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Underfloor Heating Not Working? Causes and Fixes
Reviewed by the Devon Leak Detection team. Last updated June 2026
The short answer
If your underfloor heating has stopped working or one room has gone cold, the usual culprits are an airlock, a stuck actuator, an unbalanced manifold, a tired pump or a leak in a buried loop. Work through the manifold first. If the system keeps losing pressure, suspect a leak and call for non-invasive detection.
Wet underfloor heating is a quiet, comfortable way to warm a home, which is exactly why it is so noticeable when it stops doing its job. One morning a room that used to feel warm underfoot is cold, or the whole system feels lukewarm no matter what the thermostat says. The good news is that most faults are mechanical and traceable, and many can be sorted at the manifold without lifting a single tile. This guide walks through the common causes, how to narrow them down, and the point at which a hidden leak is the likely answer.
Before you start, it helps to picture how the system is laid out. A boiler or heat pump feeds warm water to a manifold, usually tucked into a cupboard or utility room. From there the water splits into separate loops of pipe, one per zone or room, each buried in screed or sat beneath the floor. Flow gauges and small electric actuators on the manifold control how much warm water reaches each loop. When a room misbehaves, the manifold is nearly always the best place to begin.
One room or loop has gone cold
If the rest of the house is warm but one zone stays cold, the fault is almost always local to that loop rather than the heat source. Start by ruling out the simplest things. Check that the room thermostat is actually calling for heat and has working batteries or power. A thermostat that has lost its signal to the manifold is a surprisingly common reason for a single dead zone.
Next, look at the manifold itself. Each loop has an actuator, a small device that opens and closes the valve when the thermostat asks for heat. Actuators can seize, and the pin underneath them can stick. With the system calling for heat, the pipework feeding a healthy loop should warm up within a few minutes. A loop that stays stone cold while its actuator should be open points to a stuck actuator, a wiring fault, or air trapped in that run of pipe.
Air is the other frequent offender. An airlock in one loop stops the water circulating, so the pipe never warms even though everything else looks fine. Purging that loop, sometimes called bleeding or flushing, clears the trapped air and usually brings the room back to life. If you are not confident draining and refilling a loop correctly, this is a sensible point to bring in a heating engineer.
Signs you might notice:
- One room stays cold while the rest of the house heats up normally
- A loop on the manifold feels cold to the touch when others are warm
- Gurgling or trickling sounds in the pipes, a classic sign of trapped air
- The thermostat is calling for heat but nothing seems to respond
- The boiler fires but a single zone never warms through
When the whole system feels lukewarm
If every room is underwhelming rather than just one, look at the parts that serve the whole system. A circulating pump that is failing or running slowly will struggle to push warm water around the loops, leaving the floor tepid. Pumps can also airlock, so listen for unusual noise or a pump that runs hot. The blending valve that mixes hot boiler water down to the lower temperature underfloor heating needs is another common weak point. If it sticks, the water reaching the floor can be too cool to make any real difference.
Manifold balancing matters here too. Each flow gauge can be adjusted so that long loops and short loops get a fair share of warm water. If the balancing has drifted, or a valve has been left partly closed, some rooms will run noticeably cooler than others. Setting the flow rates to the figures in your system manual is often enough to even things out.
Sludge and debris build up over the years and can partly block a loop or clog the pump and valves. A system that has never been flushed and has no inhibitor in the water is more prone to this. A power flush and fresh inhibitor can restore circulation, and fitting a magnetic filter helps keep the problem from returning.
How to narrow it down
A methodical check saves a lot of guesswork. Start at the thermostat and confirm the room is actually asking for heat. Move to the manifold and feel each loop with the system running: the flow side should be warm, and a healthy loop will warm along its length. Compare the loop that is misbehaving against one that works, and you can often tell at a glance whether the issue is a cold actuator, an airlock or a blockage.
If you suspect circulation rather than a single loop, isolate the system so only the suspect zone is running. If the boiler fires and the pump runs but the loop stays cold, the trouble is in that loop, an actuator or an airlock. If the boiler does not fire at all, the problem is more likely the thermostat or the wiring. Working from the heat source outwards keeps the diagnosis logical and avoids replacing parts that were never the cause.
Losing pressure points to a leak
If you keep topping up the boiler pressure and it keeps dropping, the system is losing water somewhere. Air being released and a faulty expansion vessel can both nudge pressure down, but a steady or rapid fall usually means a leak. Visible leaks tend to show on the exposed pipework near the manifold or the boiler, which is the first place to inspect. The harder cases are leaks in the buried loops themselves, where a slow weep can hide under screed or tiles for a long time.
A buried leak often shows up before you ever see water. You might notice a damp or discoloured patch in the floor, a section of floor that feels warmer than it should, a musty smell, or pressure that simply will not hold. Because the pipe is sealed into the structure, lifting the floor to go looking is the last thing you want to do. That is exactly where specialist detection earns its keep.
How we find a leak under the floor
Finding a leak in a wet underfloor heating system without ripping up the floor is what non-invasive leak detection is built for. Thermal imaging is often the first tool out of the bag. A thermal camera reads the surface temperature of the floor and shows it as an image, and because warm water spreading from a leak heats the surrounding screed, the leak frequently appears as a telltale warm or wet pattern. Damp screed also conducts heat differently to dry screed, which makes the affected area stand out to a trained eye.
Where the thermal picture is unclear, or the loop has cooled down, tracer gas confirms the exact spot. The loop is drained and pressurised with a safe hydrogen and nitrogen mix. The gas is light enough to escape through the smallest fault and rise up through the floor, where a sensitive detector picks it up directly above the leak. Used together, thermal imaging and tracer gas let us pinpoint the failure to a small area rather than a whole room, so any repair is targeted and the disruption is kept to a minimum. You can read more on our underfloor heating leak detection service page, and browse our other guides in the articles section.
Underfloor heating problems range from a simple stuck pin to a genuine pipe failure under the screed. Plenty can be solved at the manifold, and it is always worth checking thermostats, actuators, balancing and air before assuming the worst. But when the pressure will not hold and a damp patch appears, accurate detection is the difference between a neat, located repair and a floor torn up on guesswork. If that sounds like your situation across Devon, we are here to help.
Frequently Asked Questions
A single cold room usually means a fault in that one loop rather than the heat source. The common causes are a stuck actuator on the manifold, an airlock stopping the water circulating, a thermostat not calling for heat, or a partly closed flow valve. Check the manifold and thermostat first, as these account for most single-zone faults.
The clearest sign is pressure that keeps dropping however often you top it up. You may also see a damp or discoloured patch in the floor, smell something musty, or feel a warm area where a buried pipe is weeping. Air and a faulty expansion vessel can also lower pressure, so a steady loss is worth investigating properly.
Yes. Non-invasive methods are designed for exactly this. Thermal imaging reads heat patterns through the floor surface, and tracer gas escapes through the smallest fault to pinpoint the spot. Used together they locate the leak to a small area, so any repair is targeted rather than a floor lifted across a whole room.
An airlock is trapped air in a loop that blocks the warm water from circulating, so the pipe stays cold even when everything else looks right. It is cleared by purging or flushing the affected loop to push the air out. If you are not confident draining and refilling a loop correctly, a heating engineer can do this safely.
When every room underperforms, look at the shared parts. A weak or airlocked pump, a stuck blending valve sending water through too cool, drifted manifold balancing, or sludge restricting flow can all leave the floor tepid. Checking the pump, the mixing valve and the flow gauges usually reveals the cause.
Pressure dropping or a room gone cold?
We find underfloor heating leaks without lifting your floor, then point you to a precise repair.