A heat pump is a mechanical device that uses a refrigeration cycle to extract heat from a source (the outside air, for example) and deliver that heat to the building. It’s the same process a refrigerator uses to extract heat from the fridge compartment and release it into the kitchen. The heat pump uses electricity to run the refrigeration cycle but, crucially, the amount of heat energy that‘s delivered is greater than the amount of electrical energy consumed. This makes heat pumps a highly efficient mode of heating. Heat pumps are considered to be low carbon because Scotland’s electricity grid is on a pathway to decarbonisation.
The different types of heat pumps can be described according to the source of heat which they tap into, the way in which they deliver the heat, and the type of refrigerant that they use. Heat pumps are first identified by their source:
- air source
- water source
- ground source.
And then by their delivery:
- Air-to-air, which use external air as a source and blow warm air from a unit indoors.
- Water-to-water, which use a watercourse as a source and generate hot water to feed a radiator.
- Air-to-water, which use external air as a source and generate hot water to feed a radiator.
- Ground-to-water, which use the earth as a source and generate hot water to feed a radiator.
And by their refrigerant, which could include:
- Synthetic hydrofluorocarbons (HFC), which are being phased out due to their global warming potential (ie the potential to cause global warming if the gas is released into the atmosphere).
- Synthetic hydrofluoro-olefins (HFO).
- Natural refrigerants, including propane and CO2.
Air source heat pumps are the most common type of heat pump used today, and it’s expected that this will continue to be the case in the future. Ground source and water source heat pumps have the potential to be a little more efficient since both ground source and water source heat pumps tend to be warmer than air during the winter. However, for most community projects, the minor efficiency benefits will be small compared to the additional installation cost. This guide will focus on air source heat pumps for this reason.
The most technically diverse and complicated part of a heat pump is its refrigerant. Different refrigerants have different properties relating to the temperature they can generate and the efficiency with which they can do it. Refrigerant properties also affect the cost of purchase, the cost of maintenance and the effect on the environment. This guide doesn’t discuss all the ins and outs, but it’s very important to discuss refrigerant specifications with your installer or consultant.
Heat pumps generate more heat energy than they use in electrical energy. The ratio of the electrical energy used and heat energy produced is an indication of a heat pump’s efficiency. For example, if a heat pump takes three units of heat energy from the air and one unit of electrical energy from the grid to make a total of four units of heat energy, it will have a Coefficient of Performance (COP) of four. The higher the COP, the higher the heat return is on each unit of electrical energy you are paying for. The COP does change depending on the temperature, so it will vary depending on the season.
Manufacturers must test their equipment at a set of standard temperatures, and they use those results to determine an average performance, which is called the Seasonal Coefficient of Performance (SCOP). This standardisation makes it possible to roughly compare two different heat pump models. Please note that the weather in your location is unlikely to completely reflect those of the test conditions, so the SCOP is not a guaranteed prediction of the performance you will experience.
The heat pump’s efficiency depends on the temperature at which it provides heat. For example, an air-to-water heat pump, which generates hot water at 60°C, will use around 10% less electricity than a heat pump generating water at 70°C. That’s why there is a focus on lowering the working temperatures of connected heating systems.
