How to Calculate the COP and EER of a Geothermal Heat Pump


!WARNING! These tests must be performed by a qualified person! Hazardous voltages will be present!


Calculating the COP


The Measurements

To calculate COP, you need to measure 5 things: the exact temperature of the water entering the geothermal heat pump's water coil, the exact temperature of the water leaving the geothermal heat pump's water coil, the flow rate of the water through the geothermal in gallons per minute (GPM), the amps usage of the unit, and the exact voltage at the unit.

You need to watch out for some things that can skew your measurements when you take them, and make your calculated COP useless. First, all of your measurements have to be made after the unit has been running for a minimum of 10 minutes, and while you take all your measurements. The temperature of the air entering the unit at the air coil (EAT) should be within 2 degrees of 68°F while you are measuring. The water flow through the unit must be the exact same gallons per minute the entire time you are taking all of your measurements. If you have an open loop, this means your well pump cannot start and stop; it must be running continuously (if it is starting and stopping, try increasing the water flow to the geothermal heat pump's maximum rated flow; this might keep the pump running).

When everything is running correctly, you are ready to take your five measurements:

  1. Entering Water Temperature/EWT - measure the temperature of the water going into the water coil (the coil marked water in, or source water in).

  2. Leaving Water Temperature/LWT - measure the temperature of the water coming out of the water coil (water out, or source water out).

  3. Flow Rate in Gallons Per Minute/GPM - measure your loop's flow rate.

  4. Amperage - measure the amperage used by the unit at each hot terminal, then add the amperages together and divide the answer by 2.

  5. Voltage - measure the voltage at the unit; make sure you measure across the two hot terminals.

The COP Equations

First, we plug our measurements into these two equations to give us Absorbed BTUH and Electricity BTUH:

  • (EWT - LWT) x GPM x 60 x 8.35 = Absorbed BTUH (from earth loop)

  • amperage x voltage x .85 (power factor) x 3.412 (BTUH per watt) = Electricity BTUH (from the unit's motors' electric usage)

Absorbed BTUH and Electricity BTUH are plugged into the final equation to give us our COP:

  • (Absorbed BTUH + Electricity BTUH) ÷ Electricity BTUH = COP

Here is an example taken from a Florida Heat Pump geothermal heat pump GT042-1HZC-FLE.

Measurements:
  • EWT - 54.8°F

  • LWT - 47.8°F

  • Gallons per minute water flow - 9

  • EAT - 68°F

  • Amperage - 13.4

  • Voltage - 243

(54.8 EWT - 47.8 LWT) x 9 GPM x 60 x 8.35 = 31,563 Absorbed BTUH

13.4 amps x 243 volts x .85 x 3.412 = 9,446.40 Electricity BTUH

(31,563 Absorbed BTUH + 9,446.40 Electricity BTUH) ÷ 9,446.40 Electricity BTUH = 4.34 COP


Calculating the EER


The Measurements

To calculate EER, you need to measure five things: the exact temperature of the water entering the geothermal heat pump's water coil, the exact temperature of the water leaving the geothermal heat pump's water coil, the flow rate of the water through the geothermal in gallons per minute (GPM), the amps usage of the unit and the exact voltage at the unit.

You need to watch out for some things that can skew your measurements when you take them, and make your calculated EER useless. First, all of your measurements have to be made after the unit has been running for a minimum of 10 minutes, and while you take all your measurements. The temperature of the air entering the unit at the air coil (EAT) should be within 2 degrees of 80°F. dry bulb, and 67° F. wet bulb when you are measuring. The water flow through the unit must be the exact same gallons per minute the entire time you are taking all of your measurements. If you have an open loop, this means your well pump cannot start and stop; it must be running continuously (if it is starting and stopping, try increasing the water flow to the geothermal heat pump's maximum rated flow; this might keep the pump running).

When everything is running correctly, you are ready to take your five measurements:

  1. Entering Water Temperature/EWT - measure the temperature of the water going into the water coil (the coil marked water in, or source water in).

  2. Leaving Water Temperature/LWT - measure the temperature of the water coming out of the water coil (water out, or source water out).

  3. Flow Rate in Gallons Per Minute/GPM - measure your loop's flow rate.

  4. Amperage - measure the amperage used by the unit at each hot terminal, then add the amperages together and divide the answer by 2.

  5. Voltage - measure the voltage at the unit; make sure you measure across the two hot terminals.

The EER Equations

First, we plug our measurements into these three equations to give us Rejected BTUH, Electricity BTUH and Wattage:

  • (LWT - EWT) x GPM x 60 x 8.35 = Rejected BTUH (into the earth loop)

  • amperage x voltage x .85 (power factor) x 3.412 (BTUH per watt) = Electricity BTUH (from the unit's motors' electric usage)

  • amperage x voltage x .85 = Wattage

Rejected BTUH, Electricity BTUH, and Wattage are plugged into the final equation to give us our EER:

  • (Rejected BTUH - Electricity BTUH) ÷ Wattage = EER

Here is an example taken from a Florida Heat Pump geothermal heat pump GT042-1HZC-FLE

Measurements:
  • EWT - 58.2°F

  • LWT - 68.7°F

  • Gallons per minute water flow - 8.8

  • EAT dry bulb - 76°F

  • EAT wet bulb - 62°F

  • Amperage - 12

  • Voltage - 242.8

(68.7 LWT - 58.2 EWT) x 8.8 GPM x 60 x 8.35 = 46,292.4 Rejected BTUH

12 amps x 242.8 volts x .85 x 3.412 = 8,450.02 Electricity BTUH

12 amps x 242.8 volts x .85 = 2,476.56 Watts

(46,292.4 Rejected BTUH - 8,450.02 Electricity BTUH) ÷ 2,476.56 Watts = 15.28 EER


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