Can’t turn heating off

[Ian Gregson]

Hi all,
I’ve a problem with my heating. It’s a S-Plan with 2 Honeywell motorised valves. All was good till last week. Hot water runs fine and goes off when programmer tells it too. Heating will not go off even when programmer isn’t asking it to. I’ve replaced heating motorised valve and changed the programmer (Honeywell st9400c) but still the problem persists. Only way to stop heating is to knock fuse spur off then it does stay off. Any ideas Thanks

 

[picasso]

Either a wiring mistake or you have changed the wrong valve.

 

[Ian Gregson]

Thanks for the reply. It’s been working fine for 10 years. Wired into a Honeywell wiring centre. So it’s wired as it’s always been.

 

[Exedon]

Multimeter and find where permanent live is coming from
Maybe a clear picture of your wiring center?

 

[Ian Gregson]

 

[FiremanT]

Have you a Frost Thermostat (in the loft, maybe?)

Some boilers have a built in Frost stat, what model have you?

 

[Ian Gregson]

Hi, it’s an oil fired firebird. No frost stat.