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The Renewable Energy site for Do-It-Yourselfers
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.
Cover
- 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
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.
Rebates
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?
References:
Residential Heat Pump Water Heaters
-- Energy Efficiency Fact Sheet,
Washington State University Extension Energy Program, 3 pages, 2008
http://www.energy.wsu.edu/documents/building/res/ht_pmp_water_htrs.pdf