Help! Boiler firing with CH and HW off!

6 Mar 2008
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United Kingdom
Not sure what's going on.
I have an oil fired system boiler - Warmflow.
I noticed this morning it seemed a little noisy - the boiler was hot and the pump was running.
I turned off the CH and HW at the timer - the pump still continued to run. I guess this is normal to prevent the boiler overheating (?).
More strangely, the boiler then fired up even though the ch and hw were switched to OFF on the timer. How can something be calling for hot water when these are off at the timer?
It also fired when I turned the thermostat on the boiler up, even though the ch and hw are turned off at the timer. How can this be?
Any advice please?
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Oh yes - I've got TWO of those.
The flow from the boiler goes through the Magnaclean then splits for the CH and HW.
Water pressure is a little low - about .75 bar and still cooling.

If the pump continues to run when the hw switches off, presumably this ensures the boiler doesn't overheat. There is a gate-valve by pass linking flow and return just after the Maganclean which is slightly open.

I don't understand how the boiler could actually fire when hw and ch are off, just by turning up the thermostat.

Can anyone make sense of this?
One of the valves that gas man has shown is stuck & so the switch its making is firing the boiler each time the stat calls for heat , new valve needed if it wont free off , does it have a bump on the top of the outer case
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The micro switch on one of the valves is still closed providing a signal for the boiler to fire. It also signals the pump to run as well. Identify which one it is and repair or replace.
Heres what I have.


Not sure about bumps on the top??

So when the hw tank is up to temp, it's thermostat should send a signal to the control box, which tells the valve to close.... which tells the boiler to stop?
So you're saying the microswitch in the valve is sticking, so not telling the valve to close.? Or is it sticking when the valve closes, and not telling the boiler to stop.?

So if/when the situation happens again, I would expect to find the valve open. However I can manually close it. I think.

Your comments much appreciated. Thanks.
on honeywell valves with a hump on the outer case you can change just the heads if they are faulty. Yours arent honeywell but on the later type of them theres a black button you press & the head just lifts off & you fit a new one. Dont know if that is a replaceable head , identify which valves jamming & replace it whole . The system will need draining. If it runs all the timeif you turn the room stat up its the valve on the heating side
sticking when the valve closes, and not telling the boiler to stop.?
That is exactly what is happening. On the side we can't see, if there is a black push button, the powerhead can be removed.
If it's only recently happened its more likely to be the HW valve as the CH won't have been used. The only true way to tell is by electrical testing.
Well thank you both once again for your info.

I cleaned out the Magnaclean today while checking things - seriously sludgy!

The valve tops are removeable and I took them off and blew them out - perhaps some dust build up inside could have caused the problem.

Anyway, the hw came on and went off tonight as normal, but now I know what to look for if/when it happens again.

Thanks for the education!
Its not the dust causing the issue its either the syncron motors inside the valves on its way out.... or the dirt in the system is causing the moving parts inside the valve that are in contact with the dirty water to sieze up . If it happens again remove the valve head & get a pair of grips/pliers & move the metal spindle back & to carefully to free it of off. Drain your system & let it run till its clear . Then add inhibitor & refill it . If it keeps playing up replace it whole with a honeywell valve

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