Underfloor Heating Problems: Diagnosis and Repair

Underfloor heating (UFH) is increasingly common in UK homes, both in new builds and as retrofits under extension floors. When it works, it provides comfortable, efficient heating. When it develops a fault, the concealed nature of the pipes means troubleshooting requires a systematic approach. Most UFH faults do not require opening up the floor.

Types of underfloor heating

Wet UFH (hydronic): Hot water is pumped through plastic pipe circuits (typically MLCP or polybutylene pipe) embedded in a screed floor or clipped to insulation above a joist. The water is heated by the boiler or heat pump and distributed via a manifold, which has separate circuits for each zone or room.

Electric UFH (dry): Electric resistance mats or cables are embedded under the floor covering. Controlled by a thermostat. Simpler installation, higher running cost. Electric UFH faults are usually thermostat or sensor problems; the advice below focuses on wet UFH.

How wet UFH works

The boiler or heat pump circulates warm water through the manifold to each floor circuit. The manifold has flow and return connections for each circuit, with individual actuators (electric valve heads) that open and close each circuit based on thermostat demand. A blending valve or mixing unit typically limits the flow temperature to the floor (usually 35–50°C for UFH, compared to 65–75°C for radiators).

Understanding the manifold is key — most UFH faults are in the control system (thermostats, actuators, wiring) or in the manifold (air, balance settings) rather than in the pipe circuits under the floor.

Fault 1: Zone not heating (individual room or area)

Check the thermostat first. Is it powered? Set to heat demand? Try turning it to maximum and waiting — does the actuator open? (Many actuators click audibly when opening.)

Check the actuator. The actuator is the small cylinder screwed onto the manifold valve body. When the thermostat calls for heat, the actuator opens the valve. Remove the actuator head (it usually unscrews or clips off) and check whether the valve pin beneath it moves. Press the pin — if it springs back, the valve is free. If the pin is stuck closed, gently work it free with a small spanner.

Check manifold flow rates. Most manifold systems have flow meters visible through a small window on the flow side. Each circuit should show flow when the actuator is open. A circuit with zero flow despite the actuator being open indicates a blockage or complete closure of the balance valve.

Check the circuit balance valve. Each circuit on the manifold has a balance valve (lockshield on the return side) that was set at commissioning. If it’s been accidentally fully closed, no flow will pass.

Fault 2: All zones not heating (whole-system UFH fault)

Check the manifold pump. UFH systems often have a dedicated pump at the manifold in addition to the boiler pump. If this pump has failed, no flow circulates in the UFH circuits even if the boiler is running.

Check the mixing valve. The thermostatic mixing valve (or weather compensation controller) blends boiler-temperature water with cooler return water to deliver floor temperature water. A failed mixing valve can deliver water at the wrong temperature — either too cold (no heat output) or too hot (floor feels very hot, zone thermostats shut off immediately).

Check boiler and zone valves. Confirm the boiler is running and that the zone valve to the UFH circuit (if there is one) is open. The same zone valve fault that affects radiators can affect UFH.

Fault 3: Uneven floor temperature (some areas hot, some cold)

Air in circuits: The most common cause. UFH systems are notoriously difficult to purge of air at installation. Individual circuits should be purged using the isolation valves on the manifold to drive flow one circuit at a time. This is a specialist task at commissioning but can be done by a competent plumber at any time.

Balance issues: If some circuits are much hotter than others, the manifold balance may have drifted (or was never set correctly). Balancing UFH circuits involves adjusting the flow rate through each circuit using the balance valves and flow meters until flows are equalised.

Partial blockage: A circuit that flows less than the others may have a partial blockage (rare in correctly installed UFH) or a kinked pipe at a connection point.

Fault 4: Leak in the UFH circuit

A leak in an embedded UFH pipe causes the system to lose pressure repeatedly (check the boiler pressure gauge). The floor above the leak may eventually show a damp patch, discolouration, or in screed floors, a crack.

Diagnosis: Isolate individual UFH circuits one at a time at the manifold (both flow and return) and use a hydraulic pressure test kit to pressure-test each circuit separately. The leaking circuit will show pressure loss.

Location: Once the leaking circuit is identified, leak detection using thermal imaging or acoustic equipment locates the exact position within the circuit without opening up the entire floor. This is a specialist job.

Repair: Options include complete re-pipe of the circuit (major), or — for screed floors — using a specialist UFH pipe jointing system to splice in a repair piece at the leak point. Re-pipe is the more robust option.

Fault 5: Floor warm but room not reaching setpoint

Insulation above floor level: In older properties, UFH installed under retrofit tiles or laminate may have too little insulation beneath the pipe, sending significant heat downward rather than upward. If the screed feels warm but the room doesn’t respond, this is a likely cause.

Low flow temperature: Heat pump-supplied UFH typically operates at 35–45°C flow temperature. If a weather compensation controller is set too conservatively or there’s a mixing valve fault delivering temperatures below 30°C, heat output will be insufficient.

Heat loss exceeds UFH output: UFH is a low-intensity heating system — perfect for a well-insulated property but sometimes insufficient in poorly insulated older buildings. If the room has poor wall insulation, single-glazing, or significant air infiltration, UFH may struggle to maintain setpoint in cold weather. Adding supplementary heating or improving insulation are the solutions, not a heating fault.