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The Renewable Energy site for Do-It-Yourselfers
Heat Pump Efficiency for use with
Fuel Comparison Calculator
To use the Fuel Comparison Calculator
for heat pumps, you need to find out the seasonal COP (Coefficient Of
Performance) for the heat pump, and multiply this by 100 to get the effective
efficiency to fill into the Efficiency column.
For example, if you find that the
heat pump of interest to you has a seasonal COP of 2.2 when installed in your
climate, then the effective efficiency is (2.2)(100) = 220 -- you
would enter 220 in the "Efficiency" column.
The actual heat pump COPs for both air and
ground source heat pumps depends on a number of factors. Heat pumps
do not perform in cold climates as well as they do in not so cold climates.
If it gets cold enough, heat pumps revert to simple resistance heaters with an
efficiency of around 95%.
What you are looking for is the
average COP for the full heating season for the heat pump installed in your
climate. This will likely be well below the COP when the heat pump is
operated under more favorable conditions, but it is important to get an average
seasonal COP.
Some sources provide an HSPF -- to
convert this to a COP, divide the HSPF by 3.412. For example an HSPF of
6.8 equates to a seasonal COP of (6.8 / 3.412) = 1.99.
One estimate from a seemingly
reputable source says that new air source heat pump seasonal (i.e. for the whole
heating season) have average COPs that range from 1.6 to 2.8, and new ground
source heat pump seasonal COPs range from 2.4 to 3.5. The link to the
related article is listed below. There are a few studies out in
which the actual performance of heat pumps installed in real homes was measured
-- these would be worth looking for if you are thinking of a heat pump.
From the emails I get and material I
see, ground source heat pump performance depends a lot on good design and good
installation. It appears to me that there are a significant number of GSHP
installations that don't meet customer expectations due to poor design or poor
installation -- so make sure you do your homework and get a good contractor.
More Information:
"Choosing a Heating System That Saves energy" from Home Energy Magazine ...
Article comparing heating systems and
including some discussion of heat pumps.
Comments/letters in response to
above article:
Discussion of Heat Pump COP from Home Energy Magazine ...
An Information Survival Kit for the Prospective Geothermal Heat Pump Owner,
Rafferty for the DOE...
Good information on the different
styles of GSHPs and the most frequent questions.
The EERE on heat pumps ...
To convert the HSPF to a COP divide
by 3.412 -- e.g. an HSPF of 8.2 would equate to seasonal COP of 2.4, or an
effective efficiency of 240%.
"Renewable hat -- the era of ground source heat pump", Modern Building Science
...
Some material on seasonal COPs
Definitions of COP, HSPF, and SEER ...
"Factors Influencing Space Heat and Heat Pump Efficiency from a Large-Scale
Residential Monitoring Study", from FSEC ...
A monitoring of actual homes in
Florida for actual COPs achieved.
Gary April 28, 2008, May 2, 2008