HEAT PUMP COP - CONTENTS: Definition, sources of variation in heat pump operating efficiency & cost. Definition of COP for heat pumps. Definition & explanation of the balance point in a heat pump COP. How to adjust heat pump electricity costs to include the effects of variations in outdoor temperature
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Heat pump COP, definitions, efficiency, true electricity cost calculations. This article defines heat pump COP (coefficient of performance) and explains why the operating efficiency of heat pumps varies as a function of outdoor temperature. We include a table of factors used to convert electricity costs to true electricity costs as a function of outdoor degree days in the heating season - a calculation that makes comparison of heat pump operating costs with those of other heating methods much more accurate. Illustration at page top provided by Carson Dunlop Associates.
What is the COP or Coefficient of Performance of Heat Pumps?
The COP or coefficient of performance describes the ability of a heat pump to extract heat from outdoor air down to some low temperature, typically 25 degF. for modern equipment. The COP determines how effective a heat pump can be at providing heat during cool or cold weather.
The COP or co-efficient of performance for a heat pump can be expressed as a curve showing energy consumed to operate the equipment versus the amount of heat energy provided to the building.
Because COP curves show that we cannot continue to make effective use of a heat pump at very low or very cold outdoor temperatures, a backup heating system is required where heat pumps are installed in cold climates.
[Click any image to see an enlarged version and to read details.]
Watch out: As we explain in details at HEAT PUMPS, GROUNDWATER, you can't tell which COP and EER measurement for groundwater based heat pump systems is "right" without additional data describing the testing conditions.
The balance point in a COP curve for a heat pump describes the point beyond which it is not efficient to continue to run the equipment to try to heat a building - because we are obtaining less heat energy to put into the building than the energy we are using to operate the equipment.
Where a heat pump is used to provide part of the building's heat requirements, the efficiency of the air-to-air heat pump will be less at lower temperatures.
Spies (1971, 1977)  notes that heat pump efficiency when outdoor air is warm is quite different from at cold temperatures, making its use of electricity more complex.
The coefficient of utilization may be as high as 3.0, falling to 1.0 as outdoor temperature approaches 10 degF. In 1971 when Spies wrote that note for the Small Homes Council, few heat pumps worked at temperatures that low, Also that this was in 1971, newer equipment is capable of efficient heat extraction from colder air. Spies provided a calculation to transfer heat pump efficiency or COP into electrical costs when comparing heating fuel type cost alternatives:
Table of Electricity Cost Divisors for Heat Pump Operating Cost vs Degree Days - Outdoor Temperature
Degree Days for Your Location
Electricity Price Divisor
8000 degree-day heating season
6000 degree-day heating season
4000 degree-day heating season
Henry Spies, "Fuels & Burners", Small Homes Council - Building Research Council Circular Series #G3.5, 1971. 1977
Example: If you live in a climate in which the average number of degree days in the heating season is 4000, then to compare heat pump operating costs (using electricity) to other fuels and heating methods,
divide your current electricity cost (say 5 cents per kwh) by 2.2.
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 Thanks also to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
 Henry Spies, "Fuels & Burners", Small Homes Council - Building Research Council Circular Series #G3.5, 1971. 1977 [copy on file as PDF]
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Some of the technical issues that are addressed include: Loop water flow rates and Reynolds Number, heat of extraction/rejection, heating capacity, de-superheater setup, open-loop/closed-loop, SCW, pond loops, DX, Manual-J, COP. The final chapter consists of a set of flowcharts guiding the homeowner to ask the pertinent questions needed for a successful installation.
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