How to choose the right size air source heat pump for your home

An air source heat pump can be a fantastic alternative to traditional gas boilers. The government is offering up to a £5k grant per household for the purchase and installation of greener heating systems. Now could be the perfect time to invest. 

But while a new air source heat pump may be easier on the wallet – and could even help reduce energy bills – it’s not as simple as picking one off the shelf. You’ll need to determine the right air source heat pump size for your home for the best results. In this handy heat pump sizing guide, we’ll take you through how to size an air source heat pump, and some key considerations.

Why does heat pump size matter?

You might be thinking the bigger the size of your air source heat pump, the better, right? Well, not exactly. After all, you wouldn’t need an industrial sized gas boiler to heat a 2 bed house! Instead, you’ll want to match the output of the heat pump to your heating needs.

Air source heat pumps have a rating in kW, which represents their output. For example, a 10 kW rating on a heat pump will mean it delivers 10 kW of heat (based on the air temperature outside). Most range between 4 kW to 15 kW. You’ll want to do your research here. As a general rule of thumb, the more demanding your household’s heating needs are, the more powerful you’ll need your heat pump.

A heat pump with too high a rating for your household’s heating demands will be frequently switching off and on. This can rack up high energy bills and the strain can eventually lead to faults. 

A heat pump with too low a rating won’t be as efficient, and will need to run longer. Even then, it might not be able to heat your home to a comfortable temperature. This could definitely take a nasty hit on your bills too.

How to calculate the right size air source heat pump

So, is it time to crack out the air source heat pump sizing calculator and do some complicated maths? Not quite. A Microgeneration Certification Scheme (MCS) accredited heat pump installer will be able to recommend the ideal air source heat pump size for you. 

This should be much more straightforward for new build properties. That’s because new builds have a higher standard of insulation thanks to UK building regulations – and insulation is a factor in working out your ideal air source heat pump. For older properties, you’ll need a detailed building survey to assess heat loss.

As a rough guide, you can check the recommended air source heat pump (ASHP) sizes based on your house size and level of insulation below¹:

But, there’s no one-size-fits-all solution. Your certified installer will take several different factors into account when working out the ideal size of your air source heat pump.

What is the size of your home?

While larger properties will generally need a bigger air source heat pump size, it’s not quite that cut and dried. Your installer will also need to take the type of property and room sizes into account. A detached bungalow with a sprawling layout is unlikely to have the same heating needs as a terraced house with modest rooms, for example.

It’s also likely that you’ll have different heating needs for different parts of your home. You’ll probably want your lounge to stay nice and toasty for most of the day, while the bedroom can just be heated in the mornings and evenings. You should talk to your air source heat pump installer so they can help work out your heating needs. Generally though, a heat pump with an output between 4 kW and 12 KW is suitable for most homes.

How well insulated is your home?

Another crucial factor to take into account for heat pump sizing is your home’s insulation. Unlike gas boilers – which pre-heat water to a high temperature before switching off – an air source heat pump keeps the water at a lower temperature. That means any heat loss in your home will impact the efficiency of your air source heat pump.

Remember earlier when we said heat pump sizing was more straightforward on new builds? That’s because the standard of insulation in these properties tends to be higher. As a result, heat pumps with lower outputs can be used. Older properties can still be installed with an air source heat pump. But your home will need to first meet the minimum insulation levels required by building regulations. That means you may need to carry out work to improve your current insulation first.

Don’t worry though, as you may qualify for the government’s Domestic Renewable Heat Incentive (RHI). You’ll just need to make sure you can provide a current Energy Performance Certificate (EPC). This will also be required for installers to identify potential problems with your home’s energy efficiency. 

Read more about how you can insulate your loft, walls, and windows in our handy guides. 

Do you have radiators or underfloor heating?

An air source heat pump installer will also need to understand your home’s heat delivery system. This means how heat is emitted through your home, such as through radiators or underfloor heating. An air source heat pump consistently heats water at a lower temperature than a traditional boiler. So it requires a larger surface area to keep your home feeling warm.

Underfloor heating is ideal for this. Homes with underfloor heating can use air heat pumps with lower outputs. But don’t worry if you have a more traditional heating system. You may just want to consider adding more radiators or upgrading your existing ones to a larger design. This will increase the surface area, allowing them to reach higher temperatures.

Where do you live in the UK?

We all know that not every part of the UK experiences the same conditions. Locations on the east coast, such as Canterbury and Ashford, enjoy an average of 10 hours of sunshine a day². This is in stark contrast to chilly Lancaster in the north-west of England. Residents here have to settle for just 2.2 hours of daily sunshine on average. Suffice to say that where you live will have an impact on the temperature outside your door. 

The warmer the climate, the more effectively your air source heat pump will perform. That’s because they work by extracting heat from the air outside your home. If the temperature outside is much colder, the air source heat pump will have to work harder, and a higher output will be needed.

The good news is that air source heat pumps are pretty sturdy and have been known to perform in conditions as cold as -25C! As UK weather only tends to dip below 0C for short periods, you shouldn’t have much trouble. 

That said, your installer will still need to factor the lowest annual temperatures in your area into their heat pump sizing calculations. After all, you’ll want to have complete peace of mind that your air source heat pump is fit for purpose when the worst of winter hits.

Will heat pump size affect running costs?

No one wants their green investment to land them in the red. It’s all well and good being eco-friendly with a renewable heating system. But if your air source heat pump causes your energy bills to rocket, you’re not going to be happy. That’s why determining your ideal air source heat pump size and rating is important. 

Opting for an air source heat pump with too low an output rating will increase your running costs. However, going with an air source heat pump that’s too powerful isn’t a solution either. Too high an output for your heating demands will cause the air source heat pump to short cycle. This is when it frequently switches on and off. Short cycling can lead to increased energy bills and strain on your system.

An MCS accredited heat pump installer will be able to accurately calculate your home’s heating needs to find a suitable air source heat pump. But size isn’t everything. Along with the air source heat pump size and output, your running costs will also be affected by other factors. These include your  air source heat pump efficiency (the Coefficient of Performance), property insulation, and general heating demands (like how hot you want your water). It goes without saying that the more heat you need day-to-day, the higher your running costs will be.

All in all, research has found that heat pumps are slightly more expensive to run compared to a new condensing gas boiler³. But, if you drop the heat of your water from 60C to 50C, your air source heat pump might not need to use its in-built electric heater. That could save you over £100 a year on your energy bill. 

Plus, with the efficiency of gas boilers declining over time, coupled with soaring gas bills, many customers find an air source heat pump can lead to significant savings over time. This is especially true for air source heat pump users who go off the gas grid altogether.

What size hot water cylinder do you need?

Just as important as getting your heat pump sizing right, is making sure your hot water cylinder can meet your home’s demands. An unvented indirect hot water cylinder is essential for systems with an air source heat pump. That may sound complicated but it just means that the cylinder has a coil in it which is heated by the  air source heat pump. This coil then heats the water in the cylinder. But how big does that cylinder need to be?

Generally speaking, a typical domestic household will use around 35 to 45 litres of water for every person⁴. Again though, it’s not always so straightforward. The habits of the people in the household will also need to be considered. For example, those who love soaking for a long time in the tub will use more water than those content to just take a quick shower.

The average consumption values in the table below⁵ can give you an idea of the amount of hot water needed daily for each occupant:

That means typically, a 4-person household will need a hot water cylinder that can provide around 200 litres daily. That said, you won’t necessarily need a 200-litre capacity hot water cylinder, as some heating systems may partially reheat water throughout the day. 

Your certified installer will be able to recommend the ideal hot water cylinder size for your needs, so you don’t have to worry about your taps running cold.

While there are a few factors to keep in mind when getting an ASHP installed, the process should be made all the easier with the help of an MCS accredited installer. They’ll be able to review your home’s size, insulation, heating system and location to come up with the ideal ASHP for you.

^{Sources and references}

^{1 https://www.homeheatingguide.co.uk/renewables-advice/air-source-heat-pumps-a-sizing-guide}

^{2 https://www.thesun.co.uk/travel/16083059/uk-hottest-places-revealed/}

^{3 https://www.greenmatch.co.uk/blog/2014/08/the-running-costs-of-heat-pumps}

^{4 https://www.hotwater.org.uk/sizing-a-hot-water-cylinder/}

^{5 https://www.hotwater.org.uk/sizing-a-hot-water-cylinder/}