What size heat pump do you need for underfloor heating?

Heat pump sizing matters more than boiler sizing ever did. A gas boiler is almost always oversized and gets away with it. A heat pump delivers a lower, steadier output, costs more per kW, and loses efficiency when it is pushed to high flow temperatures. Get the size right and underfloor heating is the best emitter a heat pump can have. Get it wrong and you pay for capacity you never use, or shiver in a cold snap.

This guide explains how an MCS installer sizes a heat pump, why underfloor heating changes the numbers, what the calculation looks like for a typical house, and what the sizing decision means for your manifold, pump set and pipe layout. For the system design side, see our guide to underfloor heating with a heat pump.

How is a heat pump sized for underfloor heating?

By a full heat loss calculation, done room by room, then matched against the heat pump's output at your design flow temperature. The MCS installation standard for heat pumps, MIS 3005-D, requires the heat loss to be calculated to BS EN 12831, using set indoor design temperatures and an outdoor design temperature taken from location tables. The heat pump must then supply at least 100% of that calculated heat loss at the flow temperature it operates at, without leaning on a supplementary electric heater.

1. Room-by-room heat loss. Every room's walls, floor, roof, glazing and ventilation losses are added up in watts, at the design indoor temperature for that room type. Our guide to heat loss calculations walks through the sums.
2. Design outdoor temperature. The calculation assumes a cold design day for your location, taken from the standard's tables, so the system copes with a hard winter, not an average one.
3. Emitter check. The installer confirms the underfloor heating can give out each room's heat loss at the chosen flow temperature. Pipe centres, floor finish and floor construction all feed into this.
4. Heat pump selection. A unit is chosen whose output at the design outdoor temperature and design flow temperature covers the whole load, using the manufacturer's performance data, not the headline badge rating.

That last point catches people out. A heat pump badged 8kW does not deliver 8kW in all conditions. Output falls as the outside air gets colder and as the flow temperature rises, so the selection must come from the manufacturer's tables at your actual design point. An air source unit also has to hold the indoor design temperatures through its defrost cycles.

What size heat pump does a typical house need?

There is no shortcut that replaces the calculation, but the arithmetic is simple once you have a heat loss figure. Divide the whole-house heat loss in watts by 1,000 and you have the minimum heat pump output in kW at design conditions. A house with a calculated heat loss of 7,500W needs at least 7.5kW of output on the design cold day; a well-insulated house at 4,000W needs 4kW.

For a sense of the market, Which? reports that the average air source heat pump installed under the Boiler Upgrade Scheme in October to December 2025 was 8.4kW, at an average installed cost of £13431 before the grant. Your house may need far less or more, which is exactly why the standard insists on the calculation.

Be wary of two common shortcuts:

• Sizing from the old boiler. Boilers are routinely oversized, so copying the boiler's kW rating usually buys a heat pump bigger and dearer than the house needs.
• Sizing from floor area alone. Two houses of the same size can have wildly different heat losses depending on insulation, glazing and air tightness. Watts per m² is an output check for the floor, not a sizing method for the heat pump.

Why does underfloor heating change the sizing?

Because it lets the heat pump run cool. One heat pump maker's guidance puts typical heat pump flow temperatures at 35 to 45°C and notes that underfloor heating operates efficiently at 30 to 40°C, which is ideal for a heat pump. Above roughly 50°C the compressor works harder, electricity use rises and seasonal efficiency falls. A large floor area emitting gently at low temperature is the opposite of a small radiator demanding 70°C.

Low flow temperature does two things for sizing:

• More output from the same unit. Heat pump output tables give higher kW figures at lower flow temperatures, so a unit that struggles at 55°C may cover the load comfortably at 40°C.
• Better efficiency all year. A typical air source heat pump produces around three units of heat per unit of electricity, and the seasonal figure improves as the flow temperature drops. That is running cost, not capacity, but it is the main reason to design the floor for the lowest workable temperature.

The floor has to be designed for it, though. At 35 to 40°C flow, output per m² is modest, so pipe centres of 100 to 150mm, decent floor insulation and low-tog floor coverings all matter. Our guide to UFH flow temperatures covers the trade-offs, and the heat pump ready checklist lists what to get right before the unit is ordered.

What happens if the heat pump is too big or too small?

Undersizing is the clear-cut one: MIS 3005-D does not allow a heat-pump-only system to be designed below 100% of the calculated heat loss, and any backup electric heater must not operate above the design outdoor temperature. A unit that cannot hold the house at design conditions fails the standard and will show up as cold rooms in a cold spell, or as an immersion-style backup heater quietly running up your electricity bill.

Oversizing wastes money twice. The unit itself costs more, and so can the electrical supply, pipework and buffer arrangements around it. CIBSE's analysis of heat pump sizing makes the underlying point: peak heating capacity is only needed for a few hours a year, so capacity bought for the worst hour sits idle almost all the time. A correctly sized unit spends more of the winter modulating smoothly in its efficient range.

If the calculation lands between two models in a range, most designers take the one that clears the design load with the smaller margin, then check it against the manufacturer's output table at the design point rather than rounding up by habit.

Do you need a buffer tank or a special manifold?

You need system volume and flow that match the heat pump maker's requirements, and underfloor heating is often the easiest way to provide both. A heat pump usually feeds UFH directly, without a mixing valve, because its flow temperature is already floor-safe. A non-mixing pump set gives the manifold its own circulation without blending, and an auto-balancing manifold keeps loop flows steady as zones open and close, which suits a heat pump's preference for stable flow.

Whether you need a buffer tank depends on the heat pump model, the zoning and the open system volume, and it is the installer's call against the manufacturer's data. Our guide to buffer tanks with heat pumps explains when one earns its place.

Who does the calculation, and what about the grant?

For the £7500 Boiler Upgrade Scheme grant in England and Wales, the installer must be MCS certified, and the MCS process includes the full heat loss calculation and sizing to the standard. The grant covers air source and ground source heat pumps at up to £7500 each, the system must fully replace an existing fossil fuel or electric heating system, the property needs a valid EPC, and the installer applies for the grant and takes it off your invoice. The scheme is currently open to 30 December 2029.

One VAT point to be clear about: 0% VAT on energy-saving materials applies only where a qualifying installer supplies and installs the measure, and only for qualifying measures, under rules that currently run to 31 March 2027. Buying kit supply-only from any merchant, including MaxHeatUFH, carries 20% VAT. Check gov.uk for the current rules. Our guide to UFH grants and VAT has the detail.

MaxHeatUFH supplies the underfloor heating side and can pass your project to a vetted approved installer for the install itself: use the installation enquiry form.

Frequently asked questions

Can I size a heat pump from my boiler size?

No. Boilers are usually oversized, so copying the boiler rating tends to buy too big a heat pump. MCS requires a room-by-room heat loss calculation to BS EN 12831, and the heat pump is sized from that at your design flow temperature.

What is the most common heat pump size in the UK?

The average air source unit installed under the Boiler Upgrade Scheme in late 2025 was 8.4kW, but that is an average, not a recommendation. A well-insulated house can need half that; a large or leaky one can need more.

Does underfloor heating mean I can fit a smaller heat pump?

Not smaller than the heat loss, which is fixed by the building, but UFH lets the unit deliver its output at a lower flow temperature, where heat pumps give more kW and better efficiency. The same house may need a larger, harder-worked unit on high-temperature radiators.

What flow temperature should underfloor heating run on a heat pump?

Underfloor heating operates efficiently at around 30 to 40°C flow, which sits inside the 35 to 45°C band where heat pumps perform best. The exact design figure comes from the heat loss and the floor's output.

Is it better to oversize a heat pump to be safe?

No. The standard requires 100% of the calculated heat loss to be covered, and going well beyond that buys capacity used for only a few hours a year. Size to the calculation and check the manufacturer's output table at the design point.

Do I need an MCS installer?

For the Boiler Upgrade Scheme grant, yes: the installer must be MCS certified and applies for the grant on your behalf. Heat pump system design and commissioning belong with the MCS installer either way; MaxHeatUFH supplies the UFH kit and does not install.

Does the heat pump size change my manifold or pipe?

The heat loss drives both. Low flow temperatures push the design towards closer pipe centres and enough loops per room, and the manifold needs a port per loop. Design your rooms in MaxHeatUFH ProLoop to see the loops, ports and kit for your rooms.

Sources: MCS MIS 3005-D clause 5.5 (heat pump sizing), GOV.UK Boiler Upgrade Scheme, Which? air source heat pump costs