If you are already sold on the idea of reducing your carbon footprint and updating your home heating with an air source heat pump, you may be at the point where you are trying to ascertain what size of system you need. Working out the ideal heat pump size can be tricky, especially if your property is an older one with varying differentials which can affect the size of heat pump system you need.
In general terms, the larger your home, the bigger the air source heat pump system required. As a rough estimate — and depending on the level of heat loss, the age of the property, and room types — you might expect the following heat pump size recommendation:
- A 3-bedroomed property of around 100 square metres might require a 5kW air source heat pump to heat it effectively.
- If the floor space was doubled to 200 square metres, it is fair to say the size of the heat pump would need to be doubled to 10kW.
However, it is important to note that these figures are just a guide because there are many other factors that could affect the calculation when it comes to assessing heat pump sizing. Read on to find out more about the size of air source heat pump you could potentially need for your home.
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Calculating air source heat pump size in the UK – new build and older properties
Calculating the size of an air source heat pump for newly built homes is relatively straightforward, because the level of insulation will be up to agreed level because of UK building regulations. This allows the level of heat loss to be worked out accurately, so the size of the heat pump needed is fairly easy to estimate.
Detailed heat loss calculation in older buildings is much more difficult, because there are many different factors to take into account including levels of insulation, existing radiators or underfloor heating, size of the property, number of rooms and their uses, seasons temperature fluctuations. For homes that fall into this older property category, a full survey will be required to accurately estimate heat loss levels.
The number of different types of rooms has to be taken into account because some rooms require more intense heat than others. For example, living rooms and bathrooms typically need to be kept warmer than bedrooms. So, an assessment of the number and type of rooms help determine the heating demands of the home and therefore the size of the required air source heat pump.
The most common sizes of air source heat pumps installed in UK homes are 4kW, 5kW, 6kW, 8kW, 10kW, or 12kW. You can go larger if required, such as up to 16kW, but generally it is only really needed for very large properties (usually with 5+ bedrooms).
Can I oversize or undersize my heat pump?
Using an oversized heat pump can sometimes prove be too powerful for your property, and result in inefficiencies due to short cycling (where the pump turns on and off too quickly) resulting in wasted energy and a reduced lifespan for your heat pump. You’ll not only be paying far more than you need to for your heat pump when you first buy it, but you’ll also have to replace it far sooner than if you had bought the correct sized heat pump for your home.
Undersizing, or choosing an air source heat pump that is too small, will also cost you more in the long run. How will you know your heat pump is tool small? Essentially, you will know because your appliance will be overworked and making much more noise trying to keep up with your household heating demands. You’ll see a degradation in performance and your heat pump will need replacing far sooner.
Selecting the right size heat pump is therefore important if you want to ensure the best return on your investment in terms of optimal energy production and consumption and longevity of your heat pump.
Does air source heat pump size really matter?
Air source heat pumps are affected more by fluctuations in the temperature (air temperature) than ground source heat pumps, where the ground temperature is more constant, so the power output varies more with an air source unit, which as a result has to work harder to produce the required levels of heat in the colder months.
To take into account the changes in output in the colder months, air source heat pumps are often made with larger capacities than ground source. This explains why in the above example, an air source heat pump would be 5kW in a 100 square metre house and only 4kW for a ground source heat pump.
What factors impact on the size of heat pump I need?
The size of your air source heat pump can depend on three main factors: outdoor design temperature, desired room temperature, and flow temperature.
Outdoor temperature
Air source heat pumps take in heat from the air by passing extremely cold refrigerant liquid through heat exchanger coils. Therefore, when the air temperature is warmer outside, the pump will not have to work as hard.
To make sure that the heat pump can meet your heat requirements throughout the year, including the heat pump continuing to work during winter, heat pumps are sized based on the coldest air temperatures of the year for your location. So, if coldest temperature in your locale is milder than elsewhere, generally a smaller capacity air source heat pump can be used.
Desired room temperature (the lower the temperature the better)
Desired room temperature comes down to personal preference. As a rule of thumb, a comfortable temperature in the homes is around 21 degrees centigrade, but lower room temperatures inevitably require less heat, so a smaller and less expensive heat pump may be suitable.
Unlike a gas boiler, which heats a property almost instantaneously on demand, a heat pump gradually heats a room over a period of time, and is often always operating in the background. A larger heat pump system will be required for those who like their property that bit more on the warmer side.
Flow temperature
The flow temperature is basically the temperature water needs to be circulating around your radiators in the home in order to heat your home effectively. This will be affected by the size of your radiators (and any underfloor heating) and your level of insulation (which will be reflected in your current Energy Performance Certificate).
If the radiators are too small, the flow temperature will have to be higher to heat the room to the desired temperature or you may have to use oversized or larger radiators or opt for a high temperature heat pump that potentially allows your existing radiators to be used. Your existing pipework will need to be of the standard where it is capable of carrying the heat load required.
How can I calculate what size air source heat pump I need?
If you live in a new-build property, the example numbers for air heat pumps in this article will give you a fairly accurate guideline to help you determine what size air source heat pump you will need. If you have an older property, it is advisable to contact our installation experts for further assistance.
A poorly insulated property is estimated to require a heat pump unit around 1.5 times the size of heat pump that would be required for a well-insulated property. To minimise your costs you should look to insulate your property to the best level possible first to avoid having to oversize your heat pump.
For an at a glance view the following should give you a good indication of air source heat pump sizes depending on size of property and degree of insulation.
| House size | Air source heat pump size (well-insulated property) | Air source heat pump size (poorly insulated property) |
| 1-2-bedroom | 4kW | 6kW |
| 3-bedroom | 5kW | 7.5kW |
| 4-bedroom | 10kW | 15kW |
| 5+ bedroom | 16Kw | 16kW |
Is there any other way of working out heat pump sizing?
Instead of basing your required size of air source heat pump on the number of bedrooms, you could base it on the square metreage (m2) of the property. A good rule of thumb is that you require approximately 50 watts for every square metre of property space (for example, amounting to 5kW for every 100 square metres) for a well insulated home.
However, while watts per square metre is a good starting point, there is still a need to take into account other factors including the age of property, levels of insulation, and other factors. The reality is you’ll need a professional heat loss calculation from an expert for accurate heat pump sizing to ensure your heat pump meets your needs and is not unnecessarily large.
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Heat pump sizing summary
How much does air source heat pump size matter? The right sized heat pump system is one that will meet the heat demand (and in some case hot water demand) for your household. While outdoor and indoor temperatures, along with other factors, will have a bearing on heat pump size, you should buy an appropriately sized heating system that enables you to get the best savings and reductions in energy bills.
A small heat pump may be an adequate heat distribution system for a small or newer well-insulated property, but a bigger heat pump will be required for a poorly insulated house, or one where the demand for more heat or hot water is required. Generally the bigger your home and larger your heat demand, the greater the need to have a heat pump installed with larger capacity, especially if you are requiring it to also heat a hot water cylinder.
How much heat you require, and therefore the right heat pump size, will depend on many different factors. By discussing your heat pump needs with an experienced heat pump installer will ensure that you get the best heat pump size perfectly matched to your needs.
See if you can take advantage of up to £7,500 funding available from the Boiler Upgrade Scheme and potentially have the majority of your heat pump installation covered by a grant.
