Are you heat pump ready?


If you currently heat your home with boiler and radiators, it is likely that an air source heat pump (ASHP) is the best solution for your low carbon heating. This uses electricity which can be 100% renewable and clean. There are other kinds of heat pumps, and you could opt to keep your boiler for a while and run a hybrid boiler/heat pump system - see Are hybrid heat pumps a good thing. Heat pumps are best because of the very high efficiency - at least 300%. That means for every unit of electricity you use you get two units of heat from elsewhere (for ASHP that will be the outside air) making three in total. This offsets the price of electricity being much higher than gas or oil.

ASHP are suitable for most homes but not all and some might need a bit of work first. Most installs involve some radiator upgrades, but this is not very disruptive. Some might need new pipework. Even if you have high heat loss you can heat perfectly adequately using an ASHP, however it will cost a little more to run. There is no firm definition of what heat pump ready means. We think it means:

  • Somewhere to put your heat pump
  • Somewhere for a hot water cylinder, or the equivalent
  • The pipes to your radiators are large enough (some microbore pipes are too small, though you can live with them if you are happy with reduced efficiency)
    • Microbore pipes were common in the 1970s and 1980s. They can be replaced but this is disruptive.
  • Your home is cosy enough that your heating bills will not increase.

Cosy enough depends on the ratio of electricity and gas prices and several other factors including your home energy efficiency. Typically that means:

  • No serious defects such as anything causing damp
    • If you have mouldy areas already, it could mean thermal bridging which is also a major heat loss.
  • Good roof insulation (150mm or equivalent) see Roof insulation.
  • Fully (or nearly) double glazed .
  • Reasonably draught proof see Draughts and ventilation.
  • Walls are insulated - cavity walls can usually be filled but otherwise you can insulate either inside or out, see Solid wall insulation.
    • However if you have a terraced house with not much external wall you may not need this.
    • In awkward places where there is not enough room you could have thin insulation - the first cm or two make the most difference!
    • Good heating controls (such as Homely) that optimise the heat pump performance will help reduce bills

Even if you cannot insulate your walls this is not a hard barrier, only you may have an increase in heating demand, e.g. 10-20%.

More insulation is always better, and ventilation control improves air quality as well as minimising unnecessary heat loss. Beyond basic improvements such as loft insulation and cavity wall insulation, fabric improvements can take a long time to pay back in energy bill savings - at least they did while energy prices were low. However, they will bring improved comfort and health, they reduce your exposure to energy price rises, and in some parts of the country they will increase the value of your house.

If you are heat pump ready now but you intend to do further improvements to your home energy efficiency, then ideally you should do them before you get a heat pump. This is so that it is the correct size for your needs. If you want to get the heat pump first, then do make sure it is one that can modulate its output well without losing efficiency (see Getting an Air Source Heat Pump: modulation).

Ready? Take a look at Getting an Air Source Heat Pump and also Finding installers

Do you have space for a heat pump?

ASHPs can be split or monobloc. With a split system the heating part of the heat pump is inside. It might go where your boiler is now. With a monobloc the whole heat pump is outside. The differences are explained in more detail here

Space for the outside unit

The heat pump needs to have good air flow on at least two sides and space for engineers to get in for maintenance. Ideally it should be close to the house to minimise heat loss but there is no maximum distance. Pipes from the ASHP to the house will be well insulated and may be buried in a trench.

The rules for permitted development require that is not visible from the street. It could be at the front but hidden behind a hedge. It also has to be at least 1m from the boundary of the property and must not cause a noise nuisance for neighbours. Your installer will do a noise calculation for the impact on the nearest neighbour's window. See also here for pictures and more detail

Other places are possible, as long as there is free air flow. There are trials under way in Salford for an ASHP mounted in a loft and receiving air through the roof - see UK's first roof mounted ASHP installed at the Energy House.

Space for the hot water cylinder

If you have a combi boiler, you do not currently have a hot water cylinder. However, a heat pump cannot generate hot water as fast as a combi so you will need a cylinder or the equivalent to store hot water. If you do not have enough room for a cylinder to suit your needs, then there are more compact heat storage system such as SunAmp - this is about a third the size. You do not need a cold-water tank as well - you can get an unvented cylinder that runs at mains pressure.

Are your radiators large enough?

The lower the temperature for your radiators, the more efficient the heat pump will run and your bills will be lower. However the lower temperature means the radiators have less heating power. There is a trade-off between radiator size and efficiency. Most heat pump installations involve some radiator upgrades, replacing single or double convectors with triple convectors. This rarely means more wall space - just more thickness. The most efficient radiators produce three or four times as much heat as a basic one for the same area. Your installer will do a room by room survey comparing heat loss with radiator power so that they can discuss upgrades with you.

This table shows the heating power of different kinds of radiator.

Table from 'Heat Pump Retrofit in London' from The Carbon Trust. For a gas boiler feeding radiators at 70C you can expect the mean water temperature to be 60C. For a heat pump at 55C the mean water temperature might be 50C

From experience, any rooms that are not heating up properly at the moment will certainly need an upgrade - and will be much more comfortable afterwards.

Do you have microbore pipes?

This does not affect most people, but microbore pipes were common in the 1970s and 1980s. For the purposes of an HP the usual rule is you need at least 15mm diameter (outer diameter that is). Less than this and the pipes will need to be replaced with larger ones. This is because the heat pump needs the water to flow around faster, and if you don't have large enough pipes the pump has to work very hard which will add to your bills and wear out the pump quickly. Alternatively, you can keep your microbore pipes but run the radiators hotter than is ideal with a heat pump - this will reduce your efficiency a bit but it depends on how much hotter you need to go.

Upgrading the pipes is very disruptive and it adds to the cost of installation. However it certainly can be done.

Are you sufficiently cosy? / Will it cost more to run?

Temperature on a cold day, heating morning and evening only. The green line is for a cosy house which does not lose much heat. The red line is for a large home with solid walls. The blue line shows the temperature you get with a setback of 18C. This makes hardly any difference to the cosy house because it dips only a little below 18C anyway.

A condensing boiler is typically configured to run with an outlet temperature of about 70C. To get good efficiency with an ASHP you need to run at 55C or less - the lower the better. Running your radiators cooler means they are not so good at heating your house quickly from cold. If your house is very cosy it will not get cold anyway. However, a very inefficient home can lose 4-6 degrees quite quickly. The most often recommended solution is to not let your house get cold, so rather than turning the heating off altogether overnight and while you are out, you turn it down. This is called a setback temperature. This can be 1-3C cooler than your normal heating temperature. You may not have controls for this at the moment but when you get the heat pump you will get a programmable timer that allows this.

If your home is very cosy, using a setback temperature instead of turning the heating off will hardly increase your heat demand at all. If you have reasonable draught proofing, loft insulation, double glazing and insulated walls, the increase will only be 5-10%. However if, for example you have large areas of uninsulated walls, you could see a heat demand increase of 20% or even more. This applies to your space heating demand - not your hot water demand. The total impact on heat demand should be taken into account when you estimate your bills with the heat pump.

On the other hand, you may find you can lower the thermostat setting a degree or two and this will offset some of the increase. The stable air temperature reduces air currents in the rooms and the walls will be warmer. This means you will feel warmer for the same air temperature.

The change in bills also depends on the price ratio of gas and electricity, and the efficiency you expect from your heat pump. You can increase the efficiency either by reducing heating demand (by making the house cosier) or increasing the size of your radiators. Either way this means you can run your radiators at a lower temperature.

From October 2023, the gas price (by the OFGEM price cap) is 6.9p/kWh for gas and 27.4p for electricity. Presuming 8% increase in heating demand, and your current gas boiler runs at 85% efficiency*, you need the ASHP efficiency to be at least 310% in order to break even on your bills. (This is assuming you come off gas so you do not need to pay the fixed per day charge). 310% is certainly possible, especially if your radiators are sized so that you can run the ASHP at 50C or less. It may be worth checking out other tariffs. For example if hot water is a significant part of your heating demand you could heat it off-peak.

You can test out different prices and other factors using this tool on Nicola's website

* Your boiler may be nominally much better than this, but it will not achieve its rated efficiency if it is feeding the radiators at 70C. A lower temperature, for example 55C would improve its efficiency.

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