How wet underfloor heating gets its heat
Wet underfloor heating (UFH) runs warm water from your boiler through loops of plastic pipe under the floor. It does not take water straight from the boiler at radiator temperature. Instead, the water goes to a manifold, usually in a cupboard near the heated rooms, which has several working parts:
- A pump that circulates water round the floor loops, separate from the boiler's own pump.
- A blending or mixing valve that mixes hot water from the boiler with cooler water returning from the floor, to bring it down to a safe floor temperature. Warmup's manifold guide, for example, gives an adjustable range of 20 to 60°C, set at installation to suit the floor construction and heat loss.
- Flow meters, one per loop, set by the installer to share the flow between rooms.
- Actuators, small electric heads on each loop's valve, which open when that room's thermostat calls for heat.
- A wiring centre that links the thermostats, actuators, pump and boiler. When any zone calls for heat, it starts the pump and tells the boiler to fire.
A failure at any point stops the floor warming, and working out which part has stopped is most of the job.
Why is my underfloor heating not getting warm?
Underfloor heating usually fails to get warm because hot water is not reaching the manifold, the manifold is not passing it into the loops, or a zone is not being asked for heat. The pattern of what is cold tells you which.
| What you notice | Likely cause | Where to start |
|---|---|---|
| Floors and radiators all cold | Boiler not firing | Boiler checks |
| Radiators hot, every UFH zone cold | Manifold pump, blending valve, closed isolation valves or no power to the wiring centre | Manifold checks |
| One room cold, others fine | Thermostat, actuator, stuck valve pin, closed flow meter or air in that loop | Zone checks |
| Floors warm but never warm enough | Blending valve set low, boiler flow temperature low or high tog floor covering | Settings |
| Manifold pump runs all the time | Stuck relay, failed actuator or thermostat fault | Installer or electrician |
Start at the boiler
Underfloor heating is only as warm as the water the boiler sends it. If the radiators are cold too, the problem is the boiler, and our guide to a boiler not working takes you through the checks.
Look at the boiler pressure gauge as well. Most wet UFH shares the sealed heating circuit with the boiler, so a leak or recent bleeding can drop the pressure until the boiler stops. With the system cold, it should read around 1 to 1.5 bar on most boilers. Our guide to boiler pressure too low shows how to top it up.
Check the boiler's heating flow temperature too. The blending valve can only mix water down, never up. If the boiler has been turned down low to save gas, the manifold may receive water that is barely warmer than the floor needs, and the rooms warm very slowly. On a combi, remember that the heating pauses whenever a hot tap runs, which also pauses the floor.
Finally, check the boiler's own timer or programmer. If it is in an off period, the wiring centre can ask for heat all day and the boiler will not respond. Our guide to a boiler not responding to the thermostat covers timers and controls in detail.
On a system or regular boiler, the floor is often a separate zone with its own motorised zone valve. If that valve fails to open, the floor gets nothing while the radiators or hot water carry on, the same fault described in our guide to heating not working but hot water is.
Manifold checks: pump and blending valve
If the radiators work but every UFH zone is cold, the manifold is the place to look. Nu-Heat's troubleshooting guidance lists pump failure, closed isolation valves, a stuck blending valve pin and loss of power to the wiring centre as the main causes when several zones fail at once.
- Isolation valves. The flow and return pipes into the manifold usually have lever or wheel valves. A lever in line with the pipe is open; across the pipe is closed. They are sometimes left closed after work on the system.
- Power. Check the fused spur or switch that feeds the wiring centre is on, and look for lights on the wiring centre.
- Pump. With a zone calling for heat, the manifold pump should be running. Many have an indicator light, or you can feel a light vibration on the casing. If it is silent while a zone calls, it may have failed, lost power or seized.
- Blending valve setting. Check the numbered head on the blending valve has not been turned down. The temperature gauge on the manifold flow bar shows what is going into the floor.
- Blending valve pin. Nu-Heat lists a stuck pin under the blending valve head as a common cause of every zone going cold. An installer can free or replace it.
The pump, wiring centre and actuators are usually wired at mains voltage. Do not open the wiring centre cover or disconnect any wiring. Electrical faults are a job for a qualified electrician or your UFH installer, and anything inside the boiler is for a Gas Safe registered engineer.
One zone cold: thermostat and actuator
If one room stays cold while the others warm up, the fault is almost always in that zone's control chain: the room thermostat, the signal to the wiring centre, or the actuator on that loop.
Thermostat
Replace the batteries in a wireless room thermostat, check it is not in holiday or frost mode, and turn it a few degrees above the room temperature. Some UFH thermostats show error codes such as E1 or E2. Nu-Heat's advice is to reset the thermostat as its user guide describes, and the guide also explains what each code means. The wiring centre usually has a light for each zone that shows when the zone is calling for heat. Our guide to the boiler thermostat explains the difference between a room thermostat and the boiler's own temperature control.
Actuators
Most UFH actuators are thermoelectric: a small heater inside expands a wax element that opens the valve. They take a few minutes to open after the thermostat calls, so a zone that has just been turned up will not respond instantly. Many actuators have a coloured indicator that shows when they are open.
If the zone is calling and the actuator shows closed after ten minutes or so, the actuator may have failed. If it shows open but the loop stays cold, the valve pin underneath may be stuck. Nu-Heat's guidance describes removing the actuator to check the pin moves freely, but only do this if your actuator unclips without disconnecting any wiring and your system manual shows how. Otherwise, ask the installer.
Flow meters and air
Check the flow meter for the cold loop. If it reads zero while the actuator is open, the loop may be closed at the flow meter or blocked with air. Air in a UFH loop cannot be bled from a radiator valve; the loop is flushed through the manifold's fill and drain valves, which is a job for a heating engineer. Do not change flow meter settings on loops that are working, because they were set at commissioning to balance the whole floor.
Floors that warm up but never feel warm enough
Underfloor heating works at a much lower water temperature than radiators, so the floor should feel pleasantly warm rather than hot. Screed floors in particular take hours to warm through, and most installers recommend steady, longer heating periods rather than short bursts.
If rooms never reach temperature, check the blending valve setting and the boiler flow temperature first. Floor coverings matter too: a thick carpet and underlay, or a large rug, insulates the floor and holds heat back. Check the maximum tog rating in your system's design documents before choosing new flooring. If the problem appeared after an extension or a new floor finish, the system may need its design heat output checking by the installer.
When to call an installer or engineer
Contact the original installer first if the system is still under warranty, because they hold the design documents, flow settings and wiring diagram. Call a qualified electrician for wiring centre, pump power or thermostat wiring faults. A heating engineer can flush air from the loops, replace a pump, blending valve or actuator and rebalance the flow meters. If the boiler itself will not fire, you need a Gas Safe registered engineer. When you call, say which zones are cold, whether the radiators work and what the manifold pump and actuator indicators show.