Heat Pump Water Heater for a Hydro-air system w/integrated HRV?
begreener
| Posted in Mechanicals on
Back in 1996 I built a “super-insulated” house in NH using spray foam (R30 Walls/R55 Roof) & I heated it with a hydro-air system that used a very small condensing, tankless hot water system (whose manufacturer’s name escapes me today) as the hot water supply.
Has anyone ever used a Heat Pump Water Heater to supply the input to a Lifebreath Clean Air Furnace which combines an air handler with a built-in heat recovery ventilator?
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Be Greener,
Heat pump water heaters steal heat from indoor air in order to create hot water. If you try to use a heat pump water heater for space heating, the appliance will be cooling the air near the water heater as you try to raise the temperature of the air elsewhere. It's a silly battle, and no more efficient than heating your home with electric baseboard heat, which is much cheaper to install.
Why couldn't you just thermally isolate the air handler/heat exchanger from HPWH?
The device you want is called an air to water heat pump, it pulls heat from outside to heat the water. There are parts of NH that get quite cold, you might find it hard to find one that can deliver enough heat at the temperatures you see.
You might also find the price deterring.
After doing a little research, the Harvest Thermal system seems to be the most akin to what I was trying to ask about ...
https://www.harvest-thermal.com/product
It uses an outdoor SanCO2 compressor, which works down to -27F and that's the name implies, use a CO2 as a refrigerant. I've spoken with them because they are headquartered near where I live, and they said for now they are focused on the California market.
I don't like the sound of that at all.
The reason is the efficiency of heat pumps, and the output of heat emitters, is determined entirely by temperature differences. With a tank like that, you're filling the tank at a high temperature difference -- the water you put in has to be hotter than the water in the tank, so as it fills you need hotter and hotter water. And you're taking heat out at a low temperature difference, as you take heat out the tank cools and by the end of the night it's only slightly above room temperature. This is exactly the opposite of what you want to do for maximum efficiency. It's only going to make sense if the cost of power is dramatically greater at one time of day than another.
Secondarily, systems like this typically dramatically underestimate the size of the tank needed for meaningful heat storage.
The issue with these hydro air coils is the water temperature needed. I've used something similar from Rannai before and the issue was supply air temperature. I had to run the water pretty hot (~150F) for it to deliver air that was not too cold. For a modcon, supply temperature does effect efficiency, all about return water temp but for any air to water it will kill your efficiency.
You can run these with colder water just make sure the outlets don't blow at people. This also means you won't get nowhere near nameplate BTU out of your hydro coil so it needs to be well oversized for it to heat the place.
If you look a the cost of all this VS a new ducted heat pump, you are spending more to get a system that is less efficient and mechanically much more complicated.
The SanCO2 takes cold water and brings it to hot (150 degrees) in one single pass.
Sanco use CO2 for the refrigerant and it has a different refrigerant cycle than a regular heat pump with r410. It can make 150F water but needs a supply water temperature of 50F-60F. This is fine for domestic hot water where the supply water is at soil temperature. When you feed it 130F or so water from your hydro air coil, its COP will tank, maybe in the range of 1.5. At that point might as well use resistance heat.
The low COP also means it can't supply as much heat, you might only get 4000 to 5000 BTU out of it.
Anything air to water works the best with low temperatures. For decent efficiency, you want something that can use 100F-120F or so water for space heat. This generally means larger panel rads or floor heat.
Yeah, I think the most cost-effective solution is going to be just dropping in an air-to-air air handler to replace the hydronic coil. Since it's just one coil and not a whole hydronic system it should be a straight swap out.
You’ll need to get the heat from outside the house. To me, the most logical path is installing a regular ducted heat pump then figuring out hot water. This option provides cooling too.
We can help you do a basic sanity check on a replacement. Go to this site:
https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf
Look up your county, and report back with your heating design temperature. Then follow the process laid out in this article:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
To get your heating load based on historical fuel usage.
From there we can see if there is a heat pump that will meet your needs.