Heat Pump Water Heaters



Heat pump water heaters have been around quite a while, but have never developed a large following.  There appear to be some new models coming along, and this plus the greater emphasis on energy efficiency and the tax credit program may result in more interest in this approach.









- Cold air as result


- Air Tap  (see if you can find others)


- Compare energy use, cost and CO2 emissions for HP water heater to conventional for electric and gas heaters.



What to do with the cool air output?

In normal operation, the heat pump water heater removes heat from the air and transfers it to the water.  This results in some cooling of the air around the heat pump. 


If you live in a climate with a long cooling season, and little or no heating season, this cool air is a plus, and can even be routed to the living air -- the net result is an effective improvement in the COP because you are taking some load off the AC.


But, if you live in a primarily heating climate, the cool air seems problematic.

If you just vent the cool air into the space around the water heater, then the furnace has to basically reheat this air up to room temperature, and it adds to the heating load, and reduces the effective COP.

If you vent the cool air outside, then new air from outside will be pulled in to replace this air, and again, this new outside air must be heated up to room temperature by the furnace.


I don't see any good way around this problem.  You might figure out a way to let the heat pump water heater get its air from outside, and vent it back to outside -- this will decrease the efficiency of the heat pump unit, but one manufacturer shows a COP of 1.7 at 32F.

Perhaps an unheated utility room that is vented to the outside?




Installation of the add on Air Tap heat pump water heater is covered in detail in their installation manual.  It appears to be a fairly straightforward process that a DIYer with a some plumbing experience could tackle successfully.


Installation of a heat pump water heater with the heat pump built in should be similar to a conventional hot water tank installation.


Energy and CO2 Emissions

If the heat pump water heater is added to or replaces an electric water heater, there should be about a factor of 2 reduction in CO2 emissions, since these units should use about half as much electricity. 


If the heat pump water heater replaces a gas water heater, there may be no reduction in CO2 emissions, or even an increase in emissions.    See the energy use table for my estimates.



Table showing energy consumption, CO2 emissions and energy costs for:

Electric to HP

Tankless NG to HP

Tankless Propane to HP?

Tank NG to HP

Tank Propane to HP


Have a column for coolth generated (a plus in summer, but minus in winter).

Cool air could be vented outside in the winter?



Reliability and Maintenance

These are more complex than conventional water heaters, and also more complex that most solar water heaters.  Basically, you have something of the general level of complexity of a simple refrigerator -- it seems like refrigerators can run trouble free for years, or can drive you nuts with repairs.


Here are Some reviews of an E-Tech add on residential heat pump water heater ... that touch on reliability and maintenance for that unit.




Under the new Federal Tax Credits for Energy Efficiency program heat pump water heaters with an Energy Factor (essentially COP) of 2 or greater qualify for a 30% credit up to $1500.  Energy Star seems to be initiating a program for heat pump water heaters, but has none listed at this time -- they say later in 2009.   One source states that add on heat pump water heaters will not qualify for the Energy Star program, but some state incentives may still apply.


GAMA lists two brands of heat pump water heaters that qualified under the 2005 Energy Act for a $300 tax credit.  Its not clear to me whether you are safe picking one of the GAMA listed ones, or if you need to wait for the Energy Star listing?



Residential Heat Pump Water Heaters -- Energy Efficiency Fact Sheet,
Washington State University Extension Energy Program, 3 pages, 2008