Boiler only fires for CH

timbim · 5 Jul 2013

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?

Thanks.

45yearsagasman · 5 Jul 2013

View media item 38425 View media item 44939
Got any thing like these near the cylinder?

timbim · 5 Jul 2013

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?

45yearsagasman · 5 Jul 2013

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.

timbim · 5 Jul 2013

45yearsagasman said:
I suspect a failed motor and/or a seized valve. Work with the power off while removing covers etc.

What leads you to that conclusion, and how is it consistent with the cylinder heating when the boiler is on?

45yearsagasman · 5 Jul 2013

Why not do as I suggest first? If I were on site, I would do those checks first given the symptoms.

oilboffin · 5 Jul 2013

Lock the heating valve open see if the boiler comes on that's the best way it's what I would do

diynot

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Boiler only fires for CH

Similar threads