My hot water boiler failed **On** - what would cause that?

Question

I arrived back from holiday to find the house like a sauna, everything in the kitchen too hot to touch, cutlery scalding hot etc. My central heating was on full, and must have been for some days.

Quick check of the thermostat showed it was working fine, but turning the central heating timer on or off had no effect. I have had to remove the fuse to the boiler power. Two days later with all windows open 24 hours a day it is still unbearably hot.

I had a look at this question, but the symptoms don't seem to match. If I pop the fuse back I have scalding hot water and radiators no matter what the settings on my central heating.

Answer 1

Despite scouring the Internet, I didn't find the solution - the gas man came today and after describing the symptoms to him he identified it immediately:

• Faulty zone valve contact switch

Basically, the sequence is as follows:

• Timer turns on - sends current to thermostat
• Temperature is too low - thermostat relay sends current to zone valve
• Zone valve turns fully on - contact switch triggers to instruct boiler to fire up
• Timer turns off or temperature reaches threshold - current no longer passed to zone valve
• Zone valve turns off - But faulty return spring kept switch on instructing boiler to remain on

It turns out that these zone valves have a plastic casing, rather than a metal one, so the return spring hooks around a small plastic spur... which just snapped off.

The fix: popping in a small screw to hook the spring round

Callout charge: £90

Answer 2

My fist thought would be a failed boiler control relay or boiler control circuit board. Depending on the age of the system the thermostat is wired to a relay or a control board. The thermostat supplies a signal which in turn closes the relay or tells the control board to fire the boiler. A failure of either component would allow the boiler to run regardless of the thermostat setting.