Baxi fire up ...even when CH/ water off!

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Hi, we have noticed the boiler running when CH / underfloor heating are not "on" Had the same problem a year ago....engineer said boiler was ok (2yrs old).. but a problem with Honeywell diverter valve....he showed me the spring which is slack when "On"...then spring activates when "off". I don't know what he did but it sorted it out he did not install any new component. Seems that the D Valve controlling the underfloor heating allows the boiler to work when stat is in off position.

Does this make sense?....what might he have done? thanks .
 
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If the Honeywell valve you are referring to looks like something like below. (Some types have two ports as shown, others have 3)


Then they have a small microswitch in. When the valve is open, the mechanism operates the switch which starts the heating up. These switches can stick in the on position and keep the boiler running, or the valve itself may stick and require freeing and lubrication. With the system off, when moving the lever from Auto to Man you should feel a slight resistance. If not the valve may be stuck open.

Another cause of the boiler firing is where a frost thermostat is fitted to the system. This overrides the time controls and turns on the heating to prevent freezing. These are normally fitted where the boiler or pipework are installed in an unheated area.
 
Thanks...this happened last year. Man came out....I think all he did was to move the lever a few times. I did same last night...also put new batteries in thermostat. Guess what?...this am it's all ok!! Thanks for the help.
 
PS.....If the diverter eventually fails, Is replacement straightforward? I am ok to take on simple wiring jobs. Did elec engineering some time ago.
 
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It depends on the valve. With the example above, if the existing head has a bump on the cover, the head can be removed and replaced without having to drain the system down and then it's just a matter of swapping the wires over like for like, so should be well within your skill. You can find the instructions here.
 

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