Underfloor heating playing up? Start here

Wet or electric? Thirty seconds to tell

Wet systems run warm water through pipe loops in the floor, fed from a manifold: a brass rail of valves, small sight-glass flow gauges and little white actuator heads, usually hiding in a cupboard. The heat comes from your boiler, or from a heat pump, whose winter behaviour is its own subject.

Electric systems are heating mats or cables under the floor finish, each zone with its own wall thermostat and a floor sensor buried in the screed or tile adhesive. No manifold and no pipework: the thermostat and its wiring do everything.

The split decides everything downstream. Wet faults are plumbing and controls; electric faults are electrical. Different failures, different trades, different bills.

Slow is not broken

A heated screed floor is a storage heater the size of the room. From cold, the first warm-up of the season can take the best part of a day, and even day to day it responds in hours rather than minutes. The surface should feel barely warm, around 25 to 27°C, never radiator-hot. If you are expecting toasty tiles, a perfectly healthy floor can feel broken.

Program it accordingly: gentle and early beats hard and late, and the schedule logic in the thermostat guide applies double under a slab.

When a zone goes cold

Cold floors come down to three questions: is the heat source making heat, is the distribution moving it, and is the control asking for it? The tree sorts them in order.

On a wet system, single-zone failures usually come down to the small stuff: an actuator head that no longer opens its valve (they fail shut, and they are inexpensive parts), an airlock in one loop after system work, or a wiring-centre relay that has retired. The manifold’s flow gauges tell the story: one loop showing no flow while its neighbours spin is the culprit named.

On electric systems the classic failure is the floor sensor. When it drifts or dies, the thermostat either shows an error or starts guessing from air temperature, and the floor runs too hot or not at all. Sensors can sometimes be replaced through their conduit without lifting a single tile: an electrician’s hour rather than a refit.

Electric mats: the extra rules

Electric UFH has a hard limit wet systems do not: the cable relies on the floor above to carry heat away. Trap that heat under a thick rug, a beanbag or a new wardrobe and the mat can overheat, which is why layout plans matter years after installation. If the original plan exists, keep it with the house papers.

A UFH circuit that trips the consumer unit has also earned professional attention. Repeated resetting risks turning a repairable cable fault into a dead mat, and fault-finding embedded cables is precise work for an electrician with the right test kit.

Manifold work, actuator swaps and loop bleeding are heating engineer jobs; mats, thermostats and sensors belong to an electrician; and anything on the boiler side is for a Gas Safe registered engineer. The universal homeowner contribution is evidence: photos, timings and which zones misbehave.

One more wet-system note: the water in the floor is the same water as the radiators, so the magnetite sludge that blocks radiator bottoms can silt a manifold and its narrow loops too. If every flow gauge reads low, raise water quality when you book. For everything upstream of the floor, the complete boiler guide is the map.