The energy and financial results of heat pumps depend on the temperature conditions (and perhaps humidity) of the source of heat medium and the temperature of the medium to heat.
As a rough guide, in good operating conditions, the heat pump can transfer 7 [kWh] (23,885 btu) of free energy for an electrical consumption of 3 [kWh] (10,236 btu).
For 10 [kWh] (34121 btu) heating, the bill is only for 3 [kWh] (10,236 btu) of electricity.
performance range of a heat pump drawing on the heat in the outside
air at +7 [°C] (+44.6°F)
to reheat heating circuit water from 30 to 35 [°C] (86 to 95°F)
In these operating conditions, we say that the coefficient of performance (COP) of the HP is :
An HP has a (calorific) power (value) of de 25 [kW] and a COP of 4.
How much electrical power does it absorb?
Absorbed electrical power is:
Elec P = Calorific P/COP = 25/4 = 6.25 [kW]
An HP has supplied 450,000 [kWh] (1,535,463,738 btu) with an average COP of 2.5(*).
What was its electrical consumption?
(*) As we will see heat pump performance is not constant. Performance decreases while winter passes, as the temperature difference between the source and the destination increases.
180,000 [kWh] (614,185,495 btu).
Electrical power consumed is:
Elec P = Calorific P/COP = 450,000 / 2.5 = 180,000 [kWh] (614,185,495 btu)
But be careful, as we will see in the course « Performance and Installation of Heat Pumps », heat pump capacities are not constant.
Performance decreases while winter passes, as the temperature difference between the source and the destination increases.
We will also see, especially for outside Air Source HP (ASHP) and (reversible air conditioners), that as the outside temperature decreases (or as the temperature of the heated space increases):
- Calorific power (value) supplied by the HP decreases.
- Electric consumption increases (the COP decreases).
Due to this, in renovation, it is rare that an outside air/heating water HP can provide all heating and sanitary hot water requirements.
Geothermal HP will provide this but would operate better if they were dealing with low temperature heating such as under-floor heating or fan coil units (for the operation of FCU, we can study the course « Fan coil units »).
Heating a swimming pool is also a good use for a heat pump.