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DickR · 02-01-2014, 08:03 PM

Heat pump water heaters (HPWH) perform most favorably in warm climates, where the norm is use of AC to remove heat from the house. Bear in mind that a HPWH pulls heat from the air surrounding the heater and moves it to the water inside the tank. There must be adequate air volume around the unit to provide the heat moved into the tank. That air volume will be cooled, as heat is withdrawn from it. That heat must be replaced.

In summer, in a house into which normally a fair amount of outside heat leaks and might otherwise have to be removed via air conditioning, a HPWH might be a good way of relocating that heat to water that must be heated anyway. Some dehumidification is obtained in the process (and thus there is the need for condensate draining/removal). I doubt that the summertime average efficiency would give more than twice the heat into water than otherwise provided by the electric power used by the HPWH. A COP of 3.0 is overly optimistic for this application.

In winter, the heat removed from the air around the HPWH must be replaced by whatever heating system is used to heat the house. If that heat source is cheaper, per BTU of heat provided, than straight electric resistance heating, then a HPWH will provide hot water at less expense, although the economics of running the HPWH in winter won't be anywhere near what it can be in summer. There is of course the comfort issue of the air in the space around the HPWH being cooled substantially, at a time when the house is being heated.

Overall, averaged through the year, more heat is delivered to water via a HPWH than via straight electric resistance heating, but offsetting this is the added cost of the HPWH equipment, which ultimately must be serviced or replaced after so many years of operation. The NHEC incentives help in this regard, but I have to wonder about the overall economics of adding the complication in a heating climate such as ours (climate zone 6).

There has been considerable discussion of HPWH on well-regarded sites such as www.greenbuildingadvisor.com. A search there brings up quite a number of discussions about HPWH.

02-01-2014, 08:03 PM	#2
DickR Senior Member Join Date: Apr 2008 Location: Moultonborough Posts: 765 Thanks: 4 Thanked 260 Times in 172 Posts	Heat pump water heaters (HPWH) perform most favorably in warm climates, where the norm is use of AC to remove heat from the house. Bear in mind that a HPWH pulls heat from the air surrounding the heater and moves it to the water inside the tank. There must be adequate air volume around the unit to provide the heat moved into the tank. That air volume will be cooled, as heat is withdrawn from it. That heat must be replaced. In summer, in a house into which normally a fair amount of outside heat leaks and might otherwise have to be removed via air conditioning, a HPWH might be a good way of relocating that heat to water that must be heated anyway. Some dehumidification is obtained in the process (and thus there is the need for condensate draining/removal). I doubt that the summertime average efficiency would give more than twice the heat into water than otherwise provided by the electric power used by the HPWH. A COP of 3.0 is overly optimistic for this application. In winter, the heat removed from the air around the HPWH must be replaced by whatever heating system is used to heat the house. If that heat source is cheaper, per BTU of heat provided, than straight electric resistance heating, then a HPWH will provide hot water at less expense, although the economics of running the HPWH in winter won't be anywhere near what it can be in summer. There is of course the comfort issue of the air in the space around the HPWH being cooled substantially, at a time when the house is being heated. Overall, averaged through the year, more heat is delivered to water via a HPWH than via straight electric resistance heating, but offsetting this is the added cost of the HPWH equipment, which ultimately must be serviced or replaced after so many years of operation. The NHEC incentives help in this regard, but I have to wonder about the overall economics of adding the complication in a heating climate such as ours (climate zone 6). There has been considerable discussion of HPWH on well-regarded sites such as www.greenbuildingadvisor.com. A search there brings up quite a number of discussions about HPWH.