On Fri, 1 Nov 2019 08:54:23 +0000, The Marquis Saint Evremonde
wrote:

Couple of times yesterday our boiler (Grant Vortex Eco) started up when
it shouldn't have (timer-controller in 'off' phase for both HW and CH,
no tell-tale 'on' lights on).

Only came on for a couple of minutes, warmed up and then stopped. Normal
service resumed later when timer came on.

First thought was it went into frost protection mode by mistake (it's
not cold here). Not really anything in the manual about it (except the
bare mention that the boiler may have a frost protection stat or pipe
stat). How would one check?

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.