A heat pump is a device that moves heat from outside (or under the ground) into your home. In basic terms you don’t pay for the heat – you just pay to move it. As a result of that we don’t talk about efficiency (heat you produce divided by energy you bought) but instead talk about coefficient of performance (heat you have moved divided by energy you bought). We can move a lot of heat for relatively little energy and the coefficient of performance can get quite high as a result.

The most important factor of a heat pump is that most are electrically driven. This means as the electricity grid becomes low carbon, your heating becomes low carbon. Indeed, if you buy renewable energy today, the system can have a carbon footprint of near zero today.

A heat pump works in quite a different way to a conventional gas boiler and it is worthwhile understanding those differences to gain the best benefit.

The first thing is that we don’t want to work too hard raising the temperatures too high. If you are used to boiling hot radiators, then you’re going to have to adjust your expectations with a heat pump! Ideally we get the best coefficient of performance when the indoor heaters are operating at fairly low temperatures.

The second thing to note is that the coefficient of performance deteriorates when the outside temperatures fall. Manufacturers are obliged to quote for a seasonally adjusted coefficient of performance that takes into account the coldest nights in your climate zone to give an effective single figure for the year.

There are four main types of heat pump available in the UK, although the fourth type is very new.

1. Air Source Heat Pump (air-to-water). The heat is extracted from the outside air with an outside unit (looking like an air-conditioner outside unit). This type uses your existing radiator system to provide heat with warm water circulating through the property. Older houses designed with gas boilers in mind may not have sufficient surface area of the radiators and you might need larger radiators to allow the water temperature to be reduced. Ideally a water temperature of 35°C gives the lowest running costs but you may think the system is not working if you touch the radiators as they are lower than blood heat! The system will need to run fairly continuously in the background. The cost to install the system can be quite expensive and currently the UK has a subsidy scheme called the Renewable Heat Incentive (RHI) to help pay for the cost of the unit (the repayments are spread over seven years).

2. Air Source Heat Pump (air-to-air). Often called a room air-conditioner. Indoor units generally blow warm air into the room. These can be purchased at affordable prices and can be run in multi-split mode, meaning one single outdoor unit can run up to about four indoor units spread around the house. You may notice this style of unit in modern hotel rooms.

3. Ground Source Heat Pump (ground-to-water). The heat source is the ground so this type needs excavation and laying of pipework typically in the garden. This is a good option for a new build with a sufficiently large garden. The extra cost for groundwork is often reclaimed through a higher coefficient of performance (the source temperature being under the ground remains fairly warm even when the outside air temperature drops in winter, so it is easier and cheaper to transfer heat on those cold nights).

4. Heat Pump with the thermal source being solar thermal panels in a PV-T system (hybrid Photo Voltaic and Thermal panels). This is a variant of a Solar Thermal panel where the heated water is primarily used as the heat source for the heat pump. This is relatively new and the PV-T panels comprise a layer of photovoltaic (PV) cells on top of the solar thermal heat exchanger behind, giving a particularly efficient solar energy collection device.

The systems providing warm water to circulate work well with wet underfloor heating.

It might be worth noting two other heat pump configurations: one is when the heating is fully integrated with ventilation control and filtering (and heat recovery is used to extract heat from air leaving the building) – becoming more common in central Europe; the other is when the thermal source is ground water (a variant of the ground source heat pump).

The Renewable Heat Incentive Scheme in the UK is still open for domestic dwellings until March 2022. This helps with the capital costs of installing a complete replacement heating system, with grant payments spread over seven years. There are conditions: the insulation actions listed in the Energy Performance Certificate (EPC) would need to be completed for the property, the installer would need to be registered with the Microgeneration Certification Scheme (MCS) and the heat pump would be listed on the approved list. Full details of the domestic Renewable Heat Incentive scheme are available in the scheme reference document. The official RHI scheme is available from ofgem here and the link to the RHI factsheet is here.

So how do I work out the running costs of a new heat pump and how does that compare to a new gas boiler? Click here to work out the cost benefits and click here to work out the carbon dioxide reduction. Click here for an overview of some useful heat pump knowledge.