The Wiring Centre Is the Heart of It
The wiring centre is the box, normally near the manifold, that everything plugs into. Every room thermostat wires into it, every actuator on the manifold wires out of it, and it also drives the manifold pump and sends the call for heat to the boiler. Nearly every fault on this page is a fault in that box or in something connected to it.
The important thing to understand is that a thermostat calling for heat does not, by itself, make a boiler fire. The wiring centre has to pass that demand on. When that link is missing or wrong, everything looks like it is working right up until the point where nothing gets warm.
Before you go any further
In most homes the underfloor control circuit is at mains voltage. Do not open a wiring centre, remove its cover, or disturb any wiring. Everything on this page is here so you understand what is happening and can describe it accurately, not so it can be worked on. Diagnosis and repair here is work for a competent electrician or heating engineer, and anything involving the boiler is Gas Safe registered work.
Common Wiring Faults and What They Mean
A thermostat calls, but the boiler never fires
You can hear or feel the system trying: the actuator may open and the manifold pump may even run, but no heat ever arrives because the boiler was never asked for any.
- Most likely cause
- The call-for-heat link between the wiring centre and the boiler is not wired, is wired to the wrong terminals, or the volt-free contact has failed.
- Also possible
- The relay inside the wiring centre that makes that connection has failed, which often follows years of use or a power surge.
- Tell-tale sign
- The pump runs and the actuators open, but the boiler stays silent and the water in the loops never gets warm.
- What the fix involves
- Testing whether the demand reaches the boiler terminals, then repairing the link or replacing the failed component.
The pump or boiler runs constantly and never switches off
The heating never rests, even with every thermostat turned right down and the house already warm.
- Most likely cause
- A relay in the wiring centre has stuck closed, holding a permanent demand on.
- Also possible
- A thermostat wired to the wrong terminal, so it is effectively calling all the time regardless of what it displays.
- Also possible
- A permanent live connected where a switched live should be, which is a wiring error rather than a failure.
- Why it matters
- Beyond the running cost, a floor being heated continuously is uncomfortable and can overheat floor coverings.
- What the fix involves
- Establishing where the permanent demand is coming from, then replacing the stuck component or correcting the connection.
Turning up one thermostat heats a different room
The system responds, but not in the room you asked for. Sometimes two rooms are simply swapped.
- Most likely cause
- Thermostat channels crossed against manifold ports in the wiring centre.
- Root cause
- The loops at the manifold were never labelled during installation, so which pipe served which room was guessed at when the wiring was done.
- What the fix involves
- Identifying which loop actually serves which room by warming them one at a time, correcting the wiring to match, and labelling the manifold permanently so it is never guesswork again.
Every actuator opens whenever any thermostat calls
The entire floor heats up whenever a single room asks for heat, so zoning effectively does not exist.
- Most likely cause
- All the actuators have been wired to a single channel rather than to their own, so the wiring centre cannot control them separately.
- Also possible
- Live and common connections confused, so powering one output powers them all.
- Why it matters
- You lose the whole benefit of zoning, and rooms that should be cool get heated every time anywhere in the house calls.
- What the fix involves
- Rewiring the actuators onto their correct individual channels and verifying each one against its room.
Nothing works after building work or a new consumer unit
The underfloor heating was fine, then stopped entirely after unrelated electrical work.
- Most likely cause
- The supply to the wiring centre has been lost, or the fused spur feeding it has been switched off or left disconnected.
- Also possible
- An RCD tripping because of a fault elsewhere is cutting the circuit the heating shares.
- What the fix involves
- Confirming the supply is present and the circuit is intact, then finding the underlying reason it was lost.
An actuator failed immediately after being replaced
A new part was fitted and either did nothing at all or failed within moments.
- Most likely cause
- A voltage mismatch. Underfloor systems run either 230V or 24V control circuits, and the actuator has to match the wiring centre.
- What happens
- A 24V actuator on a 230V circuit is destroyed straight away. A 230V actuator on a 24V circuit simply never opens.
- Also possible
- The actuator does not suit the valve bodies on that particular manifold, so it is not actually lifting the pin even though it is powered.
- What the fix involves
- Confirming the system voltage and the manifold type, then fitting the correct actuator.
Adding underfloor heating to an existing radiator system
Not a fault as such, but the point at which more wiring problems are created than anywhere else.
- The difficulty
- The existing system already has its own zone valves and controls calling the boiler. The underfloor wiring centre has to work with that, not against it.
- Common outcome when rushed
- Hot water priority stops working, one system permanently calls the boiler, or the two fight so that neither controls temperature properly.
- What doing it properly involves
- Interfacing the underfloor wiring centre with the existing arrangement so each can call for heat independently, hot water priority is preserved, and the whole thing is documented.
Inherited and Undocumented Wiring
A large share of the wiring work we do is on systems somebody else installed, where nothing was labelled and no drawing was ever left behind. It is genuinely common to open a wiring centre and find unmarked cores, ports that do not match the rooms, and a previous repair that solved one symptom by creating another.
The work is methodical rather than dramatic: establish which loop physically serves which room, establish what each thermostat is actually connected to, correct what is wrong, then label the manifold and the wiring centre so the next person has something to work from. Systems described as never having worked properly quite often turn out to be perfectly good systems that were simply wired wrong.
Related Faults
If the boiler does fire but the floor still does not warm up, see cold spots and flow problems. If a thermostat is dead or unresponsive rather than miswired, see controls and thermostat faults. If the boiler fires but keeps stopping again, see boiler and underfloor heating problems.
Wiring Fault FAQs
Why does my thermostat call for heat but the boiler never fires?
The demand is not reaching the boiler. In an underfloor system the thermostat talks to a wiring centre, and the wiring centre is what actually calls the boiler, usually through a volt-free contact. If that link is wired incorrectly, has never been connected, or the relay behind it has failed, the actuator may open and the pump may even run, but there is no hot water because the boiler was never asked for any.
Why is the wrong room heating up?
The thermostat channels and the manifold ports have been crossed, so a demand from one room opens the valve for another. It is a common find on systems where the wiring was done in a hurry, or where the loops were never labelled at the manifold and were guessed at afterwards.
Why does my pump or boiler run constantly with underfloor heating?
Something is holding a demand on permanently. The usual causes are a stuck relay in the wiring centre, a thermostat wired to the wrong terminal so it is effectively always calling, or a permanent live connected where a switched live should be.
Can 24V and 230V underfloor heating parts be mixed?
No. The thermostats, wiring centre and actuators all have to be on the same control voltage. Fitting a 24V actuator to a 230V wiring centre destroys it immediately, and the reverse simply never opens. This catches people out when replacing a single failed actuator with whatever was available.
Can you sort out underfloor heating wiring left by somebody else?
Yes, and it is a regular job for us. We trace what each thermostat and each manifold port is actually connected to, correct anything that is crossed or wrong, and label it so that the next person to look at it, including you, can understand it.
Can underfloor heating be added to an existing radiator system?
Usually yes. It generally means interfacing the underfloor wiring centre with the existing zone valve arrangement so both can call the boiler correctly, and making sure hot water priority still works. The wiring is the part that needs the most thought, because the two control systems have to cooperate rather than fight.