Why Your Programmer Reset After a Power Cut

A heating programmer that has lost its settings after a power cut is behaving normally, not breaking. Most programmers and thermostats keep time and schedule in memory that depends on mains power, so when the supply goes off the clock stops or resets, and the heating and hot water come back on the wrong schedule or not at all. The first thing to do when the power returns is check the clock and the timer, then put your schedule back.

Your boiler should restart by itself. SGN, a gas network operator, says that when the electricity supply returns, your boiler should work without any intervention¹. Most boilers use electricity for pumps, fans and electronic controls, which is why they stop during a cut, but they should come back when power does¹. If the boiler stays silent while the programmer is lit, the problem is likely the boiler rather than the programmer.

The work after an outage is mostly small: reset the clock, check the thermostat timer, reprogramme the schedule, and check that the frost protection setting is still where you left it. This page sets out why settings are lost, what to check first, how to tell a reset from a fault, and how to keep your settings through the next cut.

Why a programmer loses its settings when the power goes off

A programmer is an electric timing device. It runs a clock, holds a schedule in memory and switches the boiler and hot water on and off at set times. All of that depends on the mains supply. When the power goes off, the clock stops or resets, and the schedule may be held only while the unit has power. That is why the heating can come on at the wrong time, or not at all, after an outage.

The same applies to the thermostat. One network operator's advice after a cut is to check the timer to make sure it is correct, and to reset the timer and thermostat as soon as the electricity comes back on⁶. Another council's guidance is to reset the clocks on any electric timing devices and check the timer on your thermostat is correct². Neither is a sign of a fault: it is the expected behaviour of equipment that keeps time using mains electricity.

Modern heating systems add another layer. Heat pumps stop during a planned outage and resume normal operation once power returns⁷. Solar panels, heat pumps, EV chargers and batteries automatically switch off during a power cut to protect the home and engineers, and should not be damaged⁸. Smart equipment such as boiler controls can be affected when power goes off and comes back on, and one operator advises disconnecting smart equipment before a planned cut⁸. None of this means the equipment is broken, but it does mean the settings need checking once the power is back.

Check the clock and time first

Before you touch the schedule, check the clock. A programmer with the wrong time will run the right schedule at the wrong hours, so the heating may come on at midnight and stay off in the morning. Council guidance after a power cut is to reset the clocks on any electric timing devices and check the timer on your thermostat is correct². One network operator puts it plainly: check your timer to make sure it is correct, and reset the timer and thermostat as soon as your electricity comes back on⁶.

Some equipment will not work at all until its clock is set. One operator notes that some equipment such as ovens may not work until the clock has been reset⁹. The same principle applies to heating controls: set the time first, then the schedule.

If the power has not come back when you expected, check before you assume a fault. For a planned cut, 5 pm is the latest time one network operator will turn the power back on¹⁰. Emergency power cuts are normally restored after the three hours, unless operational reasons prevent it, and power might be switched off or reconnected around 30 minutes before or after the published time⁴. For planned cuts, one source says power should be back within one hour after the expected time of reconnection¹¹. If it is not, report it: call 105 or use the Power Cut website to be directed to the operator for your area⁵.

Resetting the heating and hot water schedule

Once the clock is right, put the schedule back. Lancashire Preparedness advises that you may need to reprogramme many of your electronic devices after a power cut³. For heating, that means the on and off times for heating and hot water, the temperature settings, and any holiday or frost mode.

Work through it in this order:

1. Set the clock to the correct time and date².
2. Check the thermostat timer and reset it if it is wrong⁶.
3. Re-enter the heating on and off times for each day.
4. Re-enter the hot water times if your system has a cylinder.
5. Check the frost protection setting is still on and set to the temperature you want.
6. You may need to reprogramme many of your electronic devices⁸.

If your system has a hot water cylinder, the programmer usually controls the cylinder's heating times as well as the radiators, so both schedules need checking. If you have a combi boiler, hot water is usually on demand and the programmer mainly controls the heating, but the clock still matters for the heating schedule.

Boiler or programmer: telling a reset from a fault

The useful test is simple. If the programmer display is lit and the clock is running but the boiler does nothing, the problem is likely at the boiler. If the boiler fires when you press the programmer's override or advance button, the programmer is working and the schedule is the issue.

SGN's guidance is that when the electricity supply returns, your boiler should work without any intervention¹. If it does not, and the problem is unrelated to the power cut, you will need to contact a Gas Safe registered engineer to come out and take a look¹². Northern Gas Networks gives the same advice: if your boiler is not working, but is unrelated to a power cut, contact a Gas Safe registered engineer, via the Gas Safe Register website or 0800 408 5500¹³.

A flashing display can be either. On one Ideal boiler, a flashing L and 4 means reset the boiler, while a flashing L and C means the boiler has been reset too many times in 15 minutes, and the maker's instruction is to turn the power to the boiler on and off again to reset¹⁴. Check your own maker's fault code list before resetting repeatedly. Our guide to boiler fault codes, flashing lights and lockouts explains what the common codes mean, and how many times it is safe to reset a boiler lockout covers the reset limit.

Keeping your settings through the next power cut

You cannot stop a power cut resetting a programmer that depends on mains power, but you can make the recovery quick and keep the house safe in the meantime.

Before a planned cut:

• Turn the temperature settings down a couple of hours before the power is due to be switched off, so the fridge and freezer hold their cold⁹.
• Disconnect smart equipment such as boiler controls before the power goes off and comes back on⁸.
• Fill a flask with hot water and make up a hot water bottle if you get advance notice¹⁵.
• Keep a mobile phone charged and write down important phone numbers¹⁵.

During the cut:

• Leave a light on so you know when power is restored¹⁶.
• Switch off all electrical appliances that should not be left unattended, ready for when the power comes back on¹⁷.
• Keep fridge and freezer doors closed².

After the cut:

• Reset the clocks on electric timing devices and check the thermostat timer².
• You may need to reprogramme many of your electronic devices³.
• Check the frost protection setting.
• Check on vulnerable neighbours or family¹⁵.

If you have a heat pump, it will stop during the outage and resume normal operation once power returns⁷. Our guide to heat pump breakdowns in cold weather covers what to check if it does not.

When to call a heating engineer

Call a Gas Safe registered engineer when the boiler does not restart after the power returns and the programmer is working, when the boiler shows a fault code you cannot clear with a single reset, or when you smell gas or see other warning signs. If you have been disconnected from the gas, electricity, oil or water supply, you must use a qualified engineer to get reconnected¹³.

If your electricity has not been restored, contact your network operator first. One operator asks customers to call its helpline on 03457643643 if supply has not been restored after four hours¹⁹. More widely, call 105 or visit the Power Cut website to be directed to the operator for your area⁵. If you spot damaged or dangerous electrical equipment, including during or after a power cut, report the problem, but call 999 in an emergency⁵.

If you rent your home and a heating appliance has been disconnected, the landlord must provide the tenant with emergency heating, contacting the local authority for more information, while arranging remedial work by a Gas Safe registered engineer²⁰. Our guide to broken boilers in rented homes sets out what your landlord must do.

If your home has been affected by a fire, and you have been disconnected from the gas, electricity, oil or water supply, you must use a qualified engineer to get reconnected²¹. Firefighters may have turned off or disconnected your supplies, and only a qualified trade person should reconnect you²².

For the wider picture, see our guides to power cuts, gas and heating failures and backup power at home.