Measure GSHP Efficiency

The first step is measuring the temperature of the fluid as it enters and leaves the heat pump. These are known as the Entering Water Temperature (EWT) and Leaving Water Temperature (LWT).

Using a digital thermometer at the pressure/temperature (P/T) ports, you can determine the "Delta T"—the amount of heat being added to or removed from the loop fluid. In a typical heating application, the LWT should be roughly 5°F to 8°F cooler than the EWT.

To calculate the total heat being moved, you must know the flow rate in Gallons Per Minute (GPM). This is determined by measuring the pressure drop across the heat exchanger.

By measuring the PSI at the inlet and outlet P/T ports, you can find the pressure differential. Manufacturers provide "Pressure Drop" tables that correlate this PSI difference to a specific GPM flow rate for your model.

Finally, you must measure the total electrical work being done. This requires a voltmeter and a clamp-on ammeter.

Measure the voltage at the unit's contactor and the amperage draw of the compressor and fan. Multiply these values (and the power factor, typically 0.9 for heat pumps) to find the total kilowatts (kW) consumed.

Once you have the Delta T, the GPM, and the kW, you can calculate the heat of extraction (or rejection) using the standard formula:

BTU/hr = GPM × 500 × Delta T

The Coefficient of Performance (COP) is then calculated by dividing the total heat output (in Watts) by the electrical input (in Watts). A COP of 4.0 means the system is 400% efficient at moving heat.