Underfloor Heating Not Working? A UK Engineer's Fault-Finding Guide

Cold zones, seized actuators, air locks and pump failures — how a Gas Safe engineer diagnoses UFH faults without ripping up your floor.

Underfloor Heating Not Working? A UK Engineer's Fault-Finding Guide — illustrative photo of UK home emergency engineers at work

Underfloor heating (UFH) is beautifully simple when it works and infuriating when it doesn't. The good news: 80% of UFH faults are electrical or air-related, not pipe-buried. Here's the order I check things in.

Start at the manifold, not the room

Every wet UFH system has a manifold — usually in a utility room or airing cupboard. It's where all the loops start. If one zone is cold, everything you need to diagnose it is at that manifold.

1. Are the actuators opening?

Actuators are the small black or white plastic caps sitting on top of each loop valve. When the room thermostat calls for heat, 230V (or 24V on some systems) energises the actuator and it lifts to open the valve. Manually lift the actuator's collar — if warm water starts flowing into that loop within 60 seconds, your actuator or wiring centre is at fault, not the pipework.

Actuators cost £15–£30 and swap in 5 minutes. Wiring centre faults are more common on systems 8+ years old.

2. Is the pump running?

The manifold pump has an LED or spinning arrow. Put your hand on it — it should be warm and lightly vibrating. Silent + cold = no power (check the wiring centre) or seized pump. Modern Grundfos UPM3 pumps last 8–12 years; a replacement is £120–£180.

3. Is the blending valve set right?

UFH runs at 35–45°C flow, not the 65–75°C your radiators use. The thermostatic mixer at the manifold blends boiler-hot flow with cooler return water. If it's stuck fully closed, no heat reaches the floor; fully open and you'll cook the screed. Adjust the dial; if it doesn't respond, replace the cartridge (£45).

4. Trapped air in a loop

UFH loops can be 100m+ of continuous pipe. Air gets trapped at high points. Every manifold has a purge valve — isolate one loop at a time and flush water through with a hose until you get a clean stream. This alone fixes a huge proportion of "dead loop" calls.

5. Room thermostat batteries or comms

Wireless thermostats fail silently. Check batteries first, then re-pair to the receiver. On smart systems (Heatmiser Neo, Wundatrade, Nest), a boot cycle on the hub fixes intermittent comms issues.

When you actually need pipe work

Genuine pipe failures in modern PE-RT or PEX-Al-PEX are rare — maybe 1% of the jobs I attend. Usually caused by a nail or screw driven into the floor after installation. Thermal imaging cameras find them without breaking up the screed.

Cover for UFH systems

Most standard boiler policies exclude UFH — check the wording. Our home emergency cover can be extended to include manifolds, pumps and actuators. Ask when you get a quote.

FAQ

Why is my UFH warm downstairs but cold upstairs?

Upstairs UFH (usually in a low-profile aluminium spreader plate) needs balancing separately from downstairs screed loops. It's rarely a fault — it's design.

Can I run UFH from a combi?

Yes for small areas (single room, up to 40m²). Beyond that a system boiler with a buffer tank is far better because combi cycling wrecks UFH efficiency.

Summary