Why is my heating coming on at the wrong time?

If your heating has started coming on an hour early, an hour late or in the middle of the night, the cause is usually a setting rather than a broken boiler. At this time of year the most common reason is the clocks going back, which leaves older timers running on summer time. Frost protection, a smart thermostat warming the house ahead of schedule, or a timer reset by a power cut explain most of the rest.

This post explains each cause, the safe checks you can make on the controls you can already see and use, and the signs that point to a fault for a Gas Safe registered engineer. None of these checks means opening the boiler casing, and you should never try to.

Start with the clock on your timer

The clocks went back an hour at 2am on Sunday 25 October 2026. Phones and most smart devices change themselves, but many heating timers don't. If your timer is still on summer time, it runs an hour ahead, so every heating period starts an hour earlier than the clock on your wall says it should.

You'll usually have one of three kinds of timer:

• A mechanical timer: a round dial with small pins or tabs around the edge. You set the time by turning the dial until the arrow points to the right hour.

• A digital programmer: a small screen with buttons, on the boiler, on a wall nearby or in an airing cupboard.

• A smart thermostat: set from an app on your phone, and normally keeps its own clock right.

Check the time, then check the day as well. A programmer that thinks it's Saturday will run your weekend times on a weekday.

A power cut can reset the timer

Many programmers lose their clock, or go back to their factory times, after a power cut or after the boiler's fused spur has been switched off. A fused spur is the switch plate, usually near the boiler, that supplies its electricity.

If the screen is flashing, showing 00:00 or running times you never set, enter the clock and your on and off times again using the instructions for your model. If you've lost the paper instructions, the manufacturer's website usually has them.

Frost protection can switch it on at night

Many heating systems have frost protection. It may be a separate frost thermostat, a small sensor often fitted in a garage or loft where the boiler or pipes are, or a frost setting on the boiler or room thermostat. It switches the heating on when the temperature near the sensor drops close to freezing, whatever the timer says.

If the heating comes on briefly in the early hours on a cold night and then goes off again, this is the likely reason. It's protecting your pipes and boiler, so leave it on. If it runs on mild nights too, the frost setting may be set too high or the sensor may be faulty, which an engineer can check.

Smart thermostats can start early on purpose

Many smart and programmable thermostats have a feature called optimum start, early start or pre-heat. It switches the heating on before your set time so the house is already at your chosen temperature by then. On a cold morning that can mean the boiler fires up noticeably earlier than you expect.

If early starts bother you, look for this feature in the app or the thermostat's settings and turn it off. While you're there, check the thermostat isn't on a manual hold or boost, and that nobody in the house has changed the schedule.

Check whether it's the heating or the hot water

If you have a hot water cylinder, your programmer probably has separate times for heating and hot water. When the boiler fires up at an odd time, the hot water side may be the one asking for heat, so check both sets of times.

A combi boiler, which heats water straight from the mains with no cylinder, also fires every time someone runs a hot tap. Some combis have a preheat setting that fires the boiler now and then to keep water ready. Both are normal. So is the pump running on for a few minutes after the heating goes off. This is called pump overrun, and it carries leftover heat away from the boiler.

Signs it's a fault, not a setting

If you've corrected the clock, checked both sets of times and ruled out frost protection, these signs point to a fault:

• the heating runs when the timer and thermostat are both set to off

• the boiler ignores the times you set, even after you've entered them again

• the programmer screen is blank or flickers, or the buttons do nothing

• radiators get hot when only hot water is selected

These usually come from a faulty programmer or room thermostat, a wiring fault between the controls, or a motorised valve sticking open. A motorised valve is the part that opens and closes to send hot water to the radiators or the cylinder. Our post on why a boiler keeps running when the thermostat is off explains those faults in more detail.

What not to try yourself

Set the clock and times on the controls you can see, using the instructions for your model. Don't take the cover off a programmer or thermostat that's wired into the mains, don't try to free a motorised valve and never open the boiler casing. Electrical work on the controls needs a qualified person, and by law anyone working on a gas boiler must be on the Gas Safe Register.

What we check on a visit

Our Gas Safe registered engineers check how your timer, thermostat and valves are talking to the boiler, find which part is sending the wrong signal and explain the fix in plain words before any work starts. In many cases the fault is in a programmer, thermostat or valve rather than inside the boiler itself. If your boiler is also due its yearly check, our boiler servicing page explains what a service covers.

If your heating still comes on at the wrong times after these checks, RDS Plumbing Ltd can find out why. Book a visit through our boiler repair service and we'll check the controls and the boiler together.