How to Calculate COP on a Geothermal Heat Pump

Direct answer: COP is heat delivered divided by electrical energy used, with both sides in the same units. For a field check, measure water-side heat transfer, airflow context, and true watts for the compressor, blower, and loop pumps at the same stable operating condition. Then compare the result with the exact model data, not an internet average.

Precise definition

ENERGY STAR defines coefficient of performance as a heating-mode efficiency ratio: total heating capacity divided by electrical energy input. The public rating number is tied to defined ISO or AHRI test conditions, so it is not automatically the same number a technician will calculate in an Oklahoma mechanical room.

In the field, COP is useful only when the measurement conditions are written down: heating or cooling mode, stage, entering water temperature, leaving water temperature, flow, antifreeze type and concentration, airflow, return-air temperature, and true electrical input.

Every term in that formula is a reading somebody has to take correctly. What EWT, LWT, GPM, PSI, and Delta T mean is where those come from.

The water-side heat formula

For water, the common heat-transfer shortcut is:

The 500 factor is for water. Closed geothermal loops often use antifreeze, so the correct fluid factor must come from the fluid properties, concentration, and temperature being measured. ClimateMaster service literature also points technicians back to pressure-drop tables and model-specific operating data when checking flow and performance.

Delta T is the temperature change across the heat pump. In heating, the loop side usually leaves colder than it entered because heat is being extracted from the loop. In cooling, the loop side usually leaves warmer because heat is being rejected to the loop. That direction matters.

Worked example – water-only field math

Example only: A water-side heating check shows 9 GPM and an 8 degrees F temperature change across the unit. With water, heat moved on that side is:

If the delivered heat used for the COP comparison is 36,000 Btu/h and the measured electrical input for the unit, blower, and loop pumps is 3.0 kW, then:

This example is not a promised target. It shows the unit conversion. Real readings have to be corrected for fluid type, stage, pump configuration, airflow, and the model data for the equipment on site.

A COP that will not come up in heating season often points at the ground, not the machine. An undersized loop field is the case where the readings look bad and the equipment is fine.

What technicians need to measure together

Field-check list

1. Confirm mode, stage, thermostat call, and fault history.
2. Let the unit run long enough to stabilize before recording readings.
3. Measure entering and leaving water temperature at the unit test ports.
4. Measure pressure drop across the heat exchanger and convert it to GPM using the model’s table.
5. Record antifreeze type and concentration, then use the right fluid factor.
6. Measure true watts for compressor, blower, loop pumps, and any separately powered pump required for operation.
7. Convert heat and electrical input to the same units before dividing.
8. Compare against the exact model literature or AHRI data at the measured conditions.

Safety and professional boundary

When to call Hartzell’s

Call Hartzell’s when a geothermal system has repeated lockouts, unusually low or high EWT, uncertain antifreeze concentration, unknown loop flow, high electrical use with poor comfort, or a COP number that does not match the model data after the readings are corrected.

Related geothermal pages