Underfloor heating with a combi boiler: does it work and how to connect it

Most UK homes have a combi boiler, and many people adding underfloor heating to an extension or a whole floor assume they need to change it. Usually you don't. A combi heats a sealed central heating circuit in the same way as a system boiler. The UFH manifold connects to that circuit, gets its own pump and temperature control, and tells the boiler when it needs heat.

This guide covers how the combi set-up differs from S plan and Y plan systems, the three common pipework layouts, how the controls are wired, what to set the boiler to and the mistakes that cause trouble later. For system and regular boilers too, see our wider guide to underfloor heating with a combi or system boiler.

Does underfloor heating work with a combi boiler?

Yes. There is no reason a combi cannot heat a UFH floor, as long as the floor gets water at its own lower temperature and the boiler is told when to run. You need three things:

• Temperature control at the manifold. A boiler sends out water hotter than a floor should take, so a blending valve mixes it down before it reaches the loops.
• A pump for the floor. The UFH pump pack moves water round the loops, separate from the boiler's own pump.
• A heat demand link. The wiring centre switches the boiler on when any room calls and off when none do.

The one difference from a system boiler is hot water. A combi makes hot water on demand and gives it priority, so it provides either hot water or heating, never both at once. While a shower runs, the floor gets no new heat. A screed floor stores so much heat that this rarely shows in the room.

What is the difference between S plan, Y plan and a combi system?

S plan and Y plan describe how a system or regular boiler shares its heat between the radiators and a hot water cylinder. A combi has no cylinder, so neither plan applies in full. You only zone the heating.

Layout	Valves	Used with
S plan	One 2-port zone valve for hot water and one for each heating zone	System or regular boiler and cylinder
Y plan	One 3-port mid-position valve for heating and hot water	System or regular boiler and cylinder
W plan	3-port diverter, hot water priority	Older boiler and cylinder
Combi	Diverter inside the boiler; 2-port valves for heating zones	Combi boiler, no cylinder

An S plan is the easiest to extend with UFH, because you add another 2-port valve for the floor zone. A training guide for heating engineers notes that homes over 150m² should have an extra heating zone valve, and that the S plan+ uses three zone valves. A Y plan home can take a UFH zone too, but the floor needs its own zone control, so ask your heating engineer how best to add it to the existing valve.

With a combi, the same idea gives a two-zone heating system: one 2-port valve for the radiators and one for the UFH, both wired back to the boiler. Our 2-port motorised zone valve comes in 22mm and 28mm.

How do I connect underfloor heating to a combi boiler?

Pick the layout that fits the job. All three use the boiler's central heating flow and return, never the hot water outlet.

Job	Layout	What you need
UFH only, no radiators	Boiler heating flow and return straight to the manifold	Manifold, blending valve pump pack, wiring centre
UFH and radiators	Tee off the heating flow and return, zone valve on each branch	As above, plus 2-port zone valves
One small room off a radiator circuit	Single room mixing pack, no manifold	Single room pump pack, room thermostat

For the common radiators-plus-UFH layout:

1. Take the heating flow and return from the boiler side of the radiator circuit, sized for the UFH load. Our guide to primary pipe size for a UFH manifold shows how.
2. Fit a 2-port zone valve on the UFH flow, and one on the radiator flow if the radiators are to be zoned. One boiler maker says a 2-port valve lets the radiator and UFH systems run separately, so the boiler is not trying to serve both at once.
3. Fit the manifold pump pack. A blending valve pump pack holds the floor at 20 to 55°C.
4. Fit isolating ball valves either side of the manifold so it can be serviced without draining the boiler.
5. Check the bypass. Fit an automatic bypass if the boiler maker requires one, so the boiler always has a path for water when the zones close.
6. Fill, purge, test and inhibit the whole system before commissioning.

For a single room, such as a kitchen extension, an ESBE single room mixing pack holds 35 to 60°C to one loop without a manifold. See single room UFH off radiator pipes.

Any work on the boiler itself, its gas supply or its flue must be done by a Gas Safe registered engineer.

How do I wire underfloor heating to a combi boiler?

Each room thermostat goes to a zone on the wiring centre. The wiring centre opens that room's actuators, runs the UFH pump and tells the boiler to fire. When no room calls, the boiler and pump both switch off.

• Thermostats to zones: on a Heatmiser UH8, each of the eight zones has its own actuator output.
• Pump output: the UH8 has a separate UFH pump output. Heatmiser recommends feeding it through a high limit thermostat on the manifold, so the pump stops if the floor water gets too hot.
• Boiler output: the UH8 switches the boiler through a volt-free contact rated at 3A resistive. It connects to the boiler's heating demand terminals, as shown in the boiler's own instructions.
• Zone valves: where 2-port valves are used, each one opens to let boiler water through when its zone calls for heat and closes when the thermostat is satisfied.

Approved Document L calls this boiler interlock. Controls must be wired so that when there is no call for heating or hot water, the boiler and pump switch off. Thermostatic radiator valves on their own do not count. All mains wiring should be done by a qualified electrician. See UFH zoning and wiring centres for full diagrams.

What temperature should I set my combi boiler to for underfloor heating?

Set the boiler heating temperature higher than the blending valve setting, and set the blending valve to the floor's design temperature. A blending valve can only lower the temperature, never raise it.

• UFH only: you can turn the boiler's heating temperature down close to the UFH design temperature. A cooler return helps the boiler condense, which only happens when return water is below about 55°C.
• Radiators and UFH: set the boiler for the radiators and let the blending valve control the floor.
• New systems in England: since 15 June 2022, Approved Document L asks for new and fully replaced heating systems to be sized for a maximum flow temperature of 55°C.

In England, a new or replacement gas combi must also have one of the Boiler Plus measures, in force since 2018: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. Weather and load compensation both lower the boiler flow temperature when less heat is needed.

Is my combi boiler big enough for underfloor heating?

Usually yes. A combi's output is chosen for instant hot water, so it is often larger than the house needs for heating. One heating installer gives about 11 litres a minute of hot water from a 24kW combi and about 17 litres a minute from 40kW.

The bigger risk is a boiler that is too large for a small UFH zone, so it switches on and off in short bursts. A boiler that can turn its output well down, known as modulation, copes better. The same installer contrasts a basic 30kW boiler with three output steps against a better one that steps down to 2kW. Check the boiler's minimum output against the UFH heat load.

What mistakes should I avoid with a combi and UFH?

• No blending valve. Full boiler temperature into a screed or wood floor risks damage and an uncomfortable floor.
• No boiler interlock. If the boiler is not told when the floor is satisfied, it keeps firing and cycling for nothing.
• Connecting to hot water pipes. UFH always connects to the heating circuit, never the domestic hot water.
• No zone valve where radiators share the boiler. Without one, the floor and radiators cannot be controlled separately.
• Skipping the inhibitor. The UFH water is part of the boiler's sealed circuit, so it needs the same inhibitor as the radiators.

Products for this job

• Blending Valve Pump Pack 20 to 55°C, Bastion 6m
• 2-Port Motorised Zone Valve
• Heatmiser UH8 8 Zone Wiring Centre
• ESBE Single Room UFH Mixing Pump Pack

Frequently asked questions

Can I run underfloor heating and radiators off the same combi boiler?

Yes. Tee the UFH off the heating flow and return, fit a 2-port zone valve for each zone, and put a blending valve pump pack on the manifold so the floor runs cooler than the radiators.

Do I need an S plan for underfloor heating with a combi?

Not a full S plan, because there is no cylinder. You use the same 2-port zone valves to split the heating into a radiator zone and a UFH zone.

Will the underfloor heating stop when I run a shower?

The boiler stops heating the circuit while hot water is drawn, because a combi gives hot water priority. A warm floor keeps giving out the heat it has stored, so a short pause makes little difference to the room.

Do I need a separate pump for underfloor heating on a combi?

Yes. The manifold pump pack has its own pump, which moves water round the loops at the floor temperature set by the blending valve.

Can I fit a Y plan valve for underfloor heating?

A 3-port mid-position valve shares heat between radiators and a hot water cylinder, so it is for system or regular boilers. With a combi, use 2-port zone valves.

Who should connect underfloor heating to a combi boiler?

Pipework can be done by a competent plumber, mains wiring by a qualified electrician, and any work on the boiler or gas by a Gas Safe registered engineer. MaxHeatUFH can pass installs to vetted approved installers.

Does underfloor heating make a combi boiler more efficient?

It can help. Underfloor heating returns cooler water, and a condensing boiler recovers more heat when the return is below about 55°C.

Sources: City and Guilds 9189: types of central heating system, BEAMA Part L 2022 heating controls guidance, Heatmiser UH8 wiring centre manual, Homebuilding and Renovating: UFH with combi boilers