Unexpected boiler startup

Most frost protection is set at around 7°C so that heating comes on well before pipes start to freeze. I suspect your boiler went into frost protection mode. If the boiler is a combi then you can often hear them firing up for a few minutes just to ensure there is a reservoir of warm water for DHW when you turn the tap on. You can stop it by putting the boiler in eco mode if it has one the draw back being that when you want hot water you have to wait a while for it to heat and flush the cold out of the pipework.

Richard

This also happened with my heating system. Have a look at the motorised valve. Even though the timer called for both CH and HW off the heating was running.

The timer controls the motorised valve which in turn controls the boiler. In my case the CH valve motor gears disintegrated leaving the valve in such a position that the boiler was running and CH on.

Fortunately the motor separates from the valve easily so the repair was simple.

Blimey we have been doing that for many years. modern lads don't know they bin born laddie. grin. Brian

I had the same thing, but instead of a disintegrated valvegear, it just needed removing, cleaning, and reassembling. Definitely the first thing to check.