Why does my central heating system comes on on its own accord?

Most heating systems are setup to automatically turn on and off based upon the setting of a thermostat. When heat is required (i.e. thermostat reading lower than setting) then the heat system will come on till the thermostat reads a new temperature higher than the setting.

If your system is operating outside this automated procedure then it is possible that the any one of the following has failed in some way:

• Thermostat not reading correctly
• Wiring from thermostat to heat system is damaged
• Relay and control board on heat system has failed or gone intermittent.

You may need the aid of a qualified heating specialist to troubleshoot problems like this.

If your system also heats your domestic hot water, it may be coming on to maintain the hot water temperature. Depending on the setup, you may be getting some hot water circulation through the radiators when this happens. Is the radiator system steam, or hot water? If it's hot water, can you tell of one or more of the circulators come on when the boiler runs?

If you have a programmable thermostat check every program every day. It is possible a group program was set. I would suspect a thermostat issue 1st. Including a deflective one. If the weather has been below freezing it is possible there is a sensor over ride on a pipe that is suppose to engage only when freezing is imminent.

You might want to check the manual for your boiler: some models of boiler will automatically activate for a short period of time once per 24 hours in order to avoid issues like a pump sticking due to lack of use. If you have such a boiler, then what you're seeing is intended behaviour, and will be described in the manual.

This could also be caused by frost protection: many boilers/thermostats will attempt to warm up the system if it's at risk of freezing (as frozen pipes often cause leaks), even if it's switched off at the timer or thermostat. If the temperature is below 10°C, then this seems like a very likely cause of the issue.

Also, note that the minimum of most thermostats is normally some specific temperature (e.g. 9°C), rather than "off" (the "completely off" setting is normally set on the timer, not the thermostat). If the interior temperature falls below that value, then the thermostat will be "on" at any setting, because no matter what temperature you set the thermostat to, the interior temperature will be lower than the thermostat setting – and that's the rule the thermostat uses to decide whether to turn on or not.