What Size Air Source Heat Pump Do I Need For My Home? (Free Calculator)

You can’t just pick a heat pump off the shelf. You need the right size air source heat pump to suit the size and type of house you live in. 

In this instance, yes size does matter! 

This is because you want to match the heat output of the pump to your heating needs. If your heat pump is too big, it’ll keep flicking on and off. This may strain the system, cause faults and increase your electricity bills. 

Too small an air source heat pump and your heating needs may not be met altogether. The heat pump is likely to be running a lot of the time, working to raise the temperature inside. Again, this will impact your energy bills. 

So, let’s give you the info to make a smart decision on what size air source heat pump you need for your home.

How big are air source heat pumps?

The size of an air source heat pump is measured in kilowatts (kW). You may read about British Thermal Units (BTUs) too. 

BTU is a traditional measurement of heat energy. It’s the pounds and ounces of the kilogram world. Many radiator outputs in the UK are still measured in BTUs.

The new kid on the block when it comes to heat measurement is kilowatts. They’re much easier to understand (at least for me) and better to compute. Every air source heat pump will be sized in kilowatts (kW). 

So, how big are air source heat pumps in general?

For most homes, an air source heat pump sized between 5-15kW will do the job. Generally speaking, the bigger your home, the bigger heat pump you’ll need.

However, this is a big range. So how do you know what size air source heat pump you need? Let’s get into the specifics.

Air source heat pump size calculator

How is this heat pump size calculated?

As a quick rule of thumb, professionals recommend 1kW of heat for every 10 square metres of home. This is very rough and generally results in a big overestimation. 

There’s a much better way to calculate the size of air source heat pump you need. 

All you need to know is your annual gas usage. 

Everyone with a gas boiler can get this. If you look at your recent bill, you’ll find an annual estimate. This is usually on the ‘about your tariff’ section. You may even be able to find your actual usage, which would be better. 

Once you’ve found your annual gas usage. There are a couple of simple steps to follow. 

1. Take your annual gas usage (kWh) and divide by 57.3

Example: My estimate of annual gas use is 21,000 kWh. So 21000/57.3 = 366 watts per degrees Celsius

Explanation: This will give you your Heat Transfer Coefficient in watts per degrees Celsius. The number is how much power is required to raise the temperature in your home by 1^oC

2. Multiply this figure by 20

Example: 366 x 20 = 7,320 W or 7.3 kW

Explanation: This will give a figure of what size heat pump you’ll need to raise the internal temperature by 20^oC on the coldest days. This is the so-called ‘design temperature’ based on the most likely coldest weather. 

The calculation above estimates the heat pump size for peak demand. So for me, a 7.3 kW will be enough for pretty much the whole year round. 

The heat pump size it spits out is an estimate for the maximum power required. This is why so many air source heat pumps in homes are over-sized. 

Your calculated figure will be within roughly 10% accuracy. This may seem like quite a lot, but when we’re talking about heat pump sizes it’s not that much and will serve you well on the whole.

Quickest way to calculate air source heat pump power

Even though the above method is fairly simple, there’s an even quicker way to calculate an air source heat pump size. 

Take your annual gas usage and divide it by 2,900.

In my example, 21,000 / 2,900 = 7.24 kW

How easy is that? 

Notes on heat pump size calculation 

Calculating your heat pump size based on your annual gas usage is a much more accurate way to get an estimate than a general rule of thumb. 

No it’s not always perfect, but it’ll get you close enough. 

Although the figures of 57.3 and 2,900 may seem random, they are based on UK ‘heating degree days’ over an entire season. They also take into account boiler efficiency at around 90%. 

The calculation was devised by physicist Dr Michael de Podesta. Watch his highly informative Youtube video for a full explanation. He really knows his stuff. You can also head to the Heating Degree days website for more info on this.

What size air source heat pump per beds in a house

According to research from Boiler Guide, here are the typical air source heat pump sizes needed by house type. 

Please take the recommendations with a pinch of salt as they are only rough estimates. Calculating the proper ASHP size for your home will depend on a number of factors. 

House Type	Air Source Heat Pump Size
2 bed house/apartment	5kW
3 bed house (well insulated)	5-10kW
3 bed house (poorly insulated)	9-11kW
4 bed house (well insulated)	9-11kW
4 bed house (poorly insulated)	11-16kW
5 bed house (well insulated)	11-16kW

As you can see, the size of the air source heat pump you’ll need for your home can, and will, vary. This is because there are a number of factors at play which will determine the heat pump size for you. It will also depend on the type of air source heat pump you have.

Although this might give you a good idea of the size you’ll need, a professional heat pump installer will do a thorough consultation at your property, as well as the calculation above of course. 

You can take a closer look at air source heat pump size for a 3 bed house and a 4 bed house here.

Factors to consider when choosing an air source heat pump?

There are six main factors that will need to be considered to determine the best air source heat pump size for your home. 

1. Size of your property
2. Size of rooms and radiators
3. Insulation level 
4. Number of windows
5. Level of heating demands
6. Local year round temperatures outside

Let’s take a closer look at each of these factors.

Size of your home

The size of your home is an essential factor when it comes to air source heat pump sizing.

Ultimately, the property size directly affects the amount of heat a heat pump must produce to reach and maintain a comfortable indoor temperature. In general, a bigger property requires a larger heat pump with a higher kilowatt capacity.

Alongside heating demands, the size of a home affects the amount of heat loss. Larger properties tend to lose more heat compared to smaller ones due to their greater surface area. 

For example, a small terrace house has a smaller floor area and fewer external walls compared to a large, detached property. So, the heat loss of a terraced house will be lower.

In this example, a smaller heat pump size of say 5kW may be enough to provide adequate heating, whereas the larger detached property with greater heat loss may require a heat pump double that size. Other factors also come into play, such as levels of insulation and the number of occupants.

Number of rooms and radiators

The number of rooms and radiators is another crucial factor affecting the required size of an air source heat pump. It’s a simple one really: the more rooms you have, the more space a heat pump must heat to maintain a nice temperature. 

Most rooms tend to have a radiator in them, which affects the heat output required. Some rooms will be heated more than others, such as the living room and bathroom – no one likes a cold bathroom! Whereas others are better off a little cooler, such as bedrooms and that spare room you might not go in often.

As heat pumps operate at lower temperatures than traditional gas boilers, you’re likely to need larger radiators to reach the desired room temperatures. 

Insulation levels

Insulation is critical when discussing air source heat pump size! 

The job of insulation is to reduce heat loss. Good levels of proper insulation do just that, which keeps your home warmer for longer periods. For air source heat pumps, this reduces the pump’s workload and therefore the heat output needed. This means that a well-insulated property is likely to require a smaller sized heat pump than a similar sized home with poor insulation levels.

New build properties have excellent levels of insulation and have had heat loss assessments performed. Everything is documented and the paperwork is already there making it a simpler job when choosing the correct size air source heat pump. 

Older properties tend to have poorer levels of insulation, more draughts and greater heat loss. Improving insulation is a great, and often necessary, first step before installing a heat pump. There’s no point in generating all that energy efficient heat, if most of it is going to disappear through your walls and roof!

If you’re interested, there are some great types of eco friendly insulation.

Number of windows

Heat is lost through windows. This is because heat always move from warmer to cooler. The more windows you have, the greater the potential for heat loss. In fact, windows are known to be the weakest link in a property’s thermal envelope (any part of your home separating the indoors from the outdoors). 

The amount of heat lost through windows depends on several factors, including window size, the type of glass and type of frame. For instance, single-glazed windows lose much more heat than double-glazed or triple-glazed windows. Poorly insulated frames may also leak out heat.

So, during your heat pump assessment, the installer will take the number and type of windows into account to size up the correct air source pump. 

Levels of heating demands

Further factors impacting the size of the heat pump you need are your heating demands and the number of occupants in the home. 

With more people, you’ll need higher heating output. This is usually because more rooms are occupied and there’s an increase in water demands too. How hot you like your water is all a consideration. This will all be impacted if you have an air to water heat pump, which is the most common type in the UK. 

But age is also a factor here. If you have one or more children, it’s likely that your heating demands will be higher. Elderly individuals are likely to require higher indoor temperatures to maintain a good comfortable atmosphere. If I take my grandparents as an example, their home was warm all the time! Probably a bit too warm, but that’s another conversation…

Outside temperatures across the year

The final big factor affecting heat pump size is the year round temperature fluctuations in your local area. 

As you know, air source heat pumps get their heat energy from the outside air. Even though air source heat pumps can work in very low temperatures, it’s the temperature that determines how much heat is available. Warmer air has more heat that can be extracted. 

As the temperature drops outside, the performance of an air source heat pump decreases. So, in areas with colder temperatures, larger heat pumps are needed to compensate for the reduced efficiency of the pump. On the flip side, areas with milder temperatures may be fine with a smaller sized heat pump. 

Even around the UK, average temperatures can differ by a good few degrees across the year, so it’s worth considering.

What does an air source heat pump survey do?

A professional heat pump installer should be MCS certified. MSC stands for Microgeneration Certification Scheme, which is the body that certifies low-carbon energy technologies and contractors

An MCS installer will perform a survey that involves measuring rooms, walls, exterior walls, types of windows and more. This can take half a day or more. Once it gets inputted into a spreadsheet, out comes a recommended heating load. 

From this, the installer will be able to calculate what size heat pump will suit your home the best. 

You can use your heat pump size calculation above to verify any estimates that an installer makes. For example, if the installer comes back with a significant over-estimate or even under-estimate, you’ll be able to raise any queries confidently. 

How does air source heat pump size impact running costs?

It’s no secret that the size of an air source heat pump will impact running costs. The larger the heat pump, the more expensive it is to run. 

An ASHP generates more heat energy than it consumes in electricity. 

Let’s give an example courtesy of Protons for Breakfast. 

Across the year in London, you’re likely to require 2,000^oC heating degree days (HDD). This is worked out by how many days the temperature outside falls below a base temperature (usually a few degrees below your internal thermostat setting). The result gives you an idea of heating demand for your home. 

The average home uses around 12,000 kWh of heat per year. Working at a good efficiency of 350%, a heat pump would need to provide around 3,500 kWh of electricity. 

At the time of writing (April 2023), I’m paying 32p per kWh of electricity. 

In this example, it would cost £1,120 to heat the average home with an air source heat pump with mains electricity from the grid. 

These figures and heat energy use are all general though. Remember, it all depends on the factors described above as to how much your heat demand energy is. 

Issue with undersizing or oversizing an air source heat pump

If you undersize your air source heat pump, it may struggle to maintain the desired temperature as dictated by your thermostat. 

By trying to reach this, the heat pump will have longer operating hours and higher energy consumption. 

At the other end of things, an oversized heat pump can lead to inefficiencies during operation. This is because the compressor switches off when the heat demand is less than 40% of the maximum output, then switches back on after a while. In effect it’s short cycling, which results in wear and tear on the compressor and a lower efficiency, or coefficient of performance (COP). 

Instead of generating 3.5 times the heat output per kW of electricity, this will be greatly reduced. So, this means when a heat pump is operating at a lower COP than stated you’ll be paying more in energy costs than you should be. 

Your best bet is to accurately calculate the required heating capacity based on the property’s size and heat energy use. This will result in maximum efficiency and reduced running costs.

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