Boiler and Underfloor Have To Be Set Up Together
Most boiler-related underfloor complaints are not broken components. They are a boiler and a floor that were never set up to work as one system.
The two want quite different things. A boiler is happiest running at a reasonably high flow temperature, and most are installed set somewhere between 60C and 75C. A floor needs far cooler water, typically 35C to 50C depending on how it is built and what is on top of it. The blending valve at the manifold bridges that gap. On top of that, a modern boiler can easily be capable of 24kW or more, while the underfloor load in a few rooms might be a couple of kilowatts. When a large output meets a small demand, the boiler struggles to run steadily.
Common Boiler and Underfloor Faults
The boiler fires, runs a minute or two, stops, then starts again
Short cycling. The boiler never settles into a steady run, and you hear it starting and stopping repeatedly.
- Most likely cause
- The boiler output far exceeds what the underfloor system is drawing, so its small demand is met almost instantly and the boiler shuts down, cools, and fires again.
- Made worse by
- A high boiler flow temperature, no load or weather compensation, too little water volume in the circuit, or an incorrectly set bypass.
- Also possible
- Heat is not being carried away as it should because the manifold pump is weak or the blending valve is restricting flow.
- Why it matters
- Cycling wastes gas, adds wear to the boiler, and gives worse comfort than a boiler running gently and continuously.
- What the fix involves
- Lowering the flow temperature to suit the load, setting up compensation so the boiler modulates rather than switching, and correcting the bypass, pump or volume issue behind it.
The radiators are hot but the underfloor heating stays cold
On a system serving both, one half works perfectly and the other does nothing.
- What it rules out
- The boiler is firing and its controls are working, so the fault is downstream in the underfloor circuit rather than in the boiler.
- Most likely cause
- The manifold pump is not running, or the underfloor circuit is not receiving a demand at all.
- Also possible
- The blending valve has stuck closed, so hot water reaches the manifold but never gets into the loops.
- Also possible
- The actuators are not opening, which is a controls or wiring fault.
- What the fix involves
- Checking for flow and temperature at the manifold, which separates a pump or valve problem from a demand problem in a few minutes.
The rooms never reach temperature even though the boiler runs constantly
Plenty of heat is being produced, but very little of it is reaching the rooms, so they sit below their set temperature no matter how long the boiler runs.
- Most likely cause
- The blending valve is set too low or has failed, so the water going into the floor is barely warm.
- Also possible
- The boiler flow temperature has been turned down so far that there is not enough heat to blend from, a common side effect of turning it down to save money.
- Also possible
- Air or a blockage is preventing circulation, in which case see our page on flow problems.
- What the fix involves
- Measuring the actual flow and return temperatures at the manifold and working back to whichever stage is losing the heat.
Hot water and underfloor heating interfere with each other
Running a bath or a long shower stops the heating, or the cylinder takes a long time to recover.
- On a combi
- There is no stored hot water, so heating is suspended entirely while a tap is running. That is how a combi works rather than a fault, but it is more noticeable with underfloor heating because the floor takes so long to recover afterwards.
- On a system boiler with a cylinder
- Hot water priority may not be set up correctly, so the cylinder and the heating compete instead of the cylinder being reheated quickly and control handed back.
- What the fix involves
- Setting up priority hot water properly so the cylinder gets the boiler’s full output briefly and then heating resumes. There is more on this on our heating controls page.
The water going into the floor is too hot
The floor feels hot underfoot, and on some systems you can see a high temperature on the manifold gauge.
- Most likely cause
- The blending valve has failed or stuck open, letting close to full boiler temperature into the floor.
- Why it matters urgently
- Floors are not built for boiler temperatures. Engineered timber can shrink, cup or move, and adhesives and some coverings can be affected. It is worth turning the heating off and getting it looked at rather than living with it.
- What the fix involves
- Servicing or replacing the blending valve and confirming the flow temperature limit is genuinely holding under load. There is more on how the pump and mixing unit should be set up, and what overheating does to a floor, on our flow problems page.
Underfloor heating added to an older boiler that cannot cope
The system was extended or an underfloor zone was added, and nothing has run properly since.
- Most likely cause
- The boiler and its controls were never reassessed when the load changed, so flow temperature, pump arrangement and bypass all still suit the old system.
- Also possible
- An older non-condensing or non-modulating boiler simply cannot turn down far enough to match a small underfloor load.
- What the fix involves
- Assessing the whole system rather than the new part in isolation, and correcting the flow temperature, controls and hydraulics. Occasionally the honest answer is that the boiler is the limiting factor.
Getting the Flow Temperature Right
Flow temperature is the single setting that most affects how well a boiler and an underfloor system get on. A condensing boiler only condenses, which is where its efficiency comes from, when the water returning to it is cool enough. Underfloor heating returns water at low temperatures naturally, so the pairing is a good one, but only if the boiler is set to take advantage of it rather than being left at the factory setting.
On a system serving both radiators and underfloor heating there is a compromise to strike, since the radiators want higher temperatures than the floor does. That is exactly the problem load and weather compensation controls are designed to solve: instead of one fixed temperature, the boiler varies its flow temperature according to demand or outside conditions, running cooler and longer rather than hot and intermittent. We cover this in more detail on the advanced heating controls page.
Boiler Work Is Gas Safe Work
Anything inside the boiler, and any work on the gas installation, is Gas Safe registered work by law. Wilheat Underfloor Ltd is Gas Safe registered under number 519284, so the boiler and the underfloor system can be diagnosed and corrected together in one visit rather than two trades pointing at each other. If you need boiler or hot water work in its own right, that is covered on our boilers and hot water page.
Related Faults
If the boiler never fires at all when a thermostat calls, that is usually a wiring fault rather than a boiler fault, so start with wiring and wiring centre problems. If the boiler is clearly running but the heat is uneven across rooms, see cold spots and flow problems.
Boiler and Underfloor FAQs
Why does my boiler keep cycling on and off with underfloor heating?
Short cycling usually means the boiler produces far more heat than the underfloor system is asking for. It fires, satisfies that small demand almost immediately, shuts down, cools and fires again. The usual fixes are lowering the boiler flow temperature, setting up load or weather compensation so it modulates instead of switching, making sure the system has enough water volume and a correctly set bypass, and checking the underfloor pump and blending valve are letting heat away as intended.
What flow temperature should underfloor heating run at?
Much lower than radiators. Underfloor heating typically runs somewhere between about 35C and 50C depending on the floor construction, insulation and floor covering, against the 60C to 75C a radiator system might use. The blending valve at the manifold is what brings boiler temperature down to that level.
Can you run underfloor heating from a combi boiler?
Yes, and plenty of homes do, but it needs designing rather than assuming. A combi has no stored hot water, so a tap or shower interrupts the heating while it runs, and a large boiler serving a small underfloor load is a classic recipe for short cycling. It works well when the boiler is sized sensibly and the controls are set up for it.
Why are my radiators hot but the underfloor heating is cold?
On a system serving both, the radiators run directly at boiler temperature while the underfloor circuit depends on its own pump, blending valve and controls. If the radiators are fine, the boiler and its wiring are working, so the fault is almost always in the underfloor circuit itself: the pump, the blending valve, the actuators or the demand that drives them.
Does underfloor heating work with a heat pump?
Very well. Heat pumps are most efficient at low flow temperatures, which is exactly what underfloor heating is designed around, so the two suit each other better than radiators and heat pumps do. The controls and system design differ from a boiler installation and need setting up accordingly.
Should the boiler flow temperature be turned down for underfloor heating?
Usually yes, though it depends on whether the boiler also serves radiators and hot water. Running a condensing boiler at a lower flow temperature keeps it condensing, which is where its efficiency comes from, and it suits underfloor heating naturally. On a shared system there is a balance to strike, which is what compensation controls are for.