Could a failed HW valve cause boiler to keep running?

[lecky_bob]

(Or that might be should a failed HW valve keep boiler running)

Woke up this morning to find the boiler still running even though all timers off (HW and CH). We have a simple S-plan CH/HW system with oil-fired boiler and a 2-port zone valve for CH and a 2-port zone valve for HW. Various thermostats around but all off - including HW thermostat.

Live feed was on to the boiler from the HW valve (stuck in open position). When I manually closed the HW valve (and so closed the microswitch), it broke the live feed to boiler and everything shutdown as expected.

Would one expect the HW sticking open to call for the boiler to keep going regardless of timers?

 

[Rockingit]

Were you in my house the night before? I had exactly the same mystery! I've never actually had the need to squeeze into the cavity where my system is to work out how it's configured but I noticed the boiler was fired late at night with all CH and HW controls to off, I just turned the boiler off and went to bed, seemed to have corrected itself by morning. In my case though I've got a Nest system and half wondered if it might have been an unintended symptom of an overnight software update.

 

[Avo Mk8]

Would one expect the HW sticking open to call for the boiler to keep going regardless of timers?

Yes I believe so. The outputs of the programmer via room stat, cylinder stat operate the two port valve motors (brown wire), not the boiler directly.
There is a permanent live feed (grey wire) to the two port valves, so if the motor sticks, the boiler can receive a permanently 'on' switched live.

 

[Lucien Nunes]

Yes. The feed to the contact in the valve is permanently live, so it if jams open it will command the boiler on 24/7 independently of any other control settings.

 

[brianmoooore]

Depends on whether the valve in question is a two wire or four wire version. If it's four wire (not including any earth wire), then normally the boiler would be controlled by a microswitch connected to the grey and orange wires.