Heating control question with an oddity

[CountryFan]

It a steam powered mechanical timer!

Steam power?

 

[ericmark]

Steam power?

So likely a flash over as contacts open, still an EMC fault. There should be no connection between a smoke alarm and the heating, but cables can run in proximity to each other. The SPD should stop any spikes, but older consumer units don't have them fitted.

Cheap options first

or use an extension lead and plug the central heating into another socket, I would have a problem as my central heating uses a FCU, silly idea really, as it means can't unplug it and plug it into a power bank with mains failure, but my central heating is only item in the house not RCD protected, which is why mine does not use plug and socket.

If the smoke alarm and central heating on the same MCB/RCBO I would be looking to see if can be separated.

I’ve swapped the alarms - previously the optical alarm with 9v battery backup to multi sensor fire alarms.

That would have been my first thing to do as well. I would say the problem is done to alarms not the central heating.