Boiler Running When it Shouldn’t Have Been!

[Frayed Knot]

Advice needed please.
As I’m often away from home I leave my heating permanently on with the stat set at 8deg. I came home yesterday afternoon after 2 weeks away & the house was 24 degrees!
Boiler was hot but not running, although the pump was.
Stat (Drayton Digistat) was reading 24 but was set to 8 & not showing call for heat or rf symbols. Receiver unit was unlit (not calling for heat)
Altering the temp setting down made no difference but turning it up above 24 it showed heat & rf symbols + the receiver unit rf lamp flashed normally & the green “on” light came on.
I turned the stat down & switched the heat off at the programmer & waited for the temp to drop to 20, re set the stat to 21 & it’s been working normally ever since - turned down to 12 overnight it didn’t fire up at all & it was 15 when I got up.
I have a bulk LPG tank and estimate that I’ve lost between £300 and £400 worth of gas!
At present there’s no fault - everything is working perfectly but I have to go away again in a few days & dread the same thing happening again.
I need to know what told the boiler & pump to run; was it the programmer or the stat? Or could it be a fault in the boiler?
Boiler is a conventional Glow Worm Ultracom 15hxi
Programmer is a Danfoss
Stat is a non programmable Drayton SCR Digistat.
Any advice would be greatly appreciated - at the moment, with the weather as it is, I think I may need to drain down the entire plumbing system.

 

[terryplumb]

What motorised valves do you have on the heating pipework?

 

[Frayed Knot]

Hi. As far as I can see there is just 1 Danfoss 2 port on the heat side + the same on the cylinder side.

 

[terryplumb]

The zone valve ( two port danfoss) on the heating side may have stuck open,the zone valve can keep boiler running even if programmers and thermostats are not calling for heat.

 

[Frayed Knot]

Ah! Ok, thanks, I didn’t realise that. Forgive the simplistic wording - I’m no plumber or electrician - but does the thermostat tell the motorised valve to tell the boiler & pump to run?
Unfortunately I guess it’s just something else that can’t really be tested if it’s working at the time.
Thanks again for taking an interest.

 

[terryplumb]

Yes ,if you have a programmer ,that energises ( supplies voltage to ) the thermostat, which in turn energises the zone valve ,and the zone valve energises the boiler and pump.

 

[Frayed Knot]

Right, got it. Energises, that’s the word I couldn’t think of.
I’m going to have to try to find someone to come in & check every day or so while I’m away or until it happens again & I can get it checked out.
Thanks