What are the basic workings of a heat pump?
A heat pump is a relatively new technology in the UK although they are widely used in Scandinavia and many parts of Europe where there is an abundance of renewably sourced electricity.
A heat pump takes energy from outside and transfers it into heat to be circulated around a heating and hot water system. A heat pump uses electricity to run the components of a heat pump, principally a fan, compressor and circulating pumps to transfer the energy from the heat source into the heat sink or heating system.
What type of heat pumps are available?
For use in a domestic heating system application, there are basically two forms of heat pump; Ground Source or Air Source. A Ground Source heat pump has a collection pipe filled with a Heat Transference fluid and collects heat from the ground. The collection pipework is normally a continuous, unjointed length of pipe buried in the ground either horizontally or vertically in a borehole. For an average sized domestic property, if the collection system was sited horizontally, it would require the pipe to be buried around one metre in depth and would occupy an area of ground similar in size to a tennis court. If the collection system is to be sited vertically then either one or more boreholes equivalent to around 150 metres deep would be needed.
An Air Source heat pump takes the outside air as its heat source and is consequently easier and less expensive to install as there is no collection pipework to install into the ground. The outside unit contains a fan that draws into the unit outside air and transfers this into the refrigerant circuit which is then compressed to a high temperature and then transferred into the water within the heating system via a heat exchanger.
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There are debates over which system is the most efficient however, an air source heat pump is reliant on the outside air temperature which can be a lot cooler than the ground temperature which by comparison is relatively stable. Consequently, a ground source heat pump tends to be more efficient in all outside temperature situations however, they are significantly more expensive to install and are therefore less popular than an air source heat pump.
A heat pump is at its best when it is circulating low temperature water around the heating system in a “steady state” mode. The heat pump is best run from a weather compensation system and allowed to run at all times as dictated by the external weather temperature and the internal room temperature. Because of the low temperature of the heating system water it is not best suited to be operated intermittently like a boiler. A boiler is typically turned on for a couple of hours in the morning, switched off all day and then turned back on at night for 5 or 6 hours.
The high temperatures that a boiler can generate result in it heating the house more quickly than a heat pump can hence the need to run the heat pump in a “steady state” mode and avoid the need to heat the house quickly and from a low base temperature.
The servicing requirements of a heat pump are not as onerous as a boiler however, there are disagreements as to whether the homeowner can undertake this or a service engineer is required.
With a Ground Source heat pump the requirements are more of a check than an activity, the closed loop collection system needs to be checked for the correct levels and efficacy as the fluid also serves as an anti-freeze. The heating system water pressure needs to be maintained and, if the installation has a mains pressure unvented hot water storage cylinder, the servicing requirements of that needs to be considered.
An Air Source heat pump requires the external unit to be kept free of leaves and debris, any filters within also need cleaning or replacing as specified by the manufacturer. And similar to a Ground Source heat pump, any unvented, mains water pressure cylinder needs to be serviced to the manufacturer’s instructions.
The running costs of a heat pump will vary from house type to house type. A well-insulated house built to new building regulation standards will generally be less expensive to run than a gas or oil fi red boiler system providing the heating system flow temperatures are kept relatively low, ideally around 40ºC.
A heat pump will be less efficient and more costly to run when generating higher temperatures, either because the house isn’t well insulated, the radiators are insufficiently sized for the lower temperatures or when generating higher temperatures to produce hot water.
A heat pump can be connected to an existing heating system however, there are several conditions that will need to be checked otherwise there could be issues with customer satisfaction and running costs;
- A heat loss calculation of the property needs to be undertaken to ascertain the correct radiator or heat emitter sizes required. If the original system was sized on the presumption that a boiler running at relatively high temperatures, i.e. 70-75ºC fl ow, then unless the house has since then had additional insulation measures undertaken such as cavity wall, double glazing etc. then the radiators and distribution pipework may not be large enough to heat the rooms when operating at a heat pump temperature of around 40-45ºC. There have been instances where this has not been correctly undertaken and the heat pump is then turned up to operate at a higher fl ow temperature which results in the heat pump running at a lower efficiency or even the back-up immersion heaters operating.
- If the homeowner is used to operating a boiler system intermittently, they need to be instructed as to how differently a heat pump system needs to operate. Many of the customer complaints about the suitability of a heat pump result from their expectations of how the two systems operate differently. They will need to be informed that the radiators will not be as hot as they have been used to and that they need to be more attentive to the actual air temperature in the room rather than the surface temperature of the radiator.
- If the existing heating system is a combi boiler system, then a hot water storage cylinder will be required. There are around 17m homes in the UK with a combi boiler installed so there are considerably different aspects between this system and a heat pump system. There needs to be space within the house to site the newly required cylinder which could involve space being lost from a bedroom or landing and also education to the homeowner that the cylinder now needs to heat up before hot water is available rather than instantly heated when a hot water tap is opened.
The installation cost of a heat pump will differ from house to house and from size and type of heat pump chosen. As a rule of thumb, an air to water heat pump will cost around £8k to install and a ground source heat pump could be anything up to and around £20k. This though is very dependent on the site and as to whether the existing heating system can be used and the position of the equipment etc.
There are grants available to assist in the investment, sometimes locally and regionally, as well as the Renewable Heat Incentive (due to end March 2022) which will pay an annual sum to the purchaser of the system for 7 years.
An air source heat pump has an outdoor unit that contains a fan to induce the external air into the unit and consequently, they are noisier than a ground source heat pump. Generally, there are few complaints about the noise from a heat pump and when compared with the normal ambient external noise are quite acceptable.