Underfloor heating that warms the whole room, not just the bit by the door

A wet system runs warm water through loops of plastic pipe buried in the floor. The loops meet at a manifold, a bank of valves with a flow gauge for each loop, fed by a mixing valve that blends the boiler flow down to the 35 to 45 degrees the floor needs. Each room or zone has its own loop and its own thermostat, so the bathroom can be warm at six in the morning while the spare room stays off.

It suits new floors best: extensions, ground floors being dug out and insulated, loft conversions and full renovations where the floor is coming up anyway. In a screed floor the pipe is clipped to insulation and the screed poured over it, giving the most even heat and the slowest response. In a timber floor the pipe sits in aluminium spreader plates between the joists, or in a low-profile routed board over the existing floor where height is tight. For retrofit into a London flat with a finished floor, a low-profile overlay system at 15 to 20 mm is often the answer.

An electric system is a heating cable on a mesh mat, laid on insulation board under tile or stone, wired to a thermostat with a floor probe. There is no plumbing, no manifold and no boiler involved, which makes it the practical choice for a single bathroom, en-suite, kitchen or conservatory where running pipework is not realistic. It heats quickly, which suits rooms used for short periods.

The trade-off is running cost: electricity per unit of heat costs more than gas, so electric mats are a comfort feature for a small floor rather than the main heating for a house. Getting the wattage right matters. Too low and the floor never feels warm under tile; too high in a small, well-insulated room and the thermostat cycles constantly. The mat is tested for resistance before, during and after the tiling, and the results are recorded, because once the tiles are down a break in the cable is very hard to find.

Every design starts with the heat loss of the room. A well-insulated extension with a big glazed wall and a Victorian front room with a bay and a suspended timber floor need very different outputs, and underfloor heating has a ceiling: a floor can only give out so much heat before it is uncomfortable to walk on. Where the heat loss is higher than the floor can deliver, the design says so and a radiator or towel rail is added rather than pretending.

Pipe spacing is tightened near external walls and glazing and relaxed in the middle of the room. Loop lengths are kept balanced so each loop can be set to the same flow. The floor finish changes everything: tile and stone conduct beautifully, engineered timber is fine with the right temperature limit, thick carpet and underlay insulate the room from its own heating and need a higher flow temperature that the design has to allow for.

Installation follows the design in a fixed order: insulation board laid and taped, edge insulation around the perimeter so the screed can expand, pipe laid to the drawing and clipped or stapled, manifold mounted and every loop connected and labelled, the system filled loop by loop and pressure tested, and only then the screed poured or the boards fitted. The pressure test stays on while the screed goes down, so any damage from a trowel or a boot shows immediately.

Once the floor has cured, the system is commissioned: the mixing valve set, the flows balanced at the manifold, the thermostats and the wiring centre set up, and the floor brought up to temperature over several days in stages so the screed does not crack. You are shown how it all works and left with the loop drawing, because the next person to drill into that floor needs it.

Underfloor heating faults are usually at the manifold or in the controls rather than in the floor. A loop that is cold while the others are warm is an actuator that has stuck shut, a flow gauge closed by a previous plumber, or air in the loop. A whole system that is cold is a pump that has failed, a mixing valve that has seized, a wiring centre that has lost its signal from the thermostat, or a boiler that is not being called for heat. A floor that is lukewarm at best is a flow temperature set too low or a system that was never balanced.

All of those are diagnosed at the manifold with the system running. Leaks in the floor itself are rare in a properly installed system and are found with thermal imaging and pressure testing, then repaired through a small opening rather than a whole floor. Electric mat faults are traced with a resistance test and a thermal camera, and a break can often be repaired through a single lifted tile.

Underfloor heating and heat pumps are a natural pair: a heat pump is most efficient at exactly the low flow temperatures a floor wants, and the large surface area of a floor means it can heat a room with water at 35 degrees. On a gas boiler, the same low flow temperature keeps a condensing boiler condensing, which is where its efficiency figure actually comes from. Either way the manifold's mixing valve and the boiler's controls are set up together, so the boiler is not firing to 70 degrees for a floor that only wants 40.