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.