ADVANTAGES OF AIR-TO-WATER HEAT PUMPS

Both geothermal water-to-water heat pumps and air-to-water heat pumps should be evaluated based on "net COP" rather than the COP of the heat pump as a standalone device. Net COP is calculated by dividing the rate of heat output by the total electrical power required to run the heat pump and its associated circulators. Net COP is a better indicator of the true operating cost of either type of hydronic heat pump since the power required by the required circulators is factored in. 

For a geothermal water-to-water heat pump, this would include the circulator(s) used to create flow in the earth loop and the circulator that provides flow through the load side of the heat pump. Some earth loops require multiple circulators connected in series to provide sufficient head and flow. The total power required by these circulators can be several hundred watts. Both the geothermal water-to-water heat pump and an air-to-water heat pump require a load side circulator. 

To see the effect of net COP, consider a geothermal water to water heat pump with a heating capacity of 48,000 Btu/hr. The heat pump compressor draws 4.2 KW when operating. Its earth loop requires two circulators, piped in series, operating at 220 watts each. The load side circulator draws 180 watts. Calculate the COP of the heat pump itself, and the net COP of the heat pump with its associated circulators.

The COP of the heat pump as a standalone device is:

$$COP_{HPonly}={ {48,000 Btu / hr\over [4200] watt · 3.413 {Btu / hr\over watt}}}= 3.35$$

The net COP of the heat pump and its associated circulators is:

$$COP_{net}={ {48,000 Btu / hr\over [(2 · 220) +4200 + 180] watt · 3.413 {Btu / hr\over watt}}}= 2.92$$

In this example, the net COP is about 13% lower than the COP of the heat pump as a standalone device.

Assume that an air-to-water heat pump has an output of 48,000 Btu/hr, with a corresponding electrical input of 5.5 KW. The load side circulator requires 180 watts. Calculate the COP of this heat pump alone and the net COP of the heat pump and its load side circulator.

The COP of the air-to-water heat pump alone is:

$$COP_{HPonly}={ {48,000\, Btu / hr\over [5500]\,watt · 3.413 {Btu / hr\over watt}}}= 2.56$$

The net COP of the heat pump and its load side circulator is:

$$COP_{net}={ {48,000\, Btu / hr\over [5500 + 180]\,watt · 3.413 {Btu / hr\over watt}}}= 2.48$$

In this example, the net COP is only about 3% lower than the COP of the air-to-water heat pump as a standalone device. Even though the air-to-water heat pump draws an additional 1.3 kilowatts of electrical power compared to the geothermal heat pump, its net COP, in this example, is only about 12% lower than that of the geothermal heat pump. It’s also important to remember that the COP of any heat pump is highly dependent on its operating conditions, and thus, there are scenarios where the net COP of an air-to-water heat pump could exceed the net COP of a geothermal water- to-water heat pump.