A heat pump is most efficient when the heating water is cool. Hydronic underfloor heating (UFH) can heat a room with that cool water, because the whole floor is the radiator. That is the pairing. It only works if the floor was designed for the flow temperature the heat pump will actually run at.
This article is about wet UFH: warm water in pipes in the floor. Electric mats are a different product. They turn electricity straight into heat, so they do not get the heat pump’s advantage, and they are a poor way to heat a whole house.
Why the temperatures match
A heat pump does not make heat by burning fuel. It lifts heat from outside air or the ground up to the temperature of the heating water. The bigger that lift, the harder the compressor works, and the worse the efficiency (COP). A system designed around 35–45 °C flow will use less electricity per unit of heat than the same heat pump pushed to 55–60 °C for undersized radiators.
A traditional radiator circuit is often designed at 70–80 °C. The panels are small, so the water has to be hot to get enough heat into the room. A floor is a very large emitter. Typical hydronic UFH runs at about 35–45 °C flow, sometimes up to about 55 °C if the floor construction or the covering needs it. BS EN 1264 limits the floor surface in an occupied room to about 29 °C (often nearer 27 °C under timber; bathrooms can be a little higher). You feel even warmth from a large, slightly warm surface, not a blast from a hot panel.
That low flow is also why the same floor suits a condensing boiler. The heat pump case is stronger: every degree you can knock off the flow temperature improves the seasonal efficiency, not only the condensing margin.
The floor has to meet the heat loss
“UFH works at 35 °C” is only true if, at 35 °C, the floor still puts out enough watts for that room on the design day. Output depends on pipe spacing, floor build-up, insulation under the pipes, and the covering. A tiled screed at close centres can cover a well-insulated room at a low flow temperature. The same room with carpet, wide pipe centres, or missing insulation underneath may not — and the heat pump then has to run hotter, which is the thing you were trying to avoid.
Do the heat-loss calculation room by room, then check the UFH maker’s output tables at the flow temperature you intend to use. A new, well-insulated ground floor usually has a low enough heat loss that screed UFH copes easily. An older room, a conservatory, or a floor with thick carpet often does not, and needs either fabric improvements, a different emitter, or a higher design flow.
Design points that matter with a heat pump
- Pipe and centres. Screeded systems usually use 16 mm pipe. For a heat pump, centres are typically 100–150 mm so the floor can deliver the heat loss without raising the flow temperature. Wider centres (200 mm and above) are a common boiler-era layout and often run short of output at 35–40 °C.
- Loop length. Keep each loop inside the pipe manufacturer’s limit — often around 80–100 m for 16 mm — so the pump can still get flow through the circuit.
- Insulation under the pipes. Heat only goes upwards if it is stopped from going down. On a ground floor that means insulation under the heating layer. On an intermediate floor it stops the ceiling below becoming an accidental radiator.
- No unnecessary blending valve. When UFH shares a hot boiler with radiators, a mixing valve holds the floor at a safe temperature. A heat pump already produces that temperature. Blending it down further throws away the low flow you paid for and adds a pump. On a heat-pump-only system, connect the manifolds direct (with hydraulic separation if the heat pump needs a minimum flow rate).
- Low-profile and joist systems. Overlay boards and between-joist plates are the practical retrofit where a new screed is impossible. They usually put out less heat per square metre than a screed, so the output check matters more, not less.
Floor finishes
Tile, stone, and vinyl let heat through. Engineered wood can work if it is rated for UFH and kept inside the board maker’s maximum surface temperature. Carpet and underlay together should stay inside the limit the system was designed for. Industry guidance is typically a combined 2.5 tog ceiling; with a heat pump, keep it nearer 1.5 tog. Felt underlay is a poor choice. If the covering is already chosen and it is thick, design the flow temperature and the pipe spacing around that covering — do not assume a tile output figure.
Controls
Screed UFH is slow. It suits a house that is kept warm, with weather compensation trimming the flow temperature as the weather changes, and only a small setback overnight. A deep setback feels thrifty and then the floor is still catching up at breakfast, so the heat pump spends the morning at a higher output.
Room thermostats and manifold actuators are fine for stopping a spare room overheating. They should not short-cycle the heat pump. Leave enough loops open, or provide a buffer or low-loss header, so the unit can run for a sensible length of time and still meet its minimum flow rate. On/off control of the whole house, the way an old boiler stat works, fights both the floor’s thermal mass and the heat pump.
An existing UFH system
Older UFH was often designed for a boiler: blended down from 70 °C, 200 mm centres, carpet, and sometimes thin or missing insulation. It can still be a good emitter, but it is not automatically “heat pump ready” because water happens to run through the floor. Before swapping the heat source, get the original layout if you can (centres, loop lengths, insulation, covering) and compare output at 35–45 °C with the room heat loss. Where it falls short, the fixes are practical: a more conductive covering, closer control of unused zones, fabric improvements, or accepting a higher flow temperature and the COP that comes with it.
UFH downstairs, radiators upstairs
That mix is common, and it works with a heat pump only if both emitters are happy at the same flow temperature. A mixing valve can make the floor cooler than the radiators. It cannot make the radiators hotter than the heat pump. If the heat pump is running at 40–45 °C for the floor, upstairs panels see that same water. Panels sized for 75 °C will underheat. Size them for the design flow (or use a correction factor on the catalogue output), or keep a higher-temperature source for those rooms, as on a hybrid system.
See underfloor heating vs radiators for when a mixed layout is the right call, including timber upper floors and rooms that are heated in short bursts.
Related
- Heat pumps
- Underfloor heating vs radiators
- Air source heat pumps
- Heating system output calculator
- Radiator correction factor
Important: Information is guidance only. Design UFH to BS EN 1264 and the manufacturer’s instructions. Size the heat pump and the emitters from a room-by-room heat loss. Treat the system water to BS 7593.