Underfloor heating with a combi or system boiler: connections, mixing and controls

Most UK homes that add underfloor heating already have a gas boiler, and you don't need to change it. You need the right water temperature at the floor, a connection that doesn't fight the radiator circuit, and wiring that tells the boiler when the floor needs heat.

This guide covers combi, system and regular boilers, whole floors and single rooms, the controls Part L asks for, and a priced kit for a 30m² extension. If you are fitting a heat pump instead, read underfloor heating with a heat pump.

Can you run underfloor heating off a combi boiler?

Yes. A combi heats a sealed central heating circuit just like a system boiler, so a UFH manifold can take its flow and return from that circuit. There are two things to know before you start.

• Hot water comes first. A combi has a diverter valve that sends heat to the hot tap or to the heating, not both. Vaillant explains that when a shower runs, the valve prioritises the shower over the heating until it is turned off. A screed floor holds so much heat that a 10 minute shower makes little difference to the room.
• Check the heating output. The kW figure in a combi's name is usually its hot water rating. The central heating output is listed separately in the manual and can be lower. Compare that figure, not the headline one, with the heat loss of the house.

If you are replacing a gas combi in an existing home in England, Approved Document L (paragraph 6.2) says you must also fit at least one of: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. Weather compensation suits underfloor heating well, because it lowers the boiler flow temperature in mild weather.

Can a system or regular boiler run underfloor heating?

Yes, and they are often the easier fit for a whole house. A system boiler (sealed, with a hot water cylinder) or a regular boiler (open vented, with a feed and expansion tank) already uses motorised valves to switch heating and hot water, so the floor becomes one more zone.

• S-plan. Each zone has its own 2-port motorised valve. Add one more 2-port valve for the UFH manifold, so the floor can call for heat on its own.
• Y-plan. One 3-port mid-position valve shares heating and hot water. You can tee the UFH manifold into the heating side, or convert to S-plan with 2-port valves for more control.

Watch the boiler temperature. The Energy Saving Trust says a boiler that heats a hot water cylinder needs a flow of at least 65°C to keep the stored water safe from legionella. That is far too hot for a floor, so a cylinder system always needs a blending valve at the manifold. With a regular boiler, follow the boiler maker's open vented layout for adding a pumped circuit, so the feed and vent pipes stay where they need to be.

Do I need a blending valve with a boiler?

In almost every case, yes. Radiators work at the boiler's normal heating temperature, but a floor needs much cooler water. The Danfoss underfloor heating handbook says the supply must not exceed 55°C for cement and calcium sulphate screed and 50°C for gypsum based floors. One flowing screed maker gives 50°C as its maximum running temperature.

A blending valve pump pack bolts onto the manifold. Its thermostatic valve mixes hot boiler water with cooler return water from the floor to give a steady temperature you set between 20 and 55°C, and its pump circulates the loops. The valve needs no wiring; only the pump does. Our guide to mixing and blending valves explains how the valve works and how to set it.

The only time a boiler-fed floor might skip the blending valve is when the boiler serves nothing but the floor and is locked to a low flow temperature. That is rare, and it leaves the floor unprotected if someone turns the boiler up, so most designers fit one anyway.

How do I connect underfloor heating to an existing radiator system?

Connect the UFH pump pack to the boiler primary with closely spaced tees, so the two circuits can share the boiler without pushing water round each other. Worcester Bosch's technical bulletin on this layout gives the key details:

1. Fit the two tees for the UFH flow and return 150mm apart on the primary flow pipe.
2. Make the pipe between the tees one size larger than the primary, to keep the pressure drop low.
3. Keep a straight run of pipe before and after the tees of about 8 times the pipe diameter.
4. Fit a motorised valve on the radiator circuit so primary water can still circulate when only the floor is calling for heat. The bulletin shows a spring-return 3-port valve for this.
5. Fit isolating ball valves on the primary connections to the pump pack, so you can service it without draining down.
6. Wire the wiring centre's boiler output so the boiler fires when any UFH zone calls.

Why it matters: when two pumped circuits share pipework, flow in one can drive flow in the other. The bulletin warns this can cause noise, wasted energy and heat where you didn't ask for it. The closely spaced tees let the UFH pump be sized for the floor alone. The same bulletin says the method applies to combi boilers as well as system boilers. Always check the layout against your own boiler's manual.

Can I run underfloor heating in one room off a radiator?

Yes. A single room mixing pump pack takes its feed from a nearby radiator circuit and runs one UFH loop, with no manifold. This suits a kitchen extension, conservatory or bathroom where the rest of the house stays on radiators.

• The pack mixes the radiator water down to 35 to 60°C. Set it no higher than your floor build-up allows.
• It runs one loop: up to about 100m of 16mm pipe, which covers about 12 to 14m² at 150mm centres. Larger rooms need a manifold.
• The radiator circuit must be live when the floor calls, or the floor gets no heat.

What controls do I need with a boiler?

You need a room thermostat per zone, thermal actuators on the manifold and a wiring centre that links them to the pump and the boiler. Approved Document L paragraph 5.25 says any work on a system with a boiler should include a boiler interlock, so the boiler doesn't fire when nothing needs heat.

• A Heatmiser UH4 (4 zones) or UH8 (8 zones) wiring centre has outputs for the boiler and the UFH pump. When any thermostat calls, it opens that zone's actuators, starts the pump and switches the boiler.
• On an S-plan system, the wiring centre's boiler output usually opens the UFH zone valve, and the valve's end switch fires the boiler.
• For a new heating system in a home of 150m² or more, Approved Document L paragraph 5.14 asks for at least two independently controlled heating circuits.

Our thermostats guide covers which thermostat to put in each room.

Does underfloor heating make a boiler more efficient?

It can help. The Energy Saving Trust says boilers are more efficient when the return temperature is low, and Worcester Bosch says a condensing boiler kept below about 50°C can run up to 10% more efficiently. Water coming back from a floor circuit is cool, which helps the boiler condense.

The gain is smaller when radiators share the boiler, because their return is warmer. Turn the boiler flow down as far as the radiators allow, use weather compensation if the boiler has it, and avoid big setbacks on slow screed floors. Approved Document L paragraph 5.10 asks new or fully replaced wet heating systems to be designed for 55°C or lower.

Is my boiler big enough for underfloor heating?

Usually yes, because underfloor heating needs the same heat as the rooms it replaces, not extra. Size it from the heat loss of the rooms. A new extension of 30m² at 50W/m² needs 30 × 50 = 1,500W, or 1.5kW. A whole ground floor of 70m² at 60W/m² needs 4.2kW.

You only need a bigger boiler when you add new heated space and the boiler is already near its limit. Compare the total heat loss of every room on the boiler with its central heating output. Our guide to heat loss and UFH output shows how to check each room.

What does a boiler-fed underfloor heating kit cost?

The pipe, manifold and controls for a 30m² screeded extension on a system boiler cost about £630 from MaxHeatUFH (inc VAT, October 2026), before insulation and screed.

Example: 30m² heated area, 16mm pipe at 150mm centres. Pipe in the floor is 30 × 6.7 = 201m. Three loops with tails of about 10m each make about 77m per loop. Add 10%: about 85m per loop. A 200m coil gives two loops and a 100m coil gives the third.

Total: £628.52 inc VAT, October 2026. Add one thermostat per zone, such as a Heatmiser neoStat V3 at £77 or a DS1 dial thermostat at £24. On a combi you would normally leave out the zone valve and use closely spaced tees instead.

Products for this job

• Blending Valve Pump Pack 20 to 55°C with Bastion 6m pump
• Blending Valve Pump Pack 20 to 55°C with Grundfos UPM3 Auto
• Single Room UFH Mixing Pump Pack with Bastion 6m pump
• Dualmix Stainless Steel Manifold (2 to 12 port)
• 2-Port Motorised Zone Valve
• 3/4" Ball Valve Set for Blending Valve
• Heatmiser UH8 8 Zone Wiring Centre
• All pumps, actuators and valves

Frequently asked questions

Can underfloor heating and radiators run off the same boiler?

Yes. The radiators run at the boiler's normal flow temperature and a blending valve pump pack at the manifold cools the water for the floor. Give each its own zone and connect them with closely spaced tees or a zone valve so the circuits don't interfere.

Will a combi boiler run underfloor heating and hot water at the same time?

Not at the same moment. A combi's diverter valve gives hot water priority, so the heating pauses while a hot tap runs. A floor holds enough heat that most people don't notice.

What temperature should my boiler be set to for underfloor heating?

Set the boiler to what the radiators or cylinder need, and set the floor temperature on the blending valve, usually 35 to 50°C. If the boiler serves only the floor, set it as low as gives the floor its design temperature.

Do I need a separate pump for underfloor heating with a boiler?

Yes. The boiler's pump moves water round the primary circuit, and the pump on the blending pack circulates the floor loops. Closely spaced tees or a low loss header keep the two pumps from fighting.

Can I add underfloor heating to one room without a manifold?

Yes, with a single room mixing pump pack fed from a radiator circuit. It runs one loop, so it suits rooms up to about 12 to 14m² with 16mm pipe at 150mm centres.

Is a boiler or heat pump better for underfloor heating?

Both work. A heat pump suits a floor because both work at low temperatures, while a boiler needs a blending valve but costs less to fit. See underfloor heating with a heat pump.

Sources: Approved Document L Volume 1 (2021, 2023 amendments), Worcester Bosch TB 0137: hydraulic separation of UFH via closely spaced tees, Vaillant: what is a diverter valve, Energy Saving Trust: boiler flow temperature, Worcester Bosch: how to ensure boilers condense, Danfoss underfloor heating handbook, Gyvlon flowing screed data pack