Why isn't my underfloor heating warming up properly?

This covers wet underfloor heating — pipe loops run through or under the floor and fed from the boiler via a manifold, which is what most London extensions and refurbishments use, rather than the electric mat systems fitted under tiles in a single small room. The two work differently and a fault in one rarely explains a fault in the other — our team checks which system is fitted before diagnosing anything further.

Air trapped in the loops

The most common cause by far. Each loop needs to be balanced and purged of air when it is commissioned, and if that was not done fully — or air has worked its way back in since — you get patches of floor that never quite reach temperature even though the system is clearly running. This is a similar issue in principle to a radiator that needs bleeding; see our guide on how to bleed a radiator for the underlying cause, though underfloor loops are purged at the manifold rather than bled individually room by room.

An actuator stuck closed at the manifold

Each loop has a small electric actuator on the manifold that opens and closes it in response to the room thermostat. If one has failed shut, that loop simply will not get any flow at all, regardless of what the room stat is asking for — which shows up as one room or zone staying cold while the rest of the house is fine.

The boiler flow temperature set too low for underfloor heating

Underfloor systems are designed to run at a lower flow temperature than radiators, but there is a point below which the water genuinely is not carrying enough heat to warm the room, particularly through screed. If radiators on the same system are warm but the underfloor zones are lukewarm at best, the balance between the two circuits is usually the issue, and it needs adjusting at the manifold and boiler together rather than at either alone.

A thermostat reading the room wrong

A room stat placed near a draught, direct sunlight or another heat source will call for less heat than the room actually needs, so the floor looks like it is underperforming when it is simply doing what the (misreading) thermostat asked. Worth ruling out before assuming a fault with the loops themselves.

What is not usually the cause

A properly installed screed or insulation layer very rarely degrades on its own. If a system that has worked fine for years suddenly stops heating one area, the far more likely explanation is one of the faults above rather than a structural issue with the floor build-up.

Balancing and purging the manifold, and checking flow temperatures against a boiler that is also serviced properly, is a job for underfloor heating work rather than a general call-out — and worth pairing with the boiler’s next annual service if the two have not been looked at together before. General plumbing issues elsewhere in the system are covered under general plumbing.

Outside our coverage area, independent Gas Safe engineers such as Ninja Plumbers in Fulham also work on wet underfloor heating systems.