Boiler only fires for CH

27 Dec 2010
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United Kingdom
Odd issue with friend's hot water system. He's just moved into the house, so no idea how long it's been like this. System is a standard closed loop combined HW and CH with hot cylinder. Controller is a Honeywell ST6400C.

Basically, the boiler won't fire at all on the HW cycle of the controller, only on CH or CH and HW. On CH and HW, the tank heating loop is active, so the tank will heat. I've fiddled with the cylinder stat, it's currently set at 60 but adjusting makes no difference to anything.

Any ideas as to what's wrong or the next thing to test?

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Yep, two two port motorised valves on the output side of the pump. One into CH circuit and one into cylinder heating loop. Both Honeywell brand.

The cylinder one can't have failed closed as the tank heats when the CH is on. Is there any way of determining the valve position from outside the casing?
Not possible without removing the heads. If yours have a raised dimple on the grey case then it can be removed from the body. You can then check the spindles are free to move and you can also see the motor working.
I suspect a failed motor and/or a seized valve. Work with the power off while removing covers etc.
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Why not do as I suggest first? If I were on site, I would do those checks first given the symptoms.
Lock the heating valve open see if the boiler comes on that's the best way it's what I would do

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