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Water heating accounts for 10-20% of energy use in a typical American home. Heat pump water heaters (HPWHs) are an important technology for reducing this energy use and associated carbon emissions. HPWHs use a vapor-compression cycle similar to those in air conditioners, heat pumps, and dehumidifiers. By capturing heat from air and transferring it to the tank, HPWHs can achieve efficiencies three or more times higher than conventional electric water heaters.
The vapor-compression cycle works by manipulating the pressure of a refrigerant inside a closed loop. The compressor raises the pressure and temperature of the refrigerant vapor. In the condensing coil, the high-pressure vapor turns to liquid as it releases heat to the stored water. The liquid refrigerant then passes through a small opening in the expansion valve. This restriction causes a drop in the refrigerant’s pressure, temperature, and boiling point, allowing it to boil and absorb heat from the air in the evaporator coil. A small fan moves air across the evaporator. Electric resistance elements in the tank provide backup heat during periods of high use. Source: Energy Star
Like other vapor-compression appliances, HPWHs use refrigerant to absorb heat in one location and release it in another. The refrigerant used in most HPWHs, R-134a, is a hydrofluorocarbon (HFC) and a potent greenhouse gas. Only a small fraction of the refrigerant in HPWHs and other consumer appliances is captured during repairs and at the end of equipment life; the rest ultimately leaks into the atmosphere. Maximizing the climate benefits of HPWHs requires finding a replacement for R-134a. In this article, we’ll look at three options:
- R-513A, a blend of R-134a and a climate-friendly hydrofluoroolefin (HFO)
- R-290, propane, a highly flammable hydrocarbon
- R-744, carbon dioxide (CO2)
HPWHs that use R-513A and CO2 are already available…
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7 Comments
Thanks--I'd been eagerly awaiting this article. I'm a little disappointed to learn that things aren't as far along as I'd hoped. Maybe I'll go change that anode rod and hope I can get some more life out of my current setup.
Thanks Charlie! I think we're probably a couple years out for commercially available R-290 models. That reminds me, it's about time for me to check the anode on my 5-year old HPWH. It's inside the heat pump shroud and not easy to get to.
One reasonable solution to the issue of propane indoors is to put it outdoors. One could do a monoblock (all in one, precharged unit) outside and produce hot (or cold) water and send it indoors to a tank. If the propane is outside, who cares. Similar systems exist (Chilltrix, Space Pak, Aris, Artic, Apollo, etc) and according to at least one of these manufacturers there is a propane option coming in the near future. Of course there are the issues with water pipes freezing (and high costs, especially as the tech is new), but those are surmountable. The can also be used for space heating and cooling (or heck, why not the refrigerator?)
I agree, this is a good solution, especially when the monoblock outdoor unit can provide both space conditioning and water heating.
Is there a reason flammable refrigerant pipes In houses are treated differently than flammable fuel pipes in houses? Nearly every building in northern cities has natural gas lines running to mechanical rooms and kitchens. My mother's house in Maine has propane lines running to the kitchen for cooking.
I seem to remember Lloyd Alter mentioning on the Retrofit Podcast that corporations manufacturing refridgerants have lobbied to keep propane out of heat pumps in the US.?
There definitely is a double standard. When I was an energy auditor, I found natural gas and propane leaks in maybe a quarter of the houses I visited. Just last week I read another news story about a house explosion caused by a gas leak. I'm not sure the politics behind it, but I do think we need to be extra careful to avoid any negative publicity around A3 refrigerants.
Good point regarding negative publicity.
I suppose another point might be that in the case of natural gas, safety issues are largely addressed by the utility company. I've discovered leaks and the utility company had someone on site to diagnose and either fix the leak or shut off the service within minutes. HVAC installations certainly wouldn't have that support.
That said, there definitely seems to be a double standard, especially considering parts of the world are using A3 refrigerant successfully.
And unlike natural gas or propane service lines, a refrigerant line has a finite amount of gas that can be discharged in the event of a leak.
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