What Does COP Mean on a Geothermal Heat Pump?
COP means Coefficient of Performance. It is the ratio of heat a system
moves to the electricity it burns doing it. A geothermal heat pump running at COP 4.5 gives you about
$4.50 of heat for every $1 of electricity you buy.
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That is the whole idea, and it is the number I care about most when I am sizing a geothermal system
for an Oklahoma house. Everything else on the spec sheet is downstream of it.
Why COP can go above 1 when a furnace never can
A gas furnace burns something. A really good one converts 96 cents of every dollar of gas into heat
in your house, and the rest goes up the flue. It can never do better than the fuel you paid for,
because it is making heat.
A heat pump does not make heat. It moves heat that already exists. Moving is cheaper than making,
which is why the ratio breaks past 1 and keeps going. A geothermal heat pump at COP 4.5 is delivering
450 percent of the energy you bought, because three and a half of those units came out of the ground
for free.
People hear that and assume it is a sales number. It is not. It is the same physics that makes your
refrigerator work, just aimed at a house instead of a carton of milk.
Why a geothermal COP holds and an air-source COP does not
An air-source heat pump is doing the same trick, but it is pulling heat out of the outdoor air. In
January that air can be 15 degrees, and there is not much heat left in it, so the COP falls off exactly
when you need it most and the backup heat strips kick on. Those strips run at a COP of 1. That is the
part of the electric bill that makes people call me in February.
A ground loop does not have that problem. The dirt a few feet down in Oklahoma holds a steady
temperature through the whole year. The system starts from the same place in January that it started
from in July, so the rated COP is close to the COP you actually live with. That stability is what you
are buying. It is not a small thing, and it is a big part of why geothermal is the premium system and
priced like one.
What COP does to your wallet
The dollar version
COP 4.5 is about $4.50 of heat for every $1 of electricity. In practice, a geothermal system cuts
the energy use of your heating and cooling by roughly half compared with the conventional or propane
system it replaced. Energy use, not your whole electric bill: your lights, your well pump, and your
deep freeze do not care that you put in a geothermal system.
That distinction matters, and I would rather say it out loud than have you compare bills in
September and feel misled. If someone tells you geothermal cuts your electric bill in half, ask them
which half.
Where the honest math gets loud is a propane or electric-resistance house. Those are the homes where
the change shows up hardest, and they are most of rural Oklahoma.
See what geothermal costs in Oklahoma and
the payback math.
Where to find the COP on a real unit
It is on the nameplate and in the AHRI data, usually listed at a specific entering water
temperature, because the COP changes with how warm the loop water is coming back. That is why the loop
design is not a detail. A cheap, undersized loop makes the water swing further from ideal, and the COP
you actually get drops below the one printed on the box.
You cannot see a loop again after it is backfilled. That is why I am IGSHPA accredited and why I do
not sub the loop out to whoever is cheapest that week. The rated COP is a promise the loop either keeps
or breaks.
COP, EER, and the numbers people mix up
| Rating | Measures | Used on |
|---|---|---|
| COP | Heating output per unit of electricity in | Geothermal and water source heat pumps |
| EER | Cooling BTUs per watt-hour, at one steady condition | Geothermal and water source heat pumps |
| SEER2 | Seasonal cooling efficiency across a range of outdoor air temperatures | Air conditioners and air-source heat pumps |
| HSPF2 | Seasonal heating efficiency across a range of outdoor air temperatures | Air-source heat pumps |
The short version: geothermal is rated in COP and EER because it sits in steady water, not swinging
air. If a salesman quotes you a SEER2 number for a geothermal unit, he is reading off the wrong sheet.
I broke that down further on
what EER means.
The whole cycle, start to finish. Watch what is actually moving and you can see why the ratio goes past 1.
Questions I get about this
What is a good COP for a geothermal heat pump?
The units I install commonly land in the 3.5 to 5.0 range in heating, which is where the roughly $4.50 of heat per $1 of electricity comes from. Higher is better, but only if the loop was designed to feed it. A high rated COP on an undersized loop is a number on a sticker.
Is COP the same as efficiency percent?
Same idea, different scale. A 96 percent furnace has a COP of 0.96. A geothermal heat pump at COP 4.0 is running at 400 percent by the same arithmetic. That is only possible because it is moving heat instead of making it.
Does COP change in Oklahoma winters?
Far less than an air-source heat pump’s does, and that is the point. Your loop water is starting from steady ground temperature instead of 15 degree air, so you are not leaning on backup heat strips every cold snap.
How much does geothermal cost in Oklahoma?
It depends on your house, your ground, and your loop type, and I will not guess at it from a keyboard. New system estimates are free anywhere in Oklahoma. The ranges and the rebate math are on my geothermal cost page.
Where does COP show up on my power bill?
As heating and cooling energy use dropping by roughly half versus the system it replaced. Your other loads stay the same, so read the bill honestly and compare the same months year over year.
Keep going:
Want the COP math run on your actual house?
Tell me what you heat with now and what your worst month looks like. I will tell you what a ground loop would do about it, and I will tell you if it is not worth it for your situation. New system estimates are free, statewide.
Dave Hartzell, Hartzell's Heat & Air, 602 S Main St, Kingfisher, Oklahoma. Master HVAC License, NATE Certified, IGSHPA Accredited, ClimateMaster GeoElite. New system estimates are free anywhere in Oklahoma. Diagnosing an existing geothermal system is my $348 flat Geothermal Diagnostic, no credit and no refund, with a written report before I leave.