Heat pump size is determined by your building’s calculated heat loss, not by its floor area. Two houses of identical size can differ by several kilowatts depending on insulation, glazing, ceiling height, and orientation, which is why floor area estimates are unreliable.
This guide covers how sizing actually works, why the minimum output matters as much as the maximum, what happens when you get it wrong in either direction, and how to work out roughly where your building sits before commissioning a proper calculation.
Why Floor Area Does Not Work
The rule of thumb people reach for is watts per square metre, and it fails because it treats every building the same.
A well-insulated modern house might need 30 to 50 W/m². A poorly insulated older property with single glazing might need well over 100 W/m² for the same floor plan. That is a difference of two or three times, and it comes entirely from the building rather than its size.
What actually determines heat loss:
| Factor | Effect |
|---|---|
| Insulation standard | The largest single variable, walls, roof, and floor |
| Glazing area and type | Single glazing loses several times what triple loses |
| Air tightness | Draughts replace warm air with cold continuously |
| Ceiling height | Volume to heat, not just floor to cover |
| Exposed surfaces | A detached house loses more than a mid-terrace |
| Local design temperature | The coldest condition the system must cover |
| Hot water demand | Adds to capacity requirement, often overlooked |
Improve the building before sizing the equipment. Insulation and draught sealing reduce heat demand permanently, which means the heat pump can be smaller, cheaper to buy, and cheaper to run. It also lowers the flow temperature the emitters need, which improves efficiency directly. Doing it afterwards leaves you with a system sized for a building that no longer exists, oversized and cycling for the rest of its service life.
The Figure Everyone Misses: Minimum Output
Buyers focus on maximum capacity, because that is the number in the product name. The minimum modulated output matters at least as much, and getting it wrong causes a problem that persists all winter.
A modern heat pump modulates: it varies its output to match demand rather than switching fully on and off. But it can only throttle down so far. Below that floor, it must produce more heat than the building needs, reach the set point, shut off, and restart.
That cycling is the problem. It reduces seasonal efficiency, because a compressor starting and stopping never settles into efficient operation, and it accelerates wear, because every start places mechanical and electrical stress on it.
And it happens during mild weather, which is most of the heating season. A unit sized for the coldest week of the year spends the other twenty-odd weeks unable to throttle low enough.
| Capacity | Modulation range | Cannot go below |
|---|---|---|
| 5 kW | 2.0 to 6.0 kW | 2.0 kW |
| 6 kW | 3.0 to 8.0 kW | 3.0 kW |
| 9 kW | 3.5 to 10.0 kW | 3.5 kW |
| 16 kW | 6.5 to 18.0 kW | 6.5 kW |
Read that last row carefully. A 16 kW unit cannot produce less than 6.5 kW. In a well-insulated home needing 3 kW on a mild spring day, it would produce more than twice what is required and cycle continuously. Its minimum output alone exceeds the entire design load of many houses.
What Goes Wrong in Each Direction
Undersized
The unit runs continuously and still fails to reach temperature on the coldest days. Rooms stay below target when it matters most, and any backup heating carries the shortfall at whatever that costs to run.
Undersizing is the more visible failure, because occupants feel it immediately.
Oversized
The unit reaches the set point quickly, shuts down, and restarts. Seasonal efficiency falls, compressor wear accelerates, and comfort is worse rather than better because temperature swings around the set point instead of holding steady.
Oversizing is the more common failure, because it feels like caution. It is not. With modulating equipment, a margin is not a safety feature; it is a permanent inefficiency that shows up on every bill for the life of the system.
The instinct to “go one size up to be sure” is wrong here. With a boiler it was fairly harmless, since a boiler cycling is inefficient but tolerable. With a modulating heat pump it means the unit spends most of the heating season outside its efficient range. Size to the calculated load and let the modulation handle variation, which is what it exists for.
A Rough Estimate Before the Proper Calculation
A heat loss calculation by a qualified installer is the only reliable basis for a decision. But a rough estimate helps you know roughly what to expect and whether a quotation is plausible.
Multiply your heated floor area by an approximate figure for your building type:
| Building standard | Approximate W/m² | Typical of |
|---|---|---|
| Very well insulated | 25 to 40 | New build to current standards |
| Well insulated | 40 to 60 | Modern or well-renovated property |
| Moderately insulated | 60 to 90 | Typical older property with upgrades |
| Poorly insulated | 90 to 130+ | Older property, original glazing |
A 120 m² well-insulated house at 50 W/m² gives roughly 6 kW. The same house poorly insulated at 110 W/m² gives roughly 13 kW.
That gap illustrates the point better than any argument: the building matters more than its size. It also shows why insulation improvements change which unit you need rather than merely reducing bills.
Treat these figures as a sanity check on a quotation, not as a specification. Local climate alone shifts them considerably.
Do Not Forget Hot Water
A frequently overlooked part of the requirement.
If the heat pump will also supply domestic hot water, that demand adds to the capacity needed. A household with several bathrooms in simultaneous morning use places a meaningful load on the system that a space heating calculation alone does not capture.
| Capacity | Hot water output | Water yield | Suits |
|---|---|---|---|
| 5 kW | 6 kW | 129 L/h | One bathroom, modest demand |
| 6 kW | 8 kW | 172 L/h | Two bathrooms in regular use |
| 9 kW | 10 kW | 215 L/h | Larger family, several outlets |
| 16 kW | 18 kW | 387 L/h | Large household, guest house, commercial |
In some households, hot water demand rather than space heating decides the capacity. A compact but well-occupied home with two bathrooms may need the 6 kW unit for its 172 litres per hour even though its space heating load would suit the 5 kW.
The Emitters Change the Answer Too
Sizing does not stop at capacity. What the unit will feed determines the flow temperature required, and that determines the efficiency you actually achieve.
Underfloor heating operates at 30 to 45°C. Radiators sized for a gas boiler expect 70 to 80°C. A heat pump becomes markedly more efficient the lower the flow temperature it must produce, and the difference is substantial:
| Capacity | COP at 35°C flow | COP at 45°C flow |
|---|---|---|
| 5 kW | 4.5 | 3.6 |
| 6 kW | 4.42 | 3.43 |
| 9 kW | 4.46 | 3.45 |
| 16 kW | 4.53 | 3.65 |
That is roughly 25 to 29% more heat from the same electricity, from an identical machine, determined entirely by what it is feeding.
If your emitters demand high flow temperature, the answer is usually oversizing the radiators or improving insulation rather than buying a larger heat pump. A bigger unit producing hot water is still producing hot water inefficiently.
Cold Climate Sizing
Output declines as ambient temperature falls, which is unavoidable physics for any air source heat pump rather than a product limitation.
The practical consequence is that a unit’s nominal rating is measured at standard test conditions, typically 7°C outdoor air. At your local design temperature, whatever that is, it will deliver less.
Cold climate projects should therefore be sized against the local design temperature rather than the nominal figure. Legom units are engineered to operate down to -35°C, which covers the great majority of populated cold regions, but the capacity available at -15°C is not the capacity available at 7°C, and the design should reflect that.
In the most extreme locations, a supplementary heat source or a hybrid arrangement covering the coldest days allows the heat pump to be sized for typical rather than peak demand, which reduces its cost and avoids oversizing for the rest of the season.
Matching Capacity to Building
As a starting point, though the calculation always takes precedence:
| Model | Calculated load | Typically suits | Also note |
|---|---|---|---|
| 5 kW | Around 3 to 5 kW | Small to medium homes, 2 to 3 bedrooms | Quietest at ≤52 dB(A), lightest at 60 kg |
| 6 kW | Around 4 to 7 kW | Medium homes, higher hot water demand | 33% more hot water than the 5 kW |
| 9 kW | Around 6 to 9 kW | Medium to large homes, older properties | Same cabinet as 6 kW, steps up to DN25 ports |
| 16 kW | Around 10 to 16 kW | Large homes, light commercial | Highest COP in range, twin fan, 5 L vessel included |
Two practical notes from these. The 9 kW steps up to G1.0 DN25 water connections while the 5 kW and 6 kW use G3/4 DN20, so if the capacity might be revised upward during design, confirm pipework early. And the 16 kW achieves the highest COP in the range at 4.53, so stepping up in capacity does not cost efficiency where the load genuinely calls for it.
Questions to Ask Before Committing
What is the calculated heat loss, room by room? Based on a proper calculation rather than floor area. Ask to see it.
What flow temperature will the system need? This single number determines much of the efficiency you will achieve.
What is the minimum modulated output of the proposed unit? Compare it against your mild-weather demand, not just your peak.
Has hot water demand been included? Or was the calculation space heating only?
What would change if insulation were improved first? If the answer is a smaller and cheaper system, the sequence deserves reconsidering.
An installer who answers these clearly is engaging with your building. One who quotes a capacity without them is selling a product.
“The instinct to go one size up nearly always makes things worse. With a boiler it did not matter much, and people carry that habit over. With a modulating heat pump, the size above yours cannot throttle down to what your house needs on a mild day, so it cycles instead of running steadily, and mild days are most of the winter. The number I would tell people to ask about is the minimum output rather than the maximum. Look at the sixteen kilowatt unit: it cannot go below six and a half. That alone is more than some houses need at their coldest.”
— Maggie Shen, Director of Legom
The Legom Range
Legom manufactures air-to-water monoblock heat pumps in four capacities at our facility in Jiaxing, Zhejiang Province, supplied to partners in more than 90 countries.
All models use R32 refrigerant with full DC inverter twin rotary compressors, provide reversible heating and cooling, carry an ERP rating of A+++ at 35°C, operate down to -35°C, and hold CE and RoHS certification.
Because we also manufacture the manifolds, floor heating pipe, thermal actuators, and room thermostats that complete a hydronic system, a full installation can be sourced from one manufacturer with compatibility designed in.
See the full heat pump range, or OEM and ODM services for supply under your own brand. Contact the technical team to discuss specification for a project.
Frequently Asked Questions
What size heat pump do I need for my house?
It depends on your building’s calculated heat loss rather than its floor area, since insulation, glazing, air tightness, ceiling height, and local climate all change the answer substantially. Two houses of identical size can differ by two or three times. As a rough guide, a well-insulated property might need 40 to 60 W/m² while a poorly insulated one might need over 100 W/m². Use that only as a sanity check on a quotation, and have a qualified installer perform a proper room-by-room calculation.
Is it better to oversize a heat pump to be safe?
No, and this is the most common sizing error. With modulating equipment a margin is not a safety feature but a permanent inefficiency. An oversized unit cannot throttle down to what the building needs on mild days, so it reaches the set point, shuts off, and restarts repeatedly. That cycling reduces seasonal efficiency and accelerates compressor wear, and it happens during mild weather, which is most of the heating season. Size to the calculated load and let modulation handle variation.
What is minimum modulated output and why does it matter?
It is the lowest heat output a unit can produce without switching off, and it matters as much as the maximum. A 16 kW unit that cannot go below 6.5 kW will cycle badly in a house needing 3 kW on a spring day, and mild weather makes up most of the heating season. Buyers focus on maximum capacity because that is the number in the product name, but the minimum determines how the unit behaves for most of the year.
Does hot water demand affect what size I need?
Yes, and it is frequently omitted from sizing calculations. If the heat pump will supply domestic hot water as well as space heating, that demand adds to the requirement. A household with two bathrooms in simultaneous morning use places a real load on the system. In some homes hot water rather than space heating decides the capacity: a compact but well-occupied property may need a larger unit for its hot water output even though its heating load would suit a smaller one.
How do my radiators affect the sizing?
They determine the flow temperature required, which determines the efficiency achieved. Radiators sized for a gas boiler expect 70 to 80°C, and a heat pump producing that operates in its least efficient range: a unit achieving a COP of 4.5 at 35°C flow achieves around 3.6 at 45°C, and less again at higher temperatures. The answer is usually oversizing the radiators or improving insulation rather than buying a larger heat pump, since a bigger unit still produces hot water inefficiently.
Should I size for the coldest day of the year?
Size against your local design temperature, which is the standard cold condition for your area rather than the most extreme temperature ever recorded. Output declines as ambient temperature falls, so a unit’s nominal rating measured at 7°C does not reflect what it delivers at -15°C. In very cold locations, a hybrid arrangement covering the coldest days lets the heat pump be sized for typical demand, which reduces cost and avoids oversizing for the rest of the season.
Should I improve insulation before or after installing?
Before, and the reason is practical. Insulation reduces heat demand permanently, so the heat pump can be smaller, cheaper to buy, and cheaper to run. It also lowers the flow temperature the emitters need, which improves efficiency directly. Doing it afterwards leaves a system sized for a building that no longer exists, oversized and cycling for the rest of its service life. In many projects, insulation changes which capacity you need rather than merely reducing bills.
What should I ask an installer about sizing?
Five things. What is the calculated heat loss room by room, based on a proper calculation rather than floor area. What flow temperature will the system need given the existing emitters. What is the minimum modulated output of the proposed unit, compared against mild-weather demand. Has hot water demand been included or was the calculation space heating only. And what would change if insulation were improved first. Clear answers indicate an installer engaging with your building.
Reviewed by Maggie Shen, Director at Legom, on September 11, 2026. This heat pump sizing guide was reviewed for technical accuracy, including verified modulation ranges and COP figures across the capacity range and the consequences of oversizing modulating equipment.