Why did the heating come on an hour early?

Almost always the clock. A programmer that lost power resets to a default time, and many older units have no automatic summer time change, so twice a year the whole schedule shifts by an hour until someone corrects it.

Where should the room thermostat be?

On an internal wall in a room used regularly, roughly 1.5 metres up, away from radiators, lamps, direct sun, draughts and doorways. Anywhere it feels heat that the rest of the house does not, it will switch off too early.

Can I fit a smart thermostat myself?

The physical swap is often simple, but the wiring behind it varies enormously between systems and the old control usually has to be removed from the circuit properly. Getting it commissioned avoids two controls fighting over the same boiler.

Low risk

Updated 30 August 2026

Heating Comes On At The Wrong Time

The programmer, timer or thermostat is ignoring you — heat arrives when you did not ask for it, or never.

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What's happening?

A central heating system takes its instructions from a chain: a programmer or timer decides when, a room thermostat decides whether the house is warm enough, thermostatic radiator valves decide room by room, and a wiring centre passes the resulting demand to the boiler and the valves. A fault anywhere in that chain looks the same from the hallway — heat at the wrong time, or no heat despite the boiler being fine. The most common causes are unglamorous: a programmer that lost its clock in a power cut and is now running yesterday's schedule, a unit that never got moved off the wrong British Summer Time setting, or a wireless thermostat that has silently unpaired from its receiver.

What usually causes it?

A mechanical or digital programmer that lost its time and date in a power cut
The clock never changed for British Summer Time, so everything runs an hour out
A wireless room thermostat that has lost pairing with its receiver, or has flat batteries
A room thermostat sited over a radiator, in a draught, or in direct sun, so it reads the wrong temperature
A failed programmer relay that sticks the heating on permanently
A motorised valve microswitch welded closed, keeping the boiler firing after the demand ends
A smart control installed over old wiring, with the boiler still being called by the original thermostat too

Is it dangerous?

Low. The worst outcome is wasted fuel and a house that is uncomfortable at the wrong hours. One case is worth flagging: heating that will not switch off at all, with the boiler firing continuously, usually means a stuck valve microswitch or a welded relay, and should be looked at rather than lived with, because the system never gets the rest it is designed to have.

Can I fix it myself?

Start with the clock. Set the programmer's current day and time correctly, then re-enter the on and off periods from scratch rather than editing them. Replace the room thermostat's batteries and, if it is wireless, follow the manufacturer's pairing sequence to reconnect it to the receiver. Check the thermostat is not sitting above a radiator, next to a lamp or in a sunny window. Make sure the boiler's own switched spur has not been turned off. If all of that is right and the heating still misbehaves, it is a wiring or component fault.

When should you call a heating engineer?

Call a heating engineer if the heating runs constantly, if the programmer will not hold its settings, or if replacing batteries and re-pairing does not restore control. Also call when you want a smart or programmable control fitted properly — a smart thermostat wired alongside an old one that is still calling for heat is a common and frustrating outcome of a self-fit.

What will the heating engineer actually do?

Trace the control wiring at the wiring centre and confirm which component is calling for heat
Test the programmer, room thermostat, cylinder thermostat and valve microswitches individually
Replace a failed programmer, thermostat or valve actuator and prove the switching
Reposition a badly sited room thermostat to a spot that reads the house honestly
Set up and commission a smart or programmable control, removing redundant legacy controls
Set sensible schedules and temperatures with you and confirm the system follows them

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Hot Water But No HeatingLow risk No Heating Or Hot WaterCaution Heat Pump Not Heating The HouseLow risk Radiators Cold At The TopLow risk

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