Last updated on April 19, 2022
There are air source and ground source heat pumps. Ground source was formerly called geothermal but this term is now reserved for geothermal energy such as hot springs. Both types of heat pumps transfer heat from one location to another. In winter, heat is pumped from the air, or ground, into the building. In summer, heat is pumped from the house back into the ground or air. No GHG is emitted since nothing is burned in the transfer process. GHG will be released if the electricity required to run the pumps is not renewable, however, in Ontario, over 90% of electricity is generated by non GHG producing processes (59.6% nuclear, 25.1% hydro, 8% wind and <1% solar) .
The ground temperature, 2 to 3 metres below the surface, ranges from 7°C to 21°C depending on the season . In winter, there is heat that can be pumped into the house and in summer, the ground can accept heat from the house. Horizontal ground loops are the most common for residential houses with sufficient land for the loops. Where space is limited, vertical boreholes are used.
Air source heat pumps are not as efficient as grounds source since it’s harder to extract heat from the air as it gets cold. Until recently, air source heat pumps needed auxiliary heat when the outside air temperature dropped below -20°C. This was typically electric heat coils installed in the supply air plenum. However, recent developments in air source heat pump snow allow them to work down to -30°C and auxiliary heat is not required .
Even when outdoor temperatures are cold, a good deal of energy is still available that can be extracted and delivered to the building.The heat content of air at -18°C equates to 85% of the heat contained at 21°C. .
The Coefficient of Performance (COP) is “energy out” divided by “energy in”. The COP ignores the energy used to create the electricity or natural gas, however, it is useful to calculate the performance of the energy sources. The best natural gas furnaces are 97% efficient and that means for every unit of energy put into the furnace, 97% is used for heating and 3% is wasted as flue gas. Electric baseboard heaters are 100% efficient since every unit of energy that goes into a baseboard is used as heat.
Since it’s hard to extract heat from cold air, air source heat pumps have a lower COP than ground source heat pumps. A air source heat pump has a COP of about 3 and will return 3 units of energy for every unit of input. A ground source has a COP of 4 and return 4 units of energy for every unit of input . In practice, some ground source heat pumps with variable speed field pumps and fans, and 2 stage compressors can achieve a COP of 4.5.
Why a Heat Pump?
- No GHG released when operating since no fossil fuels are burnt.
- Much more energy efficient. Heat pumps are the only heat source that releases more energy than is put into the system
- Air conditioning is part of the heat pump cycle and no additional hardware is required.
- Ducted heat pumps can be installed using the same ducts as gas or oil furnace
- Ductless heat pumps can be installed on any outside wall.