Underfloor heating and radiators on the same system (UFH downstairs, rads upstairs)

Very few homes are all underfloor heating or all radiators. A ground floor extension gets UFH while the rest of the house keeps its radiators. A new build has a screeded ground floor and timber bedrooms upstairs. A bathroom gets a warm floor and a towel rail. In every case the two types of emitter have to share one heat source, even though they want water at different temperatures.

This guide explains how a mixed system is plumbed with a boiler and with a heat pump, what temperatures each part runs at, how to zone and wire it, and the common mistakes. For the plumbing details on boilers, see UFH with a combi or system boiler.

Can you mix underfloor heating and radiators?

Yes. The two run side by side as separate circuits from the same heat source. Radiators take water at the heat source's flow temperature, and the UFH circuit blends that down to what the floor needs.

The reason you cannot just put them on one circuit is temperature. Radiators are sized to work at a set flow temperature, often 45 to 55°C on a modern or heat pump system and hotter on older boiler systems. A screed should not see supply water above 55°C, and UFH under a heat pump is often designed around 35 to 40°C. A floor at radiator temperature would overheat its surface and its finish.

Emitter	Typical design flow	Limit to know
Radiators, new or replacement system	55°C or lower	Part L design maximum since June 2022
Radiators, heat pump	45 to 55°C	Larger radiators at lower temperatures
UFH in screed, heat pump	35 to 40°C	Supply max 55°C for cement or calcium sulphate screed
Floor surface	Set by the room	29°C in occupied areas under BS EN 1264

How do you plumb UFH downstairs and radiators upstairs?

Take two separate flow and return pairs off the boiler primaries: one to the radiators, one to the UFH manifold. Each pair gets its own zone valve, and the UFH pair goes through a blending valve and pump at the manifold.

1. Radiator circuit: flow and return to the upstairs radiators, controlled by a zone valve and room thermostat, with TRVs on the radiators.
2. UFH circuit: flow and return to the downstairs manifold, controlled by its own zone valve. On a combi, the UFH can instead take a tee off the heating flow and return.
3. Blending valve and pump: fitted on the manifold. Our blending valve pump pack sets the floor flow between 20 and 55°C.
4. Manifold actuators: one per loop, opened by the room thermostats through a wiring centre.
5. Boiler interlock: the zone valve end switches tell the boiler when either circuit needs heat.

The full component list for the manifold side is in connecting a UFH manifold. If the radiators already take the boiler's full output, the heating engineer should check the boiler and pump can cover the extra load. Any work on the boiler must be done by a Gas Safe engineer.

How does a mixed system work with a heat pump?

There are two ways. Either size every emitter, radiators included, to work at one low flow temperature and run the UFH without mixing, or separate the circuits and blend the UFH down from a warmer radiator flow.

Approach	How it works	Trade-off
One low flow temperature for everything	Radiators oversized so they work at the UFH temperature, UFH on a non-mixing pump set	Bigger radiators, best efficiency
Two temperatures	Low loss header or buffer, blending valve on the UFH, radiators at the higher temperature	Simpler radiators, heat pump runs warmer, more commissioning

A heat pump works more efficiently at lower flow temperatures, and the room that needs the hottest water sets the flow temperature for the whole system. If one room's radiator needs 50°C, the heat pump runs at 50°C even when the UFH only needs 35°C. The MCS heat emitter guide also tells designers to add 5°C to the heat pump flow temperature when a blending valve is added.

That is why the MCS guide asks how much bigger each radiator must be at a given flow temperature. Multiply the room heat loss by the oversize factor to get the radiator output you need at the standard rating.

Flow temperature	Radiator oversize factor
36 to 40°C	4.3
41 to 45°C	3.1
46 to 50°C	2.4
51 to 55°C	1.9

So at 45°C a bedroom losing 600W needs a radiator rated at about 1,860W. If that will not fit, the two-temperature approach is usually the answer. Where every emitter can run cool, a non-mixing pump set circulates the UFH without a blending valve. Our guides to non-mixing pump sets and UFH with a heat pump go further. Heat pump systems are best designed by an MCS-certified installer.

Do UFH and radiators need separate thermostats?

Yes. Each zone needs its own time and temperature control, and on new and replacement systems Part L expects each room to have thermostatic control. In practice that means:

• UFH rooms: a room thermostat in each room or zone, switching that room's actuators through a wiring centre, such as the Heatmiser UH8.
• Radiator zone: a programmer and room thermostat for the zone, plus TRVs on every radiator except in the room with the thermostat.
• Interlock: the boiler and pump must switch off when no zone is calling for heat. TRVs on their own do not give interlock.

Give the two systems different time programmes. A screeded floor responds slowly, so it works best on a steady setting with small setbacks, while radiators heat bedrooms quickly for the hours they are used. Our guides to UFH zoning and wiring and UFH warm-up times explain how to set them.

Does adding UFH make a boiler more efficient?

It can help, because a condensing boiler only condenses, and gets its rated efficiency, when the water coming back to it is below about 55°C. UFH returns are cool. But the boiler sees the mixed return of the whole system, so radiators running hot can still keep it out of condensing.

The bigger gain is lowering the radiator flow temperature too. A system designed and commissioned to a 55°C flow temperature, as Part L now requires for new and replaced systems, should return below 55°C. If your radiators are big enough to keep the house warm at a lower flow temperature, turn it down.

What are the common mistakes with hybrid UFH and radiator systems?

Most problems come from treating the floor like another radiator. Avoid these:

• No mixing on a hot boiler circuit. The floor overheats, screeds crack and wood finishes move.
• UFH on the radiator zone's thermostat. The floor then only heats when the radiator room calls, and runs on the wrong schedule.
• No boiler interlock. The boiler cycles with nothing calling, which wastes fuel and breaks Part L.
• Heat pump flow set by one small radiator. One undersized radiator can push the whole system's flow temperature up.
• Unbalanced loops. Long loops starve while short ones overheat. See balancing UFH loops.

Frequently asked questions

Can I have underfloor heating downstairs and radiators upstairs?

Yes. It is a very common layout. The UFH has its own manifold with a blending valve downstairs, and the radiators stay on their own zone upstairs.

Do I need a mixing valve if I have UFH and radiators?

With a boiler, almost always, because the radiators need hotter water than the floor. With a heat pump, not if every radiator is sized to run at the floor's flow temperature.

Can UFH and radiators share one thermostat?

They should not. They heat at different speeds, so they need separate zones, each with its own time and temperature control.

Can I add underfloor heating to an existing radiator system?

Yes. A new branch from the flow and return feeds a manifold or a single-room pack. Check the boiler has enough spare output for the new area.

What flow temperature should radiators run at with a heat pump?

As low as keeps the house warm, often 45 to 55°C. Lower temperatures need bigger radiators, as the MCS oversize factors show.

Do I need a low loss header with UFH and radiators?

Not always. Boiler systems often use zone valves and a blending valve. Heat pump systems that run two temperatures commonly use a low loss header or buffer to separate the circuits.

Sources: Approved Document L Volume 1 (paras 5.10, 5.20), BEAMA Part L 2022 heating controls guidance