Dual-fuel and multi-zone HVAC?

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archbish99
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Joined: Fri Jun 10, 2011 6:02 pm

Dual-fuel and multi-zone HVAC?

Post by archbish99 »

I suspect I know the answer to this, but I'm new to the gory details of HVAC wiring and mechanics, so I want to make sure I haven't missed something. We're meeting with the HVAC folks who did our old furnace on Thursday to talk about upgrading to a dual-fuel heat pump / furnace combo so we get air conditioning as part of the bargain. Since this house is two-story, I was also interested in exploring making the stories separate zones. (The upstairs can get quite warm when it's trying desperately to heat the lower level.)

In part because I like cool tech and in part because it can apply some smarts and cost logic to the use of gas furnace versus heat pump, we're also planning to put in Nest thermostats as part of the work.

Am I correct in concluding that a thermostat, Nest or not, can be either dual-fuel (i.e. wired to activate the heat pump or the furnace based on exterior temperature) or wired up to a zone controller with no knowledge of fuel, but not both? If so, it would be the zone controller that chooses the fuel source (presumably based on an exterior sensor with no knowledge of relative costs) and the Nest just calls for heat or cooling. In that situation, is the Nest even able to do many of its smart things like calling for less heat longer, etc.?

Assuming that I'm right about that limitation, I'd also appreciate advice on the relative merits of:

* Zone controller + two Nests
* No formal zones, but something like Flair smart vents to block heat output on the upper level
* Forego the zones; one Nest on the lower level and deal with temperature differences
I'm not a financial advisor, I just play one on the Internet.
Valuethinker
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Joined: Fri May 11, 2007 11:07 am

Re: Dual-fuel and multi-zone HVAC?

Post by Valuethinker »

archbish99 wrote:I suspect I know the answer to this, but I'm new to the gory details of HVAC wiring and mechanics, so I want to make sure I haven't missed something. We're meeting with the HVAC folks who did our old furnace on Thursday to talk about upgrading to a dual-fuel heat pump / furnace combo so we get air conditioning as part of the bargain. Since this house is two-story, I was also interested in exploring making the stories separate zones. (The upstairs can get quite warm when it's trying desperately to heat the lower level.)

In part because I like cool tech and in part because it can apply some smarts and cost logic to the use of gas furnace versus heat pump, we're also planning to put in Nest thermostats as part of the work.

Am I correct in concluding that a thermostat, Nest or not, can be either dual-fuel (i.e. wired to activate the heat pump or the furnace based on exterior temperature) or wired up to a zone controller with no knowledge of fuel, but not both? If so, it would be the zone controller that chooses the fuel source (presumably based on an exterior sensor with no knowledge of relative costs) and the Nest just calls for heat or cooling. In that situation, is the Nest even able to do many of its smart things like calling for less heat longer, etc.?

Assuming that I'm right about that limitation, I'd also appreciate advice on the relative merits of:

* Zone controller + two Nests
* No formal zones, but something like Flair smart vents to block heat output on the upper level
* Forego the zones; one Nest on the lower level and deal with temperature differences
NEST has not been the favored controller here although one would have to search threads for the alternatives.

Zoning is really about comfort I believe. Unless you have a poorly insulated house (not airtight) or live in an extreme climate (say Texas for AC; Minnesota for heating) I am not sure it will save much money.

There is also an HVAC Forum that has been referenced here with very helpful people (some of them industry practitioners).
renue74
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Re: Dual-fuel and multi-zone HVAC?

Post by renue74 »

In the past, zone HVAC used servo motors to open/close vents. I'm told by a few HVAC guys that those motors or the controllers for them tend to "go out." It's been about 5 years since I heard this...so maybe the tech has gotten better.
smackboy1
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Re: Dual-fuel and multi-zone HVAC?

Post by smackboy1 »

The forum you want to visit is here

http://hvac-talk.com/vbb/forum.php

We had a Carrier dual fuel Infinity Hybrid Heat 8 zone system installed 6 years ago. We're very happy with the comfort of the system and had no major problems. All the electric dampers are still functioning with no issues. It wasn't cheap: $10K for the HVAC + $10K for the zoning for a 2 story house. Here are some tips:

- If the house is not well insulated and airtight, get an energy audit and get that done first. A well insulated and airtight house is cheaper to heat/cool and will be more comfortable.

- Get at least 2 bids, more if possible. Each bidder needs to perform a manual J heat load calculation and go through it with you. Any contractor which uses a "rule of thumb" or doesn't measure every room in your house and every window and door for the manual J should be automatically excluded. If needed a manual D evaluates the ducts.

- Make sure you have the contractor perform calculations to figure out the cost/benefit of dual fuel heat pump vs. furnace + AC using your utility costs. Since our installation the price of electricity has gone up and the price of natural gas has plummeted. I have programmed my heat pump to lock out in the winter because it's not worth it. Looking back, for us the heat pump was unnecessary.

- Variable speed blower and multi stage furnace will increase comfort. Zoning will increase comfort. Energy savings are negligible. Personally, for me comfort is of primary importance.
Disclaimer: nothing written here should be taken as legal advice, but I did stay at a Holiday Inn Express last night.
Chip
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Re: Dual-fuel and multi-zone HVAC?

Post by Chip »

My dream system would be something like what you're talking about: dual fuel, two zones. Plus thermostats where you could program in the costs of the two fuels so it would make smart decisions about when to switch.

But, back to reality.

My house is built with one zone, one fuel. It would require serious ductwork modification to go to two zones. Two fuels is an option, but I need to run the numbers. With gas costs this low I suspect it's a lengthy payout.

My current setup has inline manual dampers in the ductwork in the unfinished basement. They're pretty low tech so you might not like them :P . But each winter I close the dampers for the 2nd floor vents and open the ones for the 1st floor. I reverse the process in the summer. It takes 5 minutes. I also open/close the floor registers at the same time. Doing this makes the temperature differential between floors pretty minimal most of the time. Plus I think some attic ventilation work I did several years back really helped with summer heat on the second floor.

One thing to add to smackboy1's thorough post: thermostat location is very important. Your builder might not have put yours in the optimum location. For example, mine is in a room that gets a good bit of solar heating on clear days. So the rest of the house is too cold. I should move it, but I've been too lazy. Easier just to turn up the heat or reduce A/C. :)
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archbish99
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Re: Dual-fuel and multi-zone HVAC?

Post by archbish99 »

The first person we had come out took one look at our ducts and said we're not a good candidate for zoning. His opinion was that our ducts are ideal for heat, but poor for cooling. He thought we might get adequate cooling upstairs if we add an extra return, but said if we want to ensure good cooling upstairs, we may need to consider a separate upstairs unit (installed in the attic).

He also said most people wind up with A/C rather than heat pumps -- paired with a high efficiency furnace, the heat pump would save roughly $50/year, cost roughly $500/more, and have a shorter expected lifespan. Potentially break even, but unlikely to come out ahead. It would make more sense paired with a lower-efficiency furnace, but we have to replace our furnace anyway (the current one can't operate its fan independently of the heat).

The second person is scheduled to come out and do measurements this week, but said on the phone that's probably the right conclusion; his ballpark estimate on how much it would take to rework the ducting to enabling zoning, plus the zoning system itself, comes to roughly the cost of a separate cooling unit upstairs, and that's before you factor in that Bryant strongly recommends using their high-end equipment for zoned systems.

So we're probably looking at dual-speed gas heat, single-speed A/C, and a Nest. Not quite what I was hoping for, but hopefully it will help a lot. If the cooling upstairs isn't sufficient, we can come back and add the attic unit separately later.

Thanks for all the feedback!
I'm not a financial advisor, I just play one on the Internet.
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archbish99
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Re: Dual-fuel and multi-zone HVAC?

Post by archbish99 »

Oh, and as to Nest versus Ecobee, which seem to be the main smart tstats: I've heard strong arguments for both. The Ecobee ($50 more, after rebates on each from our power company) has the advantage of the remote temperature sensor, but what I've heard from owners is that it really just averages the temperature from the sensor(s) and the thermostat and heats until that average is where it wants "the house". That doesn't help the room actually being hotter or colder than elsewhere, it just makes the whole house warmer/cooler than it otherwise would be.

Conversely, with either, you can do some integration with home automation stuff. I've found a relatively simple tool for SmartThings which can take an upstairs temperature sensor and run the fan if the upstairs is more than, say, two degrees warmer than the downstairs. That would actually cause air to circulate and even the temperatures out some. (Which might trigger additional heating/cooling to kick in as a side-effect.) I'm considering also adding a ceiling fan to the stairwell, which could be triggered by the same criteria.
I'm not a financial advisor, I just play one on the Internet.
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