and decided to air seal and insulate to code as much of the house as we reasonably could. As part of upgrading the kitchen appliances we decided to replace the gas ovens with electric and the gas cooktop with induction. We also decided to replace the HVAC systems with Asian cold-climate mini-splits from Fujitsu. This would mostly eliminate our use of natural gas. The mini-splits are ducted so they appear similar to our old HVAC setup. So far this winter, cost is running about the same as our old setup. We're in an area that has both cheap natural gas and cheap electricity. In the spring we will be installing a solar array to hopefully offset most of our power use and get us close to net zero on a monthly basis.

So after all of these upgrades (sounds like tens of thousands of dollars, maybe more?) ... the utility costs are basically the same as they were with the gas furnace and gas appliances? And that's WITH major upgrades in insulation?

Seems like a really good case to keep a gas furnace and gas appliances.
Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

illumination wrote: Wed Dec 08, 2021 9:21 am
mgensler wrote: Wed Dec 08, 2021 8:14 am OP - We had the setup that you described for 12 years in our home. It is typically known as dual fuel. We would run the heat pumps down to 25F or so and then the 92% natural gas furnace below that temp. We would also use the gas furnace to recover after setting back the heat at night. We used the Ecobee thermostats to handle this setup and they worked well. Furnaces and heat pumps were a mix of Carrier and Trane. We're in climate zone 4a which is mostly heating-dominated.

This year we did a major kitchen remodel and decided to air seal and insulate to code as much of the house as we reasonably could. As part of upgrading the kitchen appliances we decided to replace the gas ovens with electric and the gas cooktop with induction. We also decided to replace the HVAC systems with Asian cold-climate mini-splits from Fujitsu. This would mostly eliminate our use of natural gas. The mini-splits are ducted so they appear similar to our old HVAC setup. So far this winter, cost is running about the same as our old setup. We're in an area that has both cheap natural gas and cheap electricity. In the spring we will be installing a solar array to hopefully offset most of our power use and get us close to net zero on a monthly basis.

So after all of these upgrades (sounds like tens of thousands of dollars, maybe more?) ... the utility costs are basically the same as they were with the gas furnace and gas appliances? And that's WITH major upgrades in insulation?

Seems like a really good case to keep a gas furnace and gas appliances.
It is an argument for not replacing early. It's certainly the case that when remodelling, energy efficiency measures should be installed, where feasible, and one should look again at the options re heating. The typical energy refit for a whole house does not cost more than the cost of a good new kitchen. You can spend endless money on a kitchen remodel ;-).

There are reasons not to want to have natural gas: environmental in particular*. But also gas is a very volatile commodity (my gas costs have 4x-ed in less than 12 months - but that is in Europe) and certainly in the USA it's been as low as $2.50/mmbtu and as high as $16 in the last 20 years (just from memory).

Poster notes they live in an area with low gas and electricity costs. It's nice to have the option if there are changes in the relative pricing.

* and safety, although we have all gotten fairly blase about those. But every so often an explosion reminds us of the fundamental nature of the beast.
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illumination
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Re: [Replacing HVAC - considering heat pump]

Post by illumination »

Valuethinker wrote: Wed Dec 08, 2021 9:30 am
illumination wrote: Wed Dec 08, 2021 9:21 am
mgensler wrote: Wed Dec 08, 2021 8:14 am OP - We had the setup that you described for 12 years in our home. It is typically known as dual fuel. We would run the heat pumps down to 25F or so and then the 92% natural gas furnace below that temp. We would also use the gas furnace to recover after setting back the heat at night. We used the Ecobee thermostats to handle this setup and they worked well. Furnaces and heat pumps were a mix of Carrier and Trane. We're in climate zone 4a which is mostly heating-dominated.

This year we did a major kitchen remodel and decided to air seal and insulate to code as much of the house as we reasonably could. As part of upgrading the kitchen appliances we decided to replace the gas ovens with electric and the gas cooktop with induction. We also decided to replace the HVAC systems with Asian cold-climate mini-splits from Fujitsu. This would mostly eliminate our use of natural gas. The mini-splits are ducted so they appear similar to our old HVAC setup. So far this winter, cost is running about the same as our old setup. We're in an area that has both cheap natural gas and cheap electricity. In the spring we will be installing a solar array to hopefully offset most of our power use and get us close to net zero on a monthly basis.

So after all of these upgrades (sounds like tens of thousands of dollars, maybe more?) ... the utility costs are basically the same as they were with the gas furnace and gas appliances? And that's WITH major upgrades in insulation?

Seems like a really good case to keep a gas furnace and gas appliances.
It is an argument for not replacing early. It's certainly the case that when remodelling, energy efficiency measures should be installed, where feasible, and one should look again at the options re heating. The typical energy refit for a whole house does not cost more than the cost of a good new kitchen. You can spend endless money on a kitchen remodel ;-).

There are reasons not to want to have natural gas: environmental in particular*. But also gas is a very volatile commodity (my gas costs have 4x-ed in less than 12 months - but that is in Europe) and certainly in the USA it's been as low as $2.50/mmbtu and as high as $16 in the last 20 years (just from memory).

Poster notes they live in an area with low gas and electricity costs. It's nice to have the option if there are changes in the relative pricing.

* and safety, although we have all gotten fairly blase about those. But every so often an explosion reminds us of the fundamental nature of the beast.

We're not supposed to get into environmental (carbon footprint) type arguments on the board, but most electricity is generated by fossil fuels and I would say it's complicated and depends on a lot of factors, like where you get your electricity from. I would also say if environmental concerns are your real priority (and not saving a few dollars on your utility bills) there's better bang for your buck elsewhere. I'd rather throw solar on my roof and keep my gas furnace than have a home full of mini splits, but YMMV.

If commodities like natural gas shoots up in price, electric utility prices will likely follow. It's all very interconnected. I just know I'm really glad I have the gas option as it just works better and its incredibly cheap where I live. If I converted my home to "all electric" my utilities could easily be another few hundred a month and I would be less comfortable.
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Re: [Replacing HVAC - considering heat pump]

Post by Big Dog »

bluegill wrote: Fri Dec 03, 2021 7:29 am I live in the TN river valley. Electricity is relatively cheap here. TN Valley Authority electricity. Most homes have Heat Pumps with natural gas furnace in this area. I avoid brands that restrict access to repair parts. These companies sell parts only to Authorized dealers/repairmen. Goodman brand is manufactured in this area and parts are not restricted. My son went to HVAC school. Quality of installation is much more important the quality of hardware. Recommendation: Variable speed air handler is more comfortable and quieter than single speed air handler. It worth the money to pay for variable speed air handler.
Can I ask the value of the HP when you also have a gas furnace? Is the HP that much cheaper to run when the outside temp is ~50 than the gas furnace? Or, is it just the cleaner source of energy, i.e., environmental savings?
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TomatoTomahto
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Re: [Replacing HVAC - considering heat pump]

Post by TomatoTomahto »

Valuethinker wrote: Wed Dec 08, 2021 9:30 am There are reasons not to want to have natural gas: environmental in particular*. But also gas is a very volatile commodity (my gas costs have 4x-ed in less than 12 months - but that is in Europe) and certainly in the USA it's been as low as $2.50/mmbtu and as high as $16 in the last 20 years (just from memory).
When we moved to the area, natural gas explosions in a nearby town forced residents to turn off their natural gas for a long duration. https://en.wikipedia.org/wiki/Merrimack ... explosions

I know it’s rare, so rare as to be readily disregarded in discussions of fuel sources for generators, but I’m sure those houses got plenty cold.

For those who are on the fence about ASHP, let me suggest that a GSHP is like having an ASHP where the air is around 50-60F and the HP is indoors out of the weather :D
I get the FI part but not the RE part of FIRE.
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Re: [Replacing HVAC - considering heat pump]

Post by Big Dog »

aquaman wrote: Tue Dec 07, 2021 3:53 pm
jharkin wrote: Tue Dec 07, 2021 11:53 amAll these outdated "rules of thumb" and assumptions about heat pumps need to die if we are going to convince the majority of homeowners to move off fossil fuel heating.
In another thread on this topic, I asked whether the heat pump manufacturers have they done anything yet about their comfort. I know that it at least used to be that heat pumps produced 82F to 92F air, which felt cold and uncomfortable in the winter, as it's below our body temperature of 98.6F. These temperatures would obviously still warm things up, but the air itself felt very uncomfortable. I don't know, however, if the newest heat pumps still work the same.

Natural gas furnaces produce 130F to 140F air, which always feels nice and warm, as it's significantly above our body temperature.

In that thread, I was told that the newest heatpumps still work the exact same way, but that you can slightly alleviate the discomfort by running the blower at a lower speed. If so, that's not really an attractive option for quite a few reasons.

In other words, energy efficiency is obviously important, but so is comfort. Yet, an overwhelming majority of the posts in this thread focus solely on the energy efficiency without mentioning anything about the comfort drawbacks that people would have to accept from heat pumps.
Small nit: average skin temp is ~91 degrees, not 98.6....
mgensler
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Re: [Replacing HVAC - considering heat pump]

Post by mgensler »

illumination wrote: Wed Dec 08, 2021 9:21 am
mgensler wrote: Wed Dec 08, 2021 8:14 am OP - We had the setup that you described for 12 years in our home. It is typically known as dual fuel. We would run the heat pumps down to 25F or so and then the 92% natural gas furnace below that temp. We would also use the gas furnace to recover after setting back the heat at night. We used the Ecobee thermostats to handle this setup and they worked well. Furnaces and heat pumps were a mix of Carrier and Trane. We're in climate zone 4a which is mostly heating-dominated.

This year we did a major kitchen remodel and decided to air seal and insulate to code as much of the house as we reasonably could. As part of upgrading the kitchen appliances we decided to replace the gas ovens with electric and the gas cooktop with induction. We also decided to replace the HVAC systems with Asian cold-climate mini-splits from Fujitsu. This would mostly eliminate our use of natural gas. The mini-splits are ducted so they appear similar to our old HVAC setup. So far this winter, cost is running about the same as our old setup. We're in an area that has both cheap natural gas and cheap electricity. In the spring we will be installing a solar array to hopefully offset most of our power use and get us close to net zero on a monthly basis.

So after all of these upgrades (sounds like tens of thousands of dollars, maybe more?) ... the utility costs are basically the same as they were with the gas furnace and gas appliances? And that's WITH major upgrades in insulation?

Seems like a really good case to keep a gas furnace and gas appliances.
Our electric costs are around 9.5 cents per kwh and our natural gas is around 90 cents per therm. So cheap energy all around. We actually reduced the capacity from 8.5 tons on the heat pump side and 21 tons on the furnace side to 4 tons for the cold climate heat pumps. Due to supply issues we have one heat pump that hasn't been installed yet so we are only running on 3.25 tons right now. This means that the heat pumps we have are running at a higher speed and they use more energy per btu output at high speed. These are most efficient when run at low speed even at low outside temp.

As part of the kitchen remodel these upgrades were a fraction of the total cost and add to the overall comfort of the house. We did it primarily to reduce our carbon emissions (with the addition of solar next year) and to be a model for the neighborhood. If it was purely economics it would have made sense to just stick with the natural gas furnaces and do the air sealing/insulation.
Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

illumination wrote: Wed Dec 08, 2021 9:56 am


We're not supposed to get into environmental (carbon footprint) type arguments on the board, but most electricity is generated by fossil fuels and I would say it's complicated and depends on a lot of factors, like where you get your electricity from.
If your electricity is generated by coal, yes. If it is generated by nuclear/ wind/ hydro then of course no. If it is generated by gas then it depends on your COP. That efficiency advantage on a HP makes a big difference (off the top of my head, for a COP above about 2.8 an HP is superior to a gas furnace: you are burning 1 kwhr of electricity (generated w c. 50%% efficiency) & turning that into 2.8 kwhr of heat v burning 3 kwhr of gas and turning it into say 2.7 kwhr of heat).

(Let's complicate things, the *marginal* kwhr matters more than the *average* - and yes, there are calculators on line which take that into account (at least for the UK)).
I would also say if environmental concerns are your real priority (and not saving a few dollars on your utility bills) there's better bang for your buck elsewhere. I'd rather throw solar on my roof and keep my gas furnace than have a home full of mini splits, but YMMV.
Then it really *does* get complicated - depending on what consumption you displace (and time of day etc). Solar PV + battery would offer real savings (because you avoid using the grid in peak hours).

It's not an either-or of course - one can do both (at different times).

Many jurisdictions, it is possible to buy a "green" tariff which uses energy from renewable sources (we'd then have to have an -- off topic -- discussion).
If commodities like natural gas shoots up in price, electric utility prices will likely follow. It's all very interconnected. I just know I'm really glad I have the gas option as it just works better and its incredibly cheap where I live. If I converted my home to "all electric" my utilities could easily be another few hundred a month and I would be less comfortable.
I agree that the marginal wholesale cost of electricity is usually set in the market by the marginal cost of gas fired power (probably not if you live in the Pacific NW or possibly Midwest). However that's not the only factor so electricity prices are not 1 for 1 linked to gas prices.
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Re: [Replacing HVAC - considering heat pump]

Post by mgensler »

Valuethinker wrote: Wed Dec 08, 2021 10:59 am
illumination wrote: Wed Dec 08, 2021 9:56 am


We're not supposed to get into environmental (carbon footprint) type arguments on the board, but most electricity is generated by fossil fuels and I would say it's complicated and depends on a lot of factors, like where you get your electricity from.
If your electricity is generated by coal, yes. If it is generated by nuclear/ wind/ hydro then of course no. If it is generated by gas then it depends on your COP. That efficiency advantage on a HP makes a big difference (off the top of my head, for a COP above about 2.8 an HP is superior to a gas furnace: you are burning 1 kwhr of electricity (generated w c. 50%% efficiency) & turning that into 2.8 kwhr of heat v burning 3 kwhr of gas and turning it into say 2.7 kwhr of heat).

(Let's complicate things, the *marginal* kwhr matters more than the *average* - and yes, there are calculators on line which take that into account (at least for the UK)).
I would also say if environmental concerns are your real priority (and not saving a few dollars on your utility bills) there's better bang for your buck elsewhere. I'd rather throw solar on my roof and keep my gas furnace than have a home full of mini splits, but YMMV.
Then it really *does* get complicated - depending on what consumption you displace (and time of day etc). Solar PV + battery would offer real savings (because you avoid using the grid in peak hours).

It's not an either-or of course - one can do both (at different times).

Many jurisdictions, it is possible to buy a "green" tariff which uses energy from renewable sources (we'd then have to have an -- off topic -- discussion).
If commodities like natural gas shoots up in price, electric utility prices will likely follow. It's all very interconnected. I just know I'm really glad I have the gas option as it just works better and its incredibly cheap where I live. If I converted my home to "all electric" my utilities could easily be another few hundred a month and I would be less comfortable.
I agree that the marginal wholesale cost of electricity is usually set in the market by the marginal cost of gas fired power (probably not if you live in the Pacific NW or possibly Midwest). However that's not the only factor so electricity prices are not 1 for 1 linked to gas prices.
If you have the Sense monitor, it will show you your carbon intensity graphed over time with the fuel mix your utility is using during the time of your power consumption. Right now our utility is using 43% Natural Gas, 31% Nuclear, 19% coal, and the rest is renewables.
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

aquaman wrote: Tue Dec 07, 2021 3:53 pm
jharkin wrote: Tue Dec 07, 2021 11:53 amAll these outdated "rules of thumb" and assumptions about heat pumps need to die if we are going to convince the majority of homeowners to move off fossil fuel heating.
In another thread on this topic, I asked whether the heat pump manufacturers have they done anything yet about their comfort. I know that it at least used to be that heat pumps produced 82F to 92F air, which felt cold and uncomfortable in the winter, as it's below our body temperature of 98.6F. These temperatures would obviously still warm things up, but the air itself felt very uncomfortable. I don't know, however, if the newest heat pumps still work the same.

Natural gas furnaces produce 130F to 140F air, which always feels nice and warm, as it's significantly above our body temperature.

In that thread, I was told that the newest heatpumps still work the exact same way, but that you can slightly alleviate the discomfort by running the blower at a lower speed. If so, that's not really an attractive option for quite a few reasons.

In other words, energy efficiency is obviously important, but so is comfort. Yet, an overwhelming majority of the posts in this thread focus solely on the energy efficiency without mentioning anything about the comfort drawbacks that people would have to accept from heat pumps.
If your home is a certain air temperature, it is that air temperature.

If you blow air in at a higher temperature (130F say) it gets faster to that air temperature (say 72 F) and then it shuts down. Cools down, starts the cycle again. You are either too hot or too cold and the cycling is really uncomfortable. That is my subjective experience of office heating/ cooling systems (too much one way or the other) - in the UK forced air systems are the commercial norm (but hot water rads the residential norm).

The way you run a HP is different. It is producing air at a much closer to desired temperature. So:

1. you run it for longer ("low and slow")
2. it does not then cycle on and off as much

The point being that you are still aiming for the same air temperature, it's just how you get there.

It does mean that HPs work better in well-insulated & airtight homes. Because for example you won't turn it down when you go out (OR you need to have a fairly smart controller, that knows how long in advance of your return it needs to increase the temperature). In a poorly insulated house they won't work at all well -- you will get cold spots in the room and they won't be heated back up as quickly as with forced air gas furnace.
Last edited by Valuethinker on Wed Dec 08, 2021 11:20 am, edited 1 time in total.
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

mgensler wrote: Wed Dec 08, 2021 11:08 am
If you have the Sense monitor, it will show you your carbon intensity graphed over time with the fuel mix your utility is using during the time of your power consumption. Right now our utility is using 43% Natural Gas, 31% Nuclear, 19% coal, and the rest is renewables.
Neat. Thank you.

For Great Britain it is grid.iamkate.com Gas: 44%, wind 21%, nuclear 11%, biomass 7%, coal 4% (Transmission Interconnectors from France will be nuclear, other ICs depend on mix in the other market).

There are various calculators re emissions intensity around.
Last edited by Valuethinker on Fri Dec 10, 2021 7:13 am, edited 1 time in total.
mervinj7
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Re: [Replacing HVAC - considering heat pump]

Post by mervinj7 »

mgensler wrote: Wed Dec 08, 2021 11:08 am If you have the Sense monitor, it will show you your carbon intensity graphed over time with the fuel mix your utility is using during the time of your power consumption. Right now our utility is using 43% Natural Gas, 31% Nuclear, 19% coal, and the rest is renewables.
I have a Sense monitor and I didn't even realize I had this feature. Thanks!
Here's my carbon intensity specs in CA for my house:
40% Natural Gas, 36% Solar, 12% Nuclear, 5% Wind, 4% Geothermal, 1% Biomass, 1% Hydro, <1% Coal.

Considering the COP of most modern heat pumps, we should be a good candidate for ASHP. And that's not even considering that we currently produce excess solar energy from our rooftop panels.
talzara
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Re: [Replacing HVAC - considering heat pump]

Post by talzara »

finite_difference wrote: Tue Dec 07, 2021 6:01 pm I do agree that radiant heat is the most comfortable one though. So we need a heat pump that heats up water and circulates it through the house? Rather than slightly warm air blowing on your face.
We would need more hydronic heating first. According to the latest Residential Energy Consumption Survey, only 5.6% of homes in the United States have hydronic heating. For the other 94.4% of homes, there's nothing to connect an air-to-water heat pump to.

Even for the 5.6% of homes that have hydronic heating, it's mostly baseboard radiators. The Asian air-to-water heat pumps are designed to work with radiant floor heating, which run at lower temperatures over a large surface area. They are capable of heating water to higher temperatures for baseboard, but they will be less efficient.

Here's the European catalog for the Fujitsu Waterstage air-to-water heat pump. All the capacity specs are written for floor heating: https://www.fujitsu-general.com/shared/ ... atw-01.pdf

That's why the Asian HVAC manufacturers don't sell their air-to-water heat pumps in the United States. At least, not yet.
talzara
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Re: [Replacing HVAC - considering heat pump]

Post by talzara »

RetiredAL wrote: Tue Dec 07, 2021 6:43 pm I looked at data for Mitsubishi 36,000btu a couple of weeks ago, and is showed that BTU output drops to 22,000 by 25F, long before it switches to resistive electric. As for the statement of full output at a way low temp, I guess that is technically correct, they just don't mention that "full output" has a temperature curve below a certain temp.
You looked at the wrong model. That is not a Mitsubishi low-temperature mini-split. That is just a mini-split.

Mitsubishi's low-temperature mini-splits are the H2i Hyper Heat models. When Mitsubishi says "full output" at 5°F, it means 100% of nominal heating capacity.

Here are the maximum capacities of the Mitsubishi SUZ-KA36NA2 and the SUZ-KA36NAHZ. Both have 36,000 BTU/h of nominal capacity, but only the NA2 Hyper Heat model delivers more than 36,000 BTU/h down to 5°F.

Code: Select all

         SUZ-KA36NA2       SUZ-KA36NAHZ
 47°F      36,000             40,000
 17°F      23,200             37,000
  5°F                         37,000
-13°F                         29,600
There is a decrease in capacity at lower temperatures, but Mitsubishi has rated a 40,000 BTU/h heat pump at 36,000 BTU/h nominal capacity. You can still get 100% capacity at 5°F, but nobody is going to complain about getting more than 100% at higher temperatures.
talzara
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Re: [Replacing HVAC - considering heat pump]

Post by talzara »

vg55 wrote: Tue Dec 07, 2021 2:44 pm I just purchased and installed two HEIL air source heat pumps to replace 2 end of life compressors and to reduce reliance on fossil fuel. At the moment, i have an outside monitor that will force a change over from heat pump to oil heat at 35F. It all appears to be working fine and so I will soon lower the set point to 30F. Unfortunately, I cant seem to locate any heating efficiency information on these units, so in the absence of information am conducting a trial by error approach. If anyone has insight on the HEIL NXH5 units - I would appreciate your thoughts.
Here are the specs for the Heil NXH5 series: https://www.shareddocs.com/hvac/docs/10 ... 620204.pdf

The performance chart for the 3-ton unit is on page 10. Heating capacity is given in MBh, which is another way of writing kBTU/h. The heating capacities at 1200 CFM and 70°F indoor temperature are:
  • 35,000 BTU/h at 47°F
  • 20,800 BTU/h at 17°F
  • 15,940 BTU/h at 7°F
  • 11,590 BTU/h at -3°F
Compare that to the Mitsubishi SUZ-KA36NAHZ, which maintains a heating capacity of 37,000 BTU/h down to 5°F!

Mitsubishi is giving you 103% of nominal capacity at 5°F, and Heil is giving you 32% of nominal capacity at 7°F. That is the difference between an Asian low-temperature mini-split and a traditional American heat pump.
talzara
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Re: [Replacing HVAC - considering heat pump]

Post by talzara »

adamthesmythe wrote: Tue Dec 07, 2021 6:27 pm I was unpersuaded by the postings about newer heat pump designs which contained claims rather than explanation. The same thing was true about links from heat pump manufacturers. However I HAVE managed to turn up one link with real technical content that provides a justification for believing that improved designs, capable of useful operation at lower temperatures, are practical.

(https://www.energy.gov/sites/default/fi ... 042314.pdf)
That is an example of what is wrong with the American HVAC industry.

In that slide deck, Unico describes a design that uses two compressors as a value-engineered alternative to putting in an inverter. If you suggested this to an Asian HVAC manufacturer, they would think that it was a funny joke.

The Unico cold climate heat pump project began in 2013 and was scheduled to be completed in 2016. It is now 2021. Where is it? There's no low-temperature heat pump in the Unico catalog.

Unico is selling iSERIES mini-splits now. They're not low-temperature mini splits, but at least they're inverter-driven. However, it's not American technology inside. It's Italian. iSERIES mini-splits are actually made by Argoclima in Lombardy, Italy.

Argoclima also makes air-to-water heat pumps, but they're not available in the United States. Unico is only importing the air-to-air ductless mini-splits. Here's the Argoclima catalog for Europe: https://argoclima.com/wp-content/upload ... presso.pdf
aquaman
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Re: [Replacing HVAC - considering heat pump]

Post by aquaman »

Valuethinker wrote: Wed Dec 08, 2021 4:49 amThe mantra remains "HPs are best run "low and slow"" and gas furnaces "hard (high) and fast". As long as the HP's output temp is above the current air temp in the home, it will be heating the home.
Again though, the issue with heat pumps is that the temperature of the air coming out of them is below our body temperature. So, while they're heating the house, the air coming out of them feels cold and uncomfortable.

The very point of heating a house (beyond the need to protect it from damage, such as to prevent the pipes freezing, etc...) is to make it comfortable in the winter. Hence, the fact that the air coming out of the heat pumps is below our body temperature and, therefore, feels cold and uncomfortable is a very significant issue.

Natural gas furnaces don't have to be run fast, and there are plenty of multi-stage ones out there.
aquaman
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Re: [Replacing HVAC - considering heat pump]

Post by aquaman »

Valuethinker wrote: Wed Dec 08, 2021 11:11 amIf your home is a certain air temperature, it is that air temperature.
Actually, it's not. Sitting in front of a blowing fan in the summer makes you feel cooler even though the fan is doing nothing to reduce the temperature in the room (in fact, the fan motor slightly heats up the room). That's because of the wind chill effect. https://science.howstuffworks.com/question22.htm

Likewise, if you sit in front of a heater like this one (https://www.gopresto.com/product/heatdi ... ater-07926), you'll feel quite warm, even though the heater isn't actually doing much to warm up the room.

Hence, as I mentioned above, the fact that the heat pumps produce air temperatures below our body temperature makes you feel cold and uncomfortable even though the room temperature is going up.
If you blow air in at a higher temperature (130F say) it gets faster to that air temperature (say 72 F) and then it shuts down. Cools down, starts the cycle again. You are either too hot or too cold and the cycling is really uncomfortable.
A properly sized furnace doesn't do that, not to mention the fact that as I pointed out above, there are plenty of multi-stage residential furnaces out there.

What you are describing is short-cycling, which is a symptom of improper sizing and isn't limited to oversized natural gas furnaces.
Last edited by aquaman on Wed Dec 08, 2021 3:06 pm, edited 1 time in total.
adamthesmythe
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Re: [Replacing HVAC - considering heat pump]

Post by adamthesmythe »

talzara wrote: Wed Dec 08, 2021 2:04 pm
Unico is selling iSERIES mini-splits now. They're not low-temperature mini splits, but at least they're inverter-driven. However, it's not American technology inside. It's Italian. iSERIES mini-splits are actually made by Argoclima in Lombardy, Italy.
You have asserted but you have not explained.

What does the inverter do? The inverters I know convert DC to AC. Apparently these units run on AC, so what's going on?

More significant- HOW does an inverter result in superior low-temperature operation?

Inquiring minds want more than marketing claims.
twh
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Re: [Replacing HVAC - considering heat pump]

Post by twh »

There was a lot of talk of 2-stage A/C - the compressor runs on one of two speeds. The inverter basically turns that whole thing into variable speed rather than one or two speeds.
talzara
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Re: [Replacing HVAC - considering heat pump]

Post by talzara »

adamthesmythe wrote: Wed Dec 08, 2021 2:57 pm You have asserted but you have not explained.

What does the inverter do? The inverters I know convert DC to AC. Apparently these units run on AC, so what's going on?

More significant- HOW does an inverter result in superior low-temperature operation?

Inquiring minds want more than marketing claims.
I never said the inverter resulted in superior low-temperature operation. In fact, I said that the Unico (Argoclima) iSERIES mini-splits are not low-temperature mini-splits.

The need for inverters is explained on slide 6 of the Unico slide deck that you read:
Need better COP and capacity, need a “Boost”. How?
- Oversize the compressors
- Use multiple compressors in series
This creates a problem in cooling – need capacity control
- allows sizing for HEATING, not COOLING

https://www.energy.gov/sites/default/fi ... 042314.pdf
In heating-dominated climates, a low-temperature mini-split will be greatly oversized for cooling. It will even be greatly oversized for heating for part of the heating season. An inverter makes it possible to run at partial load without short-cycling the compressor.

Maybe you didn't piece it together because Unico didn't use an inverter. Unico solved the problem by putting in two compressors and turning one of them off at partial load. It didn't work very well, and it's not in any Unico products today.

In the 2017 slide deck from Unico, slide 10 shows that compressor 2 is still turning on at 45-50°F. Whatever control algorithm they're using, it's short-cycling compressor 2. It's going to wear out. Slide 20 congratulates themselves for beating the Japanese. What good is that when your compressor is going to fail?
Valuethinker
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Re: [Replacing HVAC - considering heat pump]

Post by Valuethinker »

aquaman wrote: Wed Dec 08, 2021 2:39 pm
Valuethinker wrote: Wed Dec 08, 2021 4:49 amThe mantra remains "HPs are best run "low and slow"" and gas furnaces "hard (high) and fast". As long as the HP's output temp is above the current air temp in the home, it will be heating the home.
Again though, the issue with heat pumps is that the temperature of the air coming out of them is below our body temperature. So, while they're heating the house, the air coming out of them feels cold and uncomfortable.

The very point of heating a house (beyond the need to protect it from damage, such as to prevent the pipes freezing, etc...) is to make it comfortable in the winter. Hence, the fact that the air coming out of the heat pumps is below our body temperature and, therefore, feels cold and uncomfortable is a very significant issue.

Natural gas furnaces don't have to be run fast, and there are plenty of multi-stage ones out there.
Thank you that is interesting. I'd have to do more reading on it. If a room is a desired temperature it is the desired temperature. If it blows out of the vent a lot hotter than that, it will simply cause itself to turn off sooner.

Re NG furnaces I was not being entirely clear. I agree they can modulate (European systems are generally indirect and certainly all the good boilers do that). But the general principle remains the same - keeping the house at lower temperature when you are out or sleeping, then turning the thermostat up when you want a warmer temperature.

The heat output of combustion is maximised when you run it at a high output temperature - hence that source of hotter air you were talking about.
furiouschads
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Re: [Replacing HVAC - considering heat pump]

Post by furiouschads »

I think I will replace my noisy central heat pump with a Daikin VRV Life heat pump. Inverter is the way to go. it serves my second floor and attic. Maybe I will use it to also replace the outdoor unit of my first floor system, which is a compressor, an efficient furnace, and a variable speed air handler. I'm in the mid-Atlantic.
Normal is an illusion. What is normal for the spider is chaos for the fly. --Charles Addams. #613 in 2022 BH prediction contest. #42 in 2023. Not that I am keeping track.
ondarvr
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Re: [Replacing HVAC - considering heat pump]

Post by ondarvr »

I installed a low temperature mini split system in August of 2020 and tracked the cost difference compared to my electric forced air system.

My temperatures ranged from +112 to -8F

My savings were as high as $324.00 per month. This didn't take into account that I supplemented the forced air system with my wood stove during the coldest times. The least I saved was $50.00 per month in the summer.

And one other nice difference is when saving that much using the mini splits is the house was kept at around 70-72 at all times, no turning it down at night.

It was 24F here yesterday, I checked the heat coming out right at the coil, it was between 120 to 128F at each unit. You can physically feel warm air blowing on you when you have it on high speed when it's 15F outside.

It doesn't completely shut off when it's really cold out, it throttles down to where it only supplies enough heat to keep the temperature constant at the desired temp. So you never really experience a temperature fluctuation.

I also put a system in my shop, this savings includes keeping a 15'x30' room in it at 68F all year.

I installed both systems myself and it wasn't difficult.

I haven't removed the existing forced air system because I thought I may need a backup, but it hasn't been turned on since the first day I powered up the mini splits.
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Re: [Replacing HVAC - considering heat pump]

Post by jharkin »

illumination wrote: Tue Dec 07, 2021 3:53 pm
jharkin wrote: Tue Dec 07, 2021 11:53 am
illumination wrote: Tue Dec 07, 2021 10:40 am
But an "average" central air heat pump in most homes needs "help" when it gets below 40 degrees outside. Most all electric ones have an electric resistance coil that turns on for "Aux" heat. It's incredibly inefficient when running that way. Even modern heat pump manufacturers have this as a disclaimer.

What are you basing this "average" on? Personal experience or something stated online (possibly by a contractor trying to upsell?).

It's coming right from the manufacturer's of heat pumps.

AT WHAT TEMPERATURES ARE HEAT PUMPS MOST EFFECTIVE?
The heat pump is most effective on its own at temperatures around 25 to 30 degrees Fahrenheit. At that point, either a gas furnace or an air handler with supplemental electric heat will kick in to help heat your home.


https://www.trane.com/residential/en/re ... r-my-home/
Thats written by the marketing department at Trane, probably before they had cold climate heat pumps in their catalog. Since they signed a partnership deal with Mitsubishi in 2018 they now have relabeled Misu hyper heats and I’d imagine that will be updated.

… or maybe not if they make a better profit margin on the the units coming out of their own factory. Doesn’t change what’s on the engineering spec sheets of the units being sold that work down to -13
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illumination
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Re: [Replacing HVAC - considering heat pump]

Post by illumination »

jharkin wrote: Fri Dec 10, 2021 6:19 am
illumination wrote: Tue Dec 07, 2021 3:53 pm
jharkin wrote: Tue Dec 07, 2021 11:53 am
illumination wrote: Tue Dec 07, 2021 10:40 am
But an "average" central air heat pump in most homes needs "help" when it gets below 40 degrees outside. Most all electric ones have an electric resistance coil that turns on for "Aux" heat. It's incredibly inefficient when running that way. Even modern heat pump manufacturers have this as a disclaimer.

What are you basing this "average" on? Personal experience or something stated online (possibly by a contractor trying to upsell?).

It's coming right from the manufacturer's of heat pumps.

AT WHAT TEMPERATURES ARE HEAT PUMPS MOST EFFECTIVE?
The heat pump is most effective on its own at temperatures around 25 to 30 degrees Fahrenheit. At that point, either a gas furnace or an air handler with supplemental electric heat will kick in to help heat your home.


https://www.trane.com/residential/en/re ... r-my-home/
Thats written by the marketing department at Trane, probably before they had cold climate heat pumps in their catalog. Since they signed a partnership deal with Mitsubishi in 2018 they now have relabeled Misu hyper heats and I’d imagine that will be updated.

… or maybe not if they make a better profit margin on the the units coming out of their own factory. Doesn’t change what’s on the engineering spec sheets of the units being sold that work down to -13

It's frankly a better authority than what you're citing. Again, I'm not saying ALL heat pumps can't work at really low temps without aid of electrical resistance or some sort of fuel furnace, I used the word "average". Most central heat pumps installed work this way, even brand new ones. A setup like you're discussing that works at subzero temps without any other aux heat is not exactly common. You also have to question that just because something can technically operate, does that mean it does a good job?

What I don't get is your making this case that the average heat pump works down to some subzero temp to comfortably heat your home, when you yourself use this unique model and concede you use propane to supplement heating your home.
jharkin wrote: Sat Dec 04, 2021 4:14 pm

I installed a 2.5 T Mitsubishi hyper heat ducted unit to supplement my propane hot water baseboard. It’s been down to 24F already and it’s not even running full time, house feels more comfortable than with the baseboard and my propane usage seems to be cut in half. Electric has gone up for sure but won’t know how much until next bill, thoug I don’t think it will be even as bad as the the 30 year old AC it replaced was in summer.
bluegill
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Re: [Replacing HVAC - considering heat pump]

Post by bluegill »

Big Dog wrote: Wed Dec 08, 2021 10:11 am
bluegill wrote: Fri Dec 03, 2021 7:29 am I live in the TN river valley. Electricity is relatively cheap here. TN Valley Authority electricity. Most homes have Heat Pumps with natural gas furnace in this area. I avoid brands that restrict access to repair parts. These companies sell parts only to Authorized dealers/repairmen. Goodman brand is manufactured in this area and parts are not restricted. My son went to HVAC school. Quality of installation is much more important the quality of hardware. Recommendation: Variable speed air handler is more comfortable and quieter than single speed air handler. It worth the money to pay for variable speed air handler.
Can I ask the value of the HP when you also have a gas furnace? Is the HP that much cheaper to run when the outside temp is ~50 than the gas furnace? Or, is it just the cleaner source of energy, i.e., environmental savings?
I don't know why, it's just the traditional thing to do. I don't do it to save the planet. If you buy a home around here they usually have heat pumps and sometimes they have natural gas. The Tn River valley is HOT many months of the year, so we need Air Conditioning, which a heat pump provides. ----- Another factor is, heat pumps are uncomfortable when it gets really cold. Air temperature at outlet vent/duct is around 80 degree with a heat pump, and around 120 degrees when natural gas furnace. 80 degree out of the duct feels cold.
Big Dog
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Re: [Replacing HVAC - considering heat pump]

Post by Big Dog »

bluegill wrote: Thu Dec 16, 2021 3:58 pm
Big Dog wrote: Wed Dec 08, 2021 10:11 am
bluegill wrote: Fri Dec 03, 2021 7:29 am I live in the TN river valley. Electricity is relatively cheap here. TN Valley Authority electricity. Most homes have Heat Pumps with natural gas furnace in this area. I avoid brands that restrict access to repair parts. These companies sell parts only to Authorized dealers/repairmen. Goodman brand is manufactured in this area and parts are not restricted. My son went to HVAC school. Quality of installation is much more important the quality of hardware. Recommendation: Variable speed air handler is more comfortable and quieter than single speed air handler. It worth the money to pay for variable speed air handler.
Can I ask the value of the HP when you also have a gas furnace? Is the HP that much cheaper to run when the outside temp is ~50 than the gas furnace? Or, is it just the cleaner source of energy, i.e., environmental savings?
I don't know why, it's just the traditional thing to do. I don't do it to save the planet. If you buy a home around here they usually have heat pumps and sometimes they have natural gas. The Tn River valley is HOT many months of the year, so we need Air Conditioning, which a heat pump provides. ----- Another factor is, heat pumps are uncomfortable when it gets really cold. Air temperature at outlet vent/duct is around 80 degree with a heat pump, and around 120 degrees when natural gas furnace. 80 degree out of the duct feels cold.
Thanks for responding. In your situation, and unless you have solar panels, I'd probably just go with a standard ac unit, a two- (or variable-) speed handler, and the gas furnace.
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