When a floor stops heating, most people assume the worst: a pipe has failed somewhere under the screed, and the floor is coming up. That assumption is almost always wrong, and acting on it is expensive.
The useful way to think about an underfloor heating repair process is by location, not by symptom. Nearly every fault sits above the screed, in the manifold, the actuators, the pump, the controls or the water itself. Those parts are accessible, the components are inexpensive, and most of them are a one-hour job. Only pipe damage sits below the screed, and that is the one scenario where the floor is genuinely at risk.
So the whole discipline of diagnosing an underfloor heating fault comes down to working through the accessible causes in order, and only reaching for the destructive option once everything else is ruled out. This article sets out that order.
A note on scope first, because this is where most published advice goes wrong. Hydronic systems, which circulate warm water through pipe, and electric systems, which run a resistance cable or mat, have almost nothing in common diagnostically. Everything below deals with hydronic systems. Electric systems get their own section near the end, kept separate on purpose. If you are not sure which you have, our comparison of electric and hydronic underfloor heating settles it quickly.
Above the Screed or Below It
This distinction decides the cost and the disruption, so establish it before anything else.
Above the screed. Manifold, flow meters, circuit valve inserts, thermal actuators, circulator pump, blending or mixing valve, wiring centre, room thermostats, system water. All of it reachable by opening a cabinet door. Components cost tens of dollars to a few hundred. Most repairs take under an hour and nothing is damaged in the process.
Below the screed. The pipe loops themselves, and the insulation under them. Repairing a pipe means locating the fault precisely, breaking out the screed, cutting and joining the pipe, re-screeding, waiting for the screed to cure, and reinstating the floor covering. Days of work, and the floor finish over the repair is usually lost.
In our experience the ratio is heavily skewed. Pipe failures do happen, usually from damage during construction rather than from the pipe degrading on its own, but they are a small minority of call-outs. The great majority of “my underfloor heating has stopped working” turns out to be a stuck actuator, an air lock, a failed pump, or a thermostat that is not calling.
Which is why the order matters so much. Diagnose outward from the cabinet.
The Diagnosis Sequence
Work through these in order. Each step is cheaper and less invasive than the one after it.
1. Confirm the Heat Source Is Working
Obvious, and skipped constantly. If domestic hot water has also gone cold, the problem is the boiler or heat pump, not the underfloor heating. Check that first and you may save yourself the rest of this list.
Also check system pressure on the gauge. If it has dropped below the normal operating range, note the figure and how often it has needed topping up, because that history becomes important at step eight.
2. Check the Circulator
Put a hand on the pump body. A running circulator vibrates faintly and the pipework either side warms within a few minutes. Silence and a cold body means it is not running.
Three reasons for that, in order of likelihood: the pump has failed, the wiring centre is not calling it because no zone is demanding heat, or the end switch arrangement has dropped it out. A failed UFH circulator is one of the single most common causes of a dead floor and it is a straightforward swap.
While you are there, check the pump speed setting. An oversized fixed speed produces noise in the flow meters; too low a setting on a proportional pressure mode can starve the index circuit. Neither is a fault exactly, but both produce complaints that look like one.
3. Check the Thermal Actuators
This is the most common UFH fault by a wide margin, and the easiest to confirm.
Thermal actuators are usually normally closed, driven by a wax thermostatic element that expands when powered. They take three to five minutes to open or close fully, so do not judge one in thirty seconds. A stuck actuator leaves a zone permanently cold if it fails closed, or permanently hot if it fails open, and a zone that will not turn off is as much a fault as one that will not turn on.
The test takes two minutes. Unscrew the actuator head from the valve insert. The valve pin underneath should spring up freely. Now run the system: if that circuit heats with the actuator removed, the actuator is your fault and nothing else is. Replacing one is a ten minute job and the part is inexpensive.
If the pin does not spring up, the valve insert itself is seized, which also happens after a long summer idle. Exercising the actuators once before each heating season prevents most of these.
Check the electrical side too. With the thermostat calling, there should be supply voltage at the actuator terminals. No voltage means the fault is upstream in the wiring centre or the thermostat, not in the actuator.
4. Check the Thermostat and Its Sensor
Turn the setpoint well above the current room temperature and watch the wiring centre. The channel LED for that zone should light, and the actuator should warm to the touch within a few minutes.
If the LED does not light, the thermostat is not calling. Thermostats are generally replaced rather than repaired, since the cost of a new unit is lower than the labour of diagnosing a board.
A particular trap: where a thermostat uses a floor sensor, a sensor reading open circuit or short circuit will make the thermostat refuse to call at all, and the display may give no useful indication of why. Checking the sensor resistance against the manufacturer’s temperature curve rules this in or out. Our note on underfloor heating sensors covers the resistance values and how they are read, and underfloor heating thermostat failures goes into the control side in more depth.
5. Purge Air From the Loops
The second most common fault, and it needs no parts at all.
Symptoms of air in a loop: gurgling or trickling sounds, a loop that is warm near the manifold and cool at the far end, and a flow meter reading low or bobbing rather than sitting steady. Air collects at high points and blocks circulation as effectively as a closed valve.
The fix is to purge loop by loop, with every other circuit isolated at the manifold so the full flow is forced through one loop at a time, until the water runs clear and no bubbles come through. Doing all loops at once rarely clears anything, because the water simply takes the easy route. We cover the procedure in how to bleed underfloor heating.
6. Check the Balance
If the complaint is not “no heat” but “this one room is never warm enough”, balance is the likely cause rather than a failure.
The signature is specific: one zone sitting one to three degrees below setpoint while neighbouring zones overshoot, and a temperature difference across the manifold flow and return thermometers much wider than the design figure. Water takes the path of least resistance, so a long loop left unthrottled against short ones gets starved.
Nothing is broken here. The flow meters need setting to the calculated figures for each circuit, which is a commissioning task rather than a repair. If the commissioning figures were never recorded, they have to be recalculated from the room heat losses.
7. Check the Mixing Valve Setpoint
Where the primary circuit runs hotter than the floor can accept, a blending valve drops the flow temperature. When it drifts, the whole floor is affected rather than one zone.
Compare the manifold flow thermometer against the valve setting. A reading well below setpoint means the valve is passing too much return water; well above means it is passing too much primary. The usual cause is scale or debris on the thermostatic element, which responds slowly and then stops responding. Our guide to underfloor heating mixing valves explains the mechanism.
8. Look at the System Water
This step diagnoses the cause behind the other faults, so it is worth doing even when something else has already been fixed.
Draw a sample from the drain valve. Dark water with suspended black particles means magnetite, the iron oxide sludge that forms when oxygen reaches the system water. It restricts flow, coats heat exchanger surfaces, wears pump bearings, and blocks valve seats. A system that keeps producing faults almost always has dirty water behind them.
Three causes, and usually all three together: pipe without an oxygen barrier, so oxygen diffuses straight through the wall; no corrosion inhibitor in the fill water; and no magnetic filter to catch what forms. The remedy is a power flush, a dose of inhibitor, and a filter fitted on the return.
This is also the strongest argument for specifying oxygen barrier pipe at the outset. Our floor heating pipe carries an EVOH barrier to DIN 4726 for exactly this reason, and the cost difference at installation is trivial against a power flush ten years later.
9. Only Now: Suspect the Pipe
Reach this step only after the eight above.
The signature of a genuine pipe leak is pressure loss that keeps coming back. A system needing a top-up every few weeks is losing water somewhere, and if the manifold, pump and valves are all dry, it is going into the floor. Supporting signs include a persistently damp or cool patch, an unexplained rise in heating consumption, and a single loop that cannot be brought up to pressure.
Confirm before you cut anything. Isolate each loop individually at the manifold, pressurise it to around 6 bar, and watch for thirty minutes. A loop that holds pressure is not your leak. This narrows the problem from an entire floor to one circuit, which is the difference between a small repair and a large one.
To find the fault within that circuit, a thermal camera is the standard tool. Warm water in the pipe shows the loop routing clearly through the floor finish, and a leak reads as a distinct hot spot or an abnormally warm area. That also tells you where the pipe runs, which matters a great deal when you are about to break into screed. Acoustic detection and tracer gas are alternatives where thermal imaging is inconclusive. Our overview of HVAC leak detection methods compares the approaches.
The repair itself is a cut and a compression coupler or an electrofusion joint, then re-screed. Straightforward work. It is the locating and the reinstatement that cost money, which is why precision at this stage pays for itself several times over.
Electric Underfloor Heating: A Different Fault Set
Kept separate because almost nothing above transfers.
An electric system has no water, no manifold, no actuators and no air. The heating element is a resistance cable or mat embedded in the floor, controlled by a thermostat with a floor probe. That gives three failure points: the thermostat, the probe, and the cable.
Thermostat and probe faults are diagnosed electrically. The probe is checked for resistance against the manufacturer’s stated temperature curve, and a reading of open or short circuit identifies it immediately. Both are accessible and inexpensive.
Cable faults are the serious case. Continuity and insulation resistance are measured with a megohmmeter against the values recorded at installation, which is why that commissioning record matters so much. Locating a break needs a time domain reflectometer or a thermal camera. And here is the hard part: the cable is the heating element, so unlike a water loop it cannot be bypassed or rerouted. A cable fault generally means opening the floor.
One honest point of scope. Legom manufactures hydronic components. We do not make electric underfloor heating cable or mats, so if your system is electric, our range covers the controls side rather than the element.
What to Actually Prepare Before the Technician Arrives
Preparation is not about clearing space. It is about having the information that shortens the diagnosis, because a technician working blind spends the first two hours establishing what you could have told them in five minutes.
Gather what you have of the following:
- The loop layout drawing, or photographs taken before the screed was poured. This is by far the most valuable document and almost nobody has it. If you are having a system installed, photograph every loop before the pour and keep the images. It can be the difference between a targeted repair and exploratory demolition.
- The commissioning record, meaning the flow meter setting for each circuit and the design flow temperature. Ideally this is on a label inside the manifold cabinet.
- Pressure history. How often have you topped the system up, and by how much? This single answer separates a controls fault from a leak.
- Which zones are affected, and since when. One zone or all of them. Gradual or sudden. After any building work.
- Make and model of the thermostats, pump, manifold and heat source.
- Clear access to the manifold cabinet. If it sits behind a built-in cupboard, open it up beforehand.
If the system is under warranty, find the paperwork and check what it covers before any work begins. Our note on underfloor heating installation methods is a useful reference for understanding what was built.
Preventing the Repairs You Can Prevent
Most of the faults above are avoidable, and the measures are cheap relative to the repair.
- Specify oxygen barrier pipe to DIN 4726. This prevents the magnetite problem at source rather than managing it later.
- Dose the system with inhibitor and fit a magnetic filter on the return.
- Exercise the actuators with an all-zones call before each heating season. Prevents the stuck-wax fault entirely.
- Record the commissioning figures on a label inside the cabinet. Costs nothing, saves an hour every future visit.
- Check flow meter readings annually against those figures. Drift tells you about a developing problem before it becomes a complaint.
- Photograph the loops before the screed goes down, and pressure test at 6 bar for 24 hours before and during the pour so screeding damage shows up immediately rather than in year three.
- Use expansion joints in large screed areas, since cracking screed is a mechanism that damages pipe. We cover this in underfloor heating expansion joints.
- Get the pipe layout right at design stage. Loops within 10 to 15 percent of each other in length are balanceable; wildly uneven loops are not, whatever anyone does at the manifold. See underfloor heating pipe layout.
Legom Underfloor Heating Components
Jiaxing Legom Technology Co., Ltd. manufactures the hydronic components this diagnosis sequence runs through, from our facility in Jiaxing, Zhejiang, supplying more than 90 countries.
That covers the parts a repair most often needs: thermal actuators in normally closed and normally open variants, the wax thermostatic elements inside them, manifolds in stainless steel and brass with integral flow meters, room thermostats and wiring centres, oxygen barrier floor heating pipe, and the valves on the blending side. The manifold specifications page carries the dimensional and material figures if you are matching a replacement to an existing installation.
Because we manufacture the wax elements ourselves rather than buying them in, we can hold actuator response consistent across a production run, which matters when a distributor is supplying spares into an installed base rather than one-off units.
For distributors and installers stocking spares, or brands specifying to their own range, our OEM and ODM service covers configuration, material specification, packaging and market documentation. Tell us the destination market alongside the specification and we will confirm the approval position before quoting.
Some of our customers have visited the factory, and a couple of those visits are below.
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Frequently Asked Questions
My underfloor heating has stopped working. Is the pipe broken?
Almost certainly not. Pipe failure is a small minority of faults. Work through the accessible causes first: heat source, circulator, thermal actuators, thermostat, air in the loops. A stuck actuator and an air lock between them account for most dead floors, and neither needs the floor opened.
Why is one room cold while the rest of the floor is fine?
Either that zone’s actuator is stuck closed, its thermostat is not calling, there is air in its loop, or the manifold was never balanced so that circuit is being starved. Test the actuator first by removing the head: if the circuit then heats, you have your answer.
How do I know if I have a leak under the floor?
Repeated pressure loss is the signature. If you are topping the system up every few weeks and the manifold, pump and valves are dry, water is going somewhere. Confirm by isolating each loop at the manifold and pressure testing individually at around 6 bar for thirty minutes before anyone cuts into screed.
Can a thermal camera find an underfloor heating leak?
Yes, and on a hydronic system it does two jobs. Warm water shows the loop routing through the floor finish, and a leak reads as a distinct hot spot. Knowing where the pipe runs before breaking out screed is worth as much as locating the fault itself.
Why does my system keep needing repairs?
Usually dirty system water. Magnetite sludge restricts flow, wears the pump and blocks valve seats, producing a succession of faults that each look separate. Draw a sample from the drain valve: dark water with black particles confirms it. The remedy is a power flush, corrosion inhibitor and a magnetic filter, plus oxygen barrier pipe on any future work.
How long does an actuator last, and how hard is it to change?
Several years in normal service, shorter in hard water or where the system sits idle all summer. Replacement takes about ten minutes: unscrew the old head, check the valve pin springs freely, screw the new one on. The part is inexpensive, which is why it is the first thing to test rather than the last.
Is electric underfloor heating repaired the same way?
No. An electric system has no water, manifold, actuators or air. Faults are in the thermostat, the floor probe, or the cable, and they are diagnosed electrically with resistance and insulation measurements. A cable fault is the serious case, because the cable is the heating element and cannot be bypassed, so it generally means opening the floor.
Reviewed by Maggie Shen, Director at Legom, on October 9, 2026. This article was reviewed for technical accuracy, including the above-screed and below-screed fault distinction, the actuator head removal test, wax element actuation timing, loop-by-loop purging and pressure testing method, magnetite formation and oxygen barrier pipe, thermal imaging for loop location, and the separation of hydronic from electric system diagnostics.