Icos HE24 / Megaflo - boiler comes on when it should not

[clubsport911]

This forum helped me recently with a fault and now I've found another. In fact, the system has been a little wierd lately... Read on.

This morning at 3 am I was woken up by missus who had been up for half an hour worrying why the boiler was on and not going off.

It seems that indeed, the boiler was on and from the heat in the utility room had been on for a while.

None of the room stats were calling for heat
No hot taps were on
The timer controls were all inactive (no red lights)

Yet, the boiler was on.

The downstairs rads were in fact a little warm but not hot.

I then went to the timer (Horstmann) and pushed the programmer to OFF on all CH and HW. Went downstairs and switched the boiler off with the mains switch.

I then switched it back on, the fan came on, and once again, it fired up.

Where on earth is the boiler getting its ON signal from ? For the last fault, I managed to prove my boiler controller was ok by substituting with my neighbours, but this has stumped me.

Surely, the timer has control over the boiler ?
How can the boiler be fired up if the timer is off ?
Where do I start looking ?

Thanks for all your help !

PS in all other ways, the system seems fine....

 

[bewsh]

Possibly a midpos/2 port valve issue.

 

[lame plumber]

frost stat been played with

 

[clubsport911]

It was one of the Honeywell valves. it was sticky so the return spring wasn't enough to remove the pressure from the microswitch.

Stripped out the valve, lubed (fitted an extra spring for good measure)

Presto... it works fine !

Thanks to BEWSH and lame plumber for your replies !