Air Source Heat Pump Sizing: Getting It Right for Your Home

TLDR: Heat pumps must match your home's heat loss—typically 3-16 kW for UK homes. Sizing requires heat loss calculation, not just floor area. Oversized pumps cycle excessively (inefficient); undersized pumps can't maintain temperature. MCS installers must perform calculations before installation. The £7,500 BUS grant applies regardless of size, making proper sizing even more critical.

When I speak with homeowners about heat pumps, one question comes up more than any other: "What size do I need?" It sounds straightforward, but it's actually the single most important technical decision in the entire installation process. Get it right, and you'll have a comfortable, efficient home for the next 20 years. Get it wrong, and you'll be stuck with higher bills, uncomfortable temperatures, or both.

The frustrating thing is that sizing isn't as simple as matching kW to floor area. I've seen 120 square metre homes that need 6 kW and others the same size that need 12 kW. The difference comes down to insulation, windows, exposure, and a dozen other factors that only a proper heat loss calculation can capture.

This guide explains how sizing works, what affects your home's heat loss, and how to ensure your installer gets it right. Because once that heat pump is installed, you're living with the consequences for decades.

What Size Heat Pump Do You Need?

Before we dive into the details, here's a rough guide to typical heat pump sizes for different UK homes. These are starting points, not prescriptions—your actual requirement depends on your specific home.

Home Type	Typical Heat Loss	Heat Pump Size	Typical Cost (Installed)
Modern flat (50-80 sqm)	2-4 kW	4-6 kW	£8,000-£11,000
Semi-detached (80-120 sqm)	5-8 kW	7-10 kW	£10,000-£14,000
Detached (120-200 sqm)	8-14 kW	10-16 kW	£12,000-£18,000
Large detached (200+ sqm)	12-20 kW	14-22 kW	£16,000-£25,000

After the £7,500 Boiler Upgrade Scheme grant, these costs drop significantly. A typical semi-detached installation might cost £10,000-£14,000 before the grant, leaving £2,500-£6,500 to pay. That's comparable to a premium gas boiler replacement.

Real Homeowner Experiences: Getting Sizing Right (and Wrong)

Sarah and Mark, Nottingham (1930s semi-detached): "Our installer did a thorough heat loss calculation that took about two hours. He measured every window, checked the loft insulation depth, and even asked about our cavity wall insulation certificate. The calculation showed we needed 9.2 kW, so he recommended a 10 kW Vaillant Arotherm. We were worried it wasn't big enough—our old gas boiler was 28 kW—but he explained that boilers are always oversized because they need to heat water quickly. The heat pump runs longer at lower intensity. Two winters in, and we've been perfectly comfortable even during that cold snap in January when it hit -8°C. Our running costs are about £1,100 per year for heating and hot water."

David, Leeds (Victorian terrace): "I nearly made an expensive mistake. The first company I contacted quoted for a 12 kW heat pump based on a 'rule of thumb'—basically, they measured our floor area and multiplied by a standard figure. When I got a second quote from an Octopus Energy-recommended installer, they did a full MCS heat loss calculation and found we only needed 8 kW. That's a significant difference in equipment cost, but more importantly, the smaller pump runs more efficiently at our actual heat demand. The second company also identified that our EPC was E, which would have blocked the BUS grant. They helped us get the loft topped up to 300mm, which lifted us to D and qualified us for the £7,500."

Christine, Bristol (new build, 2018): "Our house was built to modern Building Regulations, which meant the insulation and air tightness were already excellent. The heat loss calculation showed just 4.8 kW—far lower than older homes our size. We installed a 5 kW Mitsubishi Ecodan, which cost £9,500 installed. After the BUS grant, we paid £2,000. It's been brilliant—our heating bills are about £450 per year, less than half what friends in similar-sized older homes pay for gas."

Robert, Edinburgh (stone-built cottage): "Solid stone walls are beautiful but terrible for heat retention. Our cottage is only 85 square metres, but the heat loss calculation came back at 11.4 kW—higher than many much larger modern homes. We faced a choice: install a large 12 kW heat pump or invest in insulation first. We decided to do internal wall insulation on the worst-affected rooms, which brought the heat loss down to 8.2 kW. The insulation cost £6,500 through ECO4 (we qualified due to being on Universal Credit), and then we installed an 8 kW heat pump for £12,500 minus the £7,500 grant. Total out-of-pocket was about £5,000 for both projects. Our energy bills dropped from £2,800 per year on oil to about £1,200 on the heat pump."

What Affects Heat Loss

Heat loss is simply the rate at which warmth escapes from your home. Every wall, window, floor, and roof lets heat out. The more heat you lose, the bigger the heat pump you need to replace it.

Insulation Levels

Insulation is the single biggest factor in heat loss. Here's how different wall types compare:

• Solid walls (uninsulated): U-value around 2.0 W/m²K. These lose heat rapidly—common in pre-1920s homes. Solid wall insulation (internal or external) can reduce this to 0.3-0.5 W/m²K.
• Cavity walls (unfilled): U-value around 1.5 W/m²K. Still significant heat loss. Cavity fill costs £500-£1,500 and dramatically improves performance.
• Cavity walls (insulated): U-value around 0.5 W/m²K. Much better, typical of 1980s-2000s homes.
• New build (2015+): U-value 0.2-0.3 W/m²K. Modern regulations require high performance.

Loft insulation matters too. Building Regulations now require 270mm, but many older homes have 100mm or less. Topping up is cheap—often £300-£600 for a typical loft—and can reduce heat loss by 20-25%.

Windows and Doors

Windows are thermal weak points. Heat escapes quickly through glass:

• Single glazed: U-value around 5.0 W/m²K. Terrible for heat retention. If you have single glazing, consider replacing it before installing a heat pump.
• Double glazed (pre-2000): U-value around 2.8-3.0 W/m²K. Better, but older units often have failed seals.
• Modern double glazed: U-value 1.2-1.6 W/m²K. Low-E coatings and argon fill make a big difference.
• Triple glazed: U-value 0.8-1.0 W/m²K. Common in Passivhaus-standard homes.

External doors also matter. Old wooden doors with letterboxes and gaps can lose significant heat. Insulated composite doors typically achieve U-values of 1.0-1.5 W/m²K.

Other Factors

Heat loss calculations also consider:

• Floor insulation: Suspended timber floors with no insulation lose 10-15% of a home's heat.
• Draught-proofing: Air leakage through gaps around windows, doors, and service penetrations.
• Air tightness: Measured in air changes per hour. Older homes might be 10-15 ACH; new builds target under 5 ACH.
• Building orientation: North-facing rooms lose more heat than south-facing ones.
• Exposure: Hilltop homes exposed to wind have higher heat loss than sheltered valley properties.

Why Correct Sizing Matters: The Consequences of Getting It Wrong

Oversized Heat Pump Problems

It might seem logical that bigger is better—surely a larger heat pump will heat your home faster and handle cold snaps better? Unfortunately, oversizing creates real problems:

• Higher purchase cost: A 12 kW heat pump costs £2,000-£4,000 more than an 8 kW unit. That's money wasted on capacity you don't need.
• Short cycling: An oversized pump reaches your target temperature quickly, then shuts off. It cools down, starts again, shuts off again. This constant cycling wears out components and reduces efficiency.
• Reduced efficiency at part load: Heat pumps are most efficient when running steadily at moderate output. Running at 30% capacity isn't as efficient as running at 70% capacity.
• Increased wear: More start-stop cycles mean more stress on the compressor—the most expensive component to replace.
• Temperature swings: Short cycling causes rooms to overshoot then undershoot the target temperature, reducing comfort.

Modern inverter-driven heat pumps can modulate output down to 30-40% of rated capacity, which helps reduce short cycling. But they still can't efficiently run at 20% output, so a dramatically oversized pump will still cycle.

Undersized Heat Pump Problems

Undersizing is arguably worse than oversizing:

• Cannot maintain temperature: On cold days, the pump runs flat out but still can't keep your home at 21°C. You're stuck with 18°C or have to use backup heating.
• Continuous operation: Running at maximum output 24/7 during cold snaps increases wear and raises bills.
• Backup heating required: You might need to supplement with electric heaters—expensive and defeats the purpose of a heat pump.
• Higher running costs: A pump running at maximum output is less efficient than one running at 70-80% capacity.
• Customer dissatisfaction: Nothing undermines confidence in heat pumps like a system that can't keep you warm in winter.

The Heat Loss Calculation: How It Works

MCS-certified installers must perform a room-by-room heat loss calculation before quoting. This isn't optional—it's a requirement for MCS certification and, by extension, for BUS grant eligibility.

The calculation considers:

• Room dimensions: Length, width, and height of every heated room.
• Wall construction: Solid brick, cavity, timber frame, with or without insulation.
• Window sizes and types: Single, double, or triple glazed, plus frame material.
• Floor construction: Solid concrete, suspended timber, insulated or not.
• Ceiling/roof: Insulated loft, flat roof, room-in-roof.
• Ventilation: Natural ventilation, trickle vents, mechanical ventilation.
• Design temperatures: Internal target (usually 21°C living spaces, 18°C bedrooms) and external design temperature (usually -3°C to -5°C for most of England, lower for Scotland).

The output is a total heat loss figure in kilowatts—for example, "this home loses 9.4 kW at -3°C outside when maintained at 21°C inside." The heat pump is then sized to meet this load with a small margin.

Good installers use software like MCS Heat Pump Calculator, Mitsubishi Ecodan Selection Tool, or similar. The calculation typically takes 1-2 hours for a thorough assessment.

Common Sizing Mistakes to Avoid

Mistake 1: Using Floor Area Alone

Some installers use rules of thumb like "100W per square metre" or "150W per square metre." These are dangerously inaccurate. A well-insulated modern home might need 50W/m², while a poorly insulated Victorian terrace might need 200W/m².

Always insist on a proper heat loss calculation.

Mistake 2: Matching to the Old Boiler

"My gas boiler is 28 kW, so I need a 28 kW heat pump" is a common misconception. Gas boilers are deliberately oversized because they need to heat water quickly for taps and showers. A 28 kW boiler in a home with 10 kW heat loss is normal for gas systems.

Heat pumps work differently. They run at lower output for longer periods, maintaining temperature rather than boosting rapidly. A 10 kW heat pump is usually right for a home with 10 kW heat loss.

Mistake 3: Not Accounting for Hot Water

If your heat pump supplies hot water as well as heating, the calculation must account for hot water demand. High-demand households (large families, frequent baths) may need a slightly larger pump or a separate hot water strategy.

Some installers recommend a hybrid approach: heat pump for space heating, immersion heater for hot water top-up during high-demand periods.

Mistake 4: Ignoring Future Improvements

If you're planning to add insulation, replace windows, or draught-proof within the next few years, consider sizing the heat pump for your future heat loss rather than current. An 8 kW pump might be perfect after you insulate, even if you temporarily need 10 kW now.

Discuss this with your installer. Some offer a staged approach: install slightly smaller now, knowing you'll upgrade insulation within 18 months.

The BUS Grant and Sizing

The Boiler Upgrade Scheme (BUS) provides £7,500 toward air source heat pump installation. The grant applies regardless of heat pump size—you get the same amount whether installing a 5 kW or 20 kW unit.

This makes proper sizing even more critical. There's no financial benefit to oversizing (you pay more, the grant doesn't increase), and you suffer the efficiency penalties described above.

BUS requirements include:

• MCS-certified installer must perform heat loss calculation
• Property must have EPC rating of D or better
• Heat pump must be MCS-certified product
• Installation must meet MCS standards

If your EPC is E, F, or G, you'll need to improve it before qualifying. Loft insulation, LED lighting, and draught-proofing often bridge the gap from E to D for £500-£1,500.

Q&A: Heat Pump Sizing

Q: Can I use an online calculator to estimate my heat pump size?

A: Online calculators give rough estimates—useful for initial research, but not for final sizing. For actual installation, a proper calculation by a qualified MCS installer is required. The MCS certification scheme mandates this, and it's a condition of the BUS grant.

Q: What about hot water—does that affect sizing?

A: Yes. Heat pumps often supply hot water as well as space heating. Sizing may need to account for hot water demand, especially if you have high usage or want fast recovery times. Most domestic heat pumps can handle typical hot water needs alongside heating, but high-demand households may need a larger unit or supplementary immersion heater for peak times.

Q: Should I upgrade insulation before getting a heat pump?

A: Often yes, and this is one of the most valuable pieces of advice in this entire guide. Better insulation means lower heat loss, which means a smaller heat pump, which means lower equipment cost and better efficiency. A £600 loft insulation top-up might allow you to install a £2,000 cheaper heat pump while also reducing running costs. Assess insulation upgrades before committing to heat pump size.

Q: My neighbour has a 10 kW heat pump and their house is similar to mine. Should I get the same?

A: Not necessarily. "Similar" houses can have very different heat loss characteristics. Your neighbour might have cavity wall insulation; you might have solid walls. They might have modern double glazing; you might have original 1970s units. Always get your own heat loss calculation.

Q: What if my installer says calculations aren't necessary?

A: Find a different installer. Heat loss calculations are mandatory under MCS certification. An installer who skips them isn't MCS-certified or isn't following the rules—either way, you won't qualify for the BUS grant, and you're at risk of a poorly sized system.

Q: How long does a heat loss calculation take?

A: A thorough assessment takes 1-2 hours. The installer measures rooms, examines construction, checks insulation, and records window details. Some use laser measurers and thermal cameras. If an installer claims to have calculated your heat loss in 15 minutes, be sceptical.

The Bottom Line

Heat pump sizing isn't guesswork—it's engineering. It requires professional heat loss calculation specific to your home, performed by someone who understands building physics and heat pump operation.

Insist on seeing the calculation from your installer. It should list every room, the U-values used for each element, and the resulting heat loss. This documentation is required for MCS certification and BUS grant eligibility anyway, so any reputable installer will provide it.

Get your home as efficient as possible first. Top up loft insulation to 300mm. Fill cavity walls if possible. Draught-proof windows and doors. Replace single glazing. Each improvement reduces your heat loss, allowing a smaller, cheaper, more efficient heat pump.

Then size the heat pump to match your improved home. This approach minimises capital cost, maximises efficiency, and ensures you'll be comfortable for decades to come.

A properly sized heat pump, installed in a well-prepared home, is one of the best investments you can make. It protects you from rising gas prices, reduces your carbon footprint, and provides reliable, comfortable heating with running costs 30-50% lower than a gas boiler. But all of that depends on getting the size right from the start.