Why does my heating come on at the wrong time of day?

Heating that comes on at odd hours is rarely a fault. Programmers are small clocks, and small clocks drift, lose time in a power cut, and are easy to set to the wrong half of the day.

What you'll need

• The programmer's instructions if you have them
• A phone for an accurate time reference

Step by step

1. Compare the programmer's clock with your phone.Check both the time and the day. Mechanical programmers drift noticeably over months, and a power cut resets many digital ones.
2. Check AM against PM.On a 12-hour display this is the most common single error, and it produces heating that runs exactly twelve hours out. Many programmers show a small AM/PM marker that is easy to miss.
3. Check the clocks-changing adjustment.Most simple programmers do not adjust for British Summer Time. If the heating went wrong in late March or late October, that is why.
4. Check which programme is running.Many programmers have weekday and weekend settings, or a seven-day programme. Heating that is right on weekdays and wrong at weekends is a programme selection issue.
5. Check for an override left on.Advance, boost, constant, or a holiday mode. These are easy to press by accident and stay on until cancelled. Look for an indicator light or symbol.
6. Look at the mode switch.Timer, on, off, once, twice. A programmer left on 'on' runs continuously regardless of the schedule; one on 'once' gives a single period a day.
7. Check for a second control.A smart thermostat or an app added later can override the old programmer, or the old programmer can override the smart one. Two schedules in conflict produce baffling behaviour.
8. Check the battery in a wireless control.A wireless thermostat or programmer with a failing battery can lose its schedule or its link and revert to a default.
9. Write the intended schedule down before reprogramming.Decide on paper what you want, then enter it. Reprogramming on the fly at the box is how conflicting periods get created.
10. Recheck a week later.If the clock has drifted again in a week, the programmer itself is failing and is worth replacing — they are inexpensive.

Tips

• Check the clock twice a year when the clocks change. It takes a minute and prevents six months of odd behaviour.
• Photograph the programmer's settings once you have them right. It makes restoring them after a power cut trivial.
• If two controls have been fitted over the years, work out which one is actually in charge before adjusting either.

Common mistakes

• Assuming the boiler is faulty — Heating running at the wrong hours with the right duration is a clock problem, not a boiler one. Calling an engineer for it is an expensive way to have the time reset.
• Leaving the programmer on 'constant' through winter — It runs the heating whenever the room thermostat calls, all night included. It is easy to switch on during a cold snap and forget about.

If it doesn't work

Questions people ask