You’ve probably heard that geothermal heat pumps are the most efficient way to heat and cool your home. But what does that phrase actually mean for your home and your heating bills? What does energy efficiency mean? How is it measured? How is it calculated? What is heat pump COP? That’s what we’re going to look at today.
Efficiency is how much energy you get out of an appliance, compared to how much energy you put into running it. A 97% efficient furnace gives you back 97% of the energy you put into it in the form of heat for your home, the other 3% is lost up the flue. Since electric baseboard has no flue, 100% of the energy that goes in, stays in your home in the form of heat. The higher the efficiency of your system, the lower your heating bills.
Keep reading to learn about geothermal heat pump efficiency, how to calculate it, and what it means for your home heating bills.
Calculating Your Geothermal Heat Pump COP
Geothermal heat pump efficiency is traditionally measured using a ratio called “coefficient of performance” (COP). The COP of a geothermal heat pump is the ratio of the heating or cooling output to the energy input to run the machine. A high COP over 1.0 means your heat pump is performing very efficiently, and your heating bills will be low. A heat pump is the only heating and cooling device that has a COP over 1.0.
Let’s take a deeper look at how to calculate a heat pump’s heating COP.
Calculating Coefficient of Performance
First, you’ll need two things:
- Energy out, or the heat pump’s expected output.
- Energy In, or how much energy it takes to run the heat pump.
Using these two values, we can complete the formula below:
In this formula, energy out is the heat pump’s output in Btu/hr, and energy in is the energy required to run the heat pump, measured in watts.
Let’s do an example calculation using our standard four-ton water to air heat pump. All of the information we need is available in this heat pump’s manual, found on the R Series product page. The data we need is on page 35 of the manual.
A four-ton water to air heat pump operating in heating mode on a closed ground loop has an output of 35,900 Btu/hr when the compressor is running in stage 2. The first thing we need to do is convert the Btu/hr output to watts. One Btu/hr equals 0.293 watts.
35,900 Btu/hr x 0.293 = 10,518 watts
Now we have the “energy out” portion of the formula in watts, and from the manual, we see that the heat pump consumes 2,700 watts to run. This is the “energy in” part of the COP formula.
Now we can plug this into the calculation: COP = energy out / energy in
COP = 10,518 / 2,700 = 3.89
Based on our calculations, a four-ton water to air heat pump running in heating mode on stage 2 would have a COP of 3.89. That means for every watt of electricity used to run this machine; you’ll get 3.89 watts of heat energy out of the heat pump. The extra output is free energy harvested from the ground loop by the heat pump. Put in terms of the efficiency we mentioned above, a four-ton R Series heat pump is 389% efficient when heating in stage 2.
Coefficient of Performance and Your Home’s Heating Bills
Compared to electric baseboard or an oil furnace, geothermal is very, very efficient. The high COP means you’ll have lower heating bills, but just how much lower?
We’ve put together a table comparing the efficiencies of electric baseboard heating, geothermal heating and an air to water heat pump, and the effect on your heating bills:
|Electric Baseboard||Air Source Heat Pump||Geothermal Heat Pump|
|Yearly Electric Consumption||$4,000||$1,369||$1,028|
Coefficient of Performance And Changing Conditions
Your heat pump’s COP changes depending on operating conditions, here are several factors that will affect your heat pump’s effective COP:
Whether It’s Heating or Cooling Season
Since the output of the heat pump changes when in cooling mode, so does the COP. If you look in our manuals, we calculate separate COPs for every heat pump in both heating (COPh) and cooling (COPc) modes to account for this variance.
Whether the Heat Pump is in First or Second Stage
All Nordic heat pumps have two-stage scroll compressors, which allows the heat pump to vary its output depending on the heating or cooling demands. Since the output changes depending on which stage the heat pump is operating in, the formula changes, and so does the resulting COP. Our manuals account for this and we calculate separate COP’s for each stage.
For example, the four-ton water to air heat pump we looked at above has a COP of 4.10 in stage 1 and 3.89 in stage 2 while in heating mode on a closed loop. You can estimate an average COP by adding these two together and dividing by two:
(4.20 + 3.89) / 2 = 4.05
Individual Operating Conditions
There are myriad individual operating conditions that affect the actual COP of your heat pump. Variables like the temperature of your ground loop, the temperature of the buffer tank (if you’re using in-floor heat) and whether or not you’re on an open or closed loop all affect the actual COP of your heat pump.
Fortunately all of these variables will have a minimal effect on the actual performance of your heat pump, and you can rely on the reported COP’s in our heat pump manuals as a baseline for how your system will perform when it is installed correctly, or as in the example above, you can calculate it yourself!