What Size Air Source Heat Pump Do I Need?

Last Updated on June 3, 2025

Choosing the right size air source heat pump is crucial to ensure efficient heating, long-term energy savings, and optimal performance. Whether you’re a homeowner upgrading your heating system or a landlord improving property energy ratings, knowing what size heat pump you need can make or break your investment.

An oversized system could cycle on and off too frequently, leading to wear and inefficiency, while an undersized one will struggle to heat your home, especially during UK winters. 

Let’s break down what size air source heat pump you need based on your property’s specifics, including insulation levels, room sizes, number of occupants, and more.

How to Choose the Right Size Air Source Heat Pump For My Home

Air source heat pumps aren’t one-size-fits-all. To function efficiently, the system must be carefully matched to your home’s specific requirements. 

Underestimating or overestimating the heat demand can lead to higher energy bills, discomfort, and unnecessary wear on the system. Here’s a breakdown of the main factors that influence the correct heat pump size for your property:

1. Property Size and Floor Area (in m²)

The overall size of your home is the most obvious factor. A small one-bedroom flat will need significantly less heating output than a five-bedroom detached house. Most UK homes require around 1kW of heat output per 10m² of well-insulated space. However, this is just a rule of thumb — accurate calculations will always account for more than just square footage.

2. Level of Insulation

Insulation plays a massive role in heat retention. Homes that are poorly insulated will lose heat faster, requiring a more powerful system to compensate. 

On the other hand, a well-insulated property, with features like loft insulation, cavity wall insulation, and double glazing will retain heat longer and therefore need a smaller heat pump. 

This is why upgrading your insulation before installation can result in a more cost-effective and energy-efficient system.

3. Number of Occupants

The number of people living in the home affects how much hot water is used and how often rooms are heated. For instance, a family of five will typically have a higher hot water demand compared to a single occupant. 

This impacts the required domestic hot water (DHW) capacity of the system, particularly if you’re opting for an integrated hot water cylinder.

4. Number and Size of Radiators or Underfloor Heating

If your current radiators are small or spaced far apart, your heat distribution might not be efficient. Air source heat pumps work best with larger radiators or underfloor heating systems because they operate at lower flow temperatures than traditional gas boilers. 

If your radiators aren’t suitable, they may need to be upgraded to ensure your heat pump works optimally. Check out our guide on heat pump and radiator compatibility. 

5. Desired Indoor Temperature

Not everyone likes the same level of warmth. Some households prefer a cooler environment, while others like to maintain a steady 21°C or higher throughout the home. The higher your desired indoor temperature, the more energy your system will need to produce, impacting the required size of the heat pump.

6. External Climate Conditions

While air source heat pumps are designed to work even in cold weather, external temperatures still influence system performance. 

Homes in colder regions of the UK (like the Scottish Highlands or Northern England) may require slightly larger systems compared to those in milder areas like Cornwall or the South East, where the ambient air temperature remains higher year-round.

Air Source Heat Pump Sizing Table (Estimated Output Based on Property Size)

How Do I Calculate What Size Heat Pump I Need?

Calculating the right size air source heat pump for your home involves a heat loss assessment, a detailed analysis that measures how much heat your property loses and how much is required to maintain a comfortable indoor temperature all year round.

While you can estimate heat pump size using general rules of thumb, an accurate calculation should always be done by a qualified installer. However, to give you a basic idea, here’s what’s involved:

Step-by-Step: Rough Estimation for ASHP Sizing

1. Measure Your Floor Area (in m²):
   Start by measuring the total heated floor space in your home. For example, a 3-bedroom home might have around 100–120 m² of heated space.
   
2. Multiply by a Heat Demand Factor:
   Use a heat demand factor that corresponds to your insulation level:
   
   • Poor insulation: 80–100 W/m²
     
   • Average insulation: 60–70 W/m²
     
   • Good insulation: 40–50 W/m²
     
3. For example:
   A 100m² home with average insulation → 100 × 65W = 6,500 watts or 6.5kW estimated heat demand.
   
4. Factor in Hot Water Requirements:
   If your system will supply hot water too, add 1–2kW to cover daily usage. Larger families may need even more.
   
5. Consider Local Climate:
   In colder regions of the UK, like Northern Scotland or rural Wales, add 10–15% extra capacity to account for lower external temperatures.

Why You Shouldn’t Rely on DIY Sizing Alone

While the formula above gives you a ballpark figure, it doesn’t take into account:

• Heat loss through walls, windows, floors, and the roof
  
• Orientation of your home and solar gain
  
• Internal zoning and room layouts
  
• Radiator compatibility and flow temperature settings

That’s why we strongly recommend requesting a free, professional quote, not just to get the correct size system, but to ensure you’re eligible for available heat pump grants, and that the system will deliver the performance and savings you expect.

Why Getting the Right Size Air Source Heat Pump Matters

An ASHP that is too small will struggle to heat the entire property efficiently.

• Energy Efficiency: Correctly sized systems operate at optimal capacity, reducing your energy bills.
  
• Comfort: A well-matched pump keeps consistent warmth, even on cold days.
  
• Longevity: Proper sizing prevents overworking the system, which can lead to premature breakdowns.
  
• Grant Eligibility: Some heat pump grants require a formal sizing assessment.

If you’re curious about financial support, our detailed guide on heat pump grants walks you through eligibility and application processes, including the Boiler Upgrade Scheme.

Heat Pump Sizing: Why a Quote Is Essential

While the above table offers a helpful starting point, a free heat pump quote includes an in-depth heat loss assessment tailored to your property’s characteristics. This ensures you get a system that’s perfectly suited, whether you live in a compact bungalow in Birmingham or a large semi-detached house in Leeds.

Other Considerations Before ASHP Installation

• Insulation: Poorly insulated homes may require larger systems. Upgrades like loft or cavity wall insulation can reduce heating demand significantly.
  
• System Type: Monobloc vs split systems may vary in installation space and output range.
  
• Flow Temperatures: Lower flow temperatures improve efficiency but must work with your heating system design.

If you’re in the planning phase, our air source heat pump installation guide is a must-read, offering a complete walk-through from initial survey to commissioning.

Air Source Heat Pump Pros and Cons You Should Know

Before making a final decision, it’s worth weighing the air source heat pump pros and cons. While they’re excellent for reducing carbon emissions and long-term heating costs, upfront costs and insulation requirements must be considered.

Ready to Find the Right Size Heat Pump for Your Home?

Don’t leave it to guesswork. Get expert guidance and a bespoke system tailored to your property’s size, layout, and heating needs. Eco Happy offers:

Free no-obligation quotes
Accurate heat loss calculations
Access to government funding and grants
Expert installation across the UK

Request your free air source heat pump quote now and take the first step towards a warmer, more energy-efficient home.

FAQs

How big should my air source heat pump be?

The size of your air source heat pump depends on several factors, including the size of your home, insulation levels, number of occupants, and your hot water demand. 

As a rough guide, most UK homes require a heat pump between 5kW and 16kW. A small, well-insulated home might need a 5–7kW unit, while a larger or poorly insulated property could need 12kW or more. 

It’s essential to have a professional heat loss calculation carried out by a qualified installer to determine the correct size. Oversized or undersized units can lead to inefficiency, higher running costs, and reduced comfort.

Is it better to undersize or oversize a heat pump?

Neither option is ideal. The best approach is to size your heat pump correctly based on a professional heat loss calculation. However, if you had to choose, slight oversizing is generally preferred over undersizing. 

An undersized heat pump may struggle to heat your home during colder months, leading to discomfort and reliance on backup heating. 

On the other hand, an oversized unit can lead to short cycling (frequent on-off cycling), which reduces efficiency and can cause wear and tear over time. In the UK, getting the sizing right is crucial for comfort, efficiency, and long-term performance, so always consult a qualified installer.

What happens if a heat pump is too big?

If a heat pump is too big for your home, it can lead to short cycling, where the system turns on and off frequently because it heats the space too quickly. This reduces the overall efficiency of the system, increases wear and tear on components, and can shorten the lifespan of the unit. It may also lead to uneven temperatures and higher energy bills.

In the UK, where heat pumps are designed to run for long periods at lower temperatures, correct sizing is essential to ensure comfort, efficiency, and long-term reliability. Always have a qualified installer carry out a proper heat loss calculation before installation.