The Government views heat pumps (HP) as the UK’s decarbonisation and heating solution, and in many ways, they are correct to. However, although they can work in every building there remains some things to consider before taking the plunge.

Which heat pump and how do they work?

There are three types of heat pump that the market can provide, air source (ASHP), ground source (GSHP) and water source (WSHP). All function by passing a liquid called a refrigerant through a heat exchanger, capturing the liquids heat. The reason the liquid refrigerant heats up is because it has a very low boiling point, sometimes as low as -25 degrees centigrade (°C), and so when it passes through a heat source like air, the ground or water, it naturally heats up.

The liquid (refrigerant) is then fed into a compressor for pressurisation, which further increases its temperature and turning it into a gas. The final stage sees it pass through another heat exchanger to transfer all that heat to a cylinders or thermal stores for hot water and/or radiators/underfloor heating, or to be blown into a room by fans as warm air.

Unlike gas or direct electric which can heat water on demand at high temperatures, eg-85°C, heat pumps typically heat water to 55°C, and to make them as efficient as possible even lower, eg:35°C isn’t uncommon. This lower heat output is why larger radiators or underfloor heating are preferred.

It is also worth noting that warm air systems can provide hot water with an add-on, but this isn’t typically part of the system.

Heat pump efficiency

You can get heat pumps that heat to higher temperatures, eg: water to 85°C, but efficiency is compromised as more electricity is needed to achieve this. The point of a heat pump is to use as little electricity as possible by letting the refrigerant do the work in a well designed system.

Heat pump efficiency is also calculated as Coefficient of Performance (COP) or Seasonal Coefficient of Performance (SCOP). If an air to water (radiators/UFH) ASHP achieves a SCOP or COP of 4, it means for every unit of electricity you put in, you get 4 out. And this is important because gas boilers have a maximum ‘COP’ just shy of 1, so if electricity is four times more expensive than gas, your heat pump needs a COP of 4 to achieve cost parity with gas.

How you heat matters

Heat pumps which heat air to warm a room have the highest efficiency because they’re not going through another process, eg-heating water for radiators. However, when using a water system (hydronic), what you heat is an important consideration.

A radiator still heats a room’s air, plus stays warm for a period after the heating turns off but underfloor heating effectively turns the floor into a massive radiator (often referred to as thermal mass). If you can use a flooring type which heats up quickly and holds heat for longer, you can obviously use cooler water. This then starts to match the better efficiency that heating air directly gives you, plus hydronic HP systems heat hot water for bathing and cleaning.

Why does this matter for your project?

Older buildings may not be able to install underfloor heating systems easily, or at all in the case of heritage buildings, so won’t benefit from higher thermal mass. Or the cost would be prohibitive. They may also have considerable air leakage, so get colder more quickly. Many cannot install double/triple glazing or insulate walls and floors. This doesn’t discount heat pumps but means that a heat pump which heats to higher temperatures and costs more to run may be required. To help these systems keep running costs lower, solar panels (PV) and a battery may be a good addition to reduce reliance on grid electricity.

In the case of heritage buildings, some may have considerable grounds, which makes them perfect candidates for ground source heat pumps, the most efficient heat pump type. Here, the heat source heating the refrigerant is the ground, either via very deep bore holes or arrays laid flat across an area at depths of a metre or two. A rough guide is 10m2 per m2 of a buildings floorspace.

Rather than an expensive heat pump, with potentially expensive building retrofitting, some may question whether a £2,000 electric boiler which heats to 85°C and alongside PV and battery is the most economical approach. This would be a different article but a fair question.

For larger properties, you may also need a couple of heat pumps or additional/secondary heating systems for the coldest periods; something quite typical in countries where heat pumps are most popular. 

There is one other major not much discussed benefit to heat pumps in traditionally built buildings, which is that heat pumps are typically heating all the time at low temperatures. This means a drier building and reduced chance of condensation and mould. The addition of mechanical ventilation, especially if equipped with heat recovery, further increases the health benefits for building and occupant.

For commercial buildings, air to air heat pumps are already the most popular choice and in places such as Scandinavia, where much of the residential heat pump hype has come from, 80% plus of new heat pump sales are air to air.

Unfortunately, the Government has not yet understood this factor and so have backed air to water systems in government funding, regulatory encouragement and discussion points. This is changing, slowly, but this blind spot is even more bizarre when you realise we already install air to air systems nationally, we have more than 50,000 F-Gas engineers who would be qualified to install these heat pumps (rather than the sub 5,000 air to water installers, many without strong experience) and that they can cost three times less.

There are even DIY install Air to Air systems!

The planning elephant in the room

Heat pumps are fine candidates for EVERY property but not every budget because capital cost can be incredibly high and running costs prohibitive. Yet even if you can get over these hurdles, many heritage buildings need planning permission to install them, especially air source heat pumps and this scuppers far too many installs. Though some Local Planning Authorities (LPAs) are now introducing permitted development rights for HPs.

The grid, or local Distribution Network Operator (DNO) might also require you upgrade your connection to three phase, and this may require planning too. There are even instances when they flat out refuse connections due to grid capacity. And planning might stop you doing any supportive works to keep overall running costs down, such as solar panels (even tiles), or retrofitting works.

While you should consider heat pumps for all buildings and view them as valuable to a building’s health, you should probably be more concerned with whether you’ll be permitted to have one, or afford to have one because as per usual, it’s not the technology which doesn’t work but the bureaucratic system.

Rico Wojtulewicz
NFB Head of Policy and Market Insight