CH won't work without heating water?

[mjr1972]

I have a Worcester Danesmoor 20/25 boiler that is pretty old with a separate control unit/timer that controls both central heating and hot water.

Over the last few months I noticed that the CH would only work if I also lit up the Hot Water light as well.

Today I tried to put the CH on (by manually advancing both the CH & HW to 24hrs) and noticed that for 3 hours I could not get the CH to work. It started to warm the rads but then stopped (no lockout light).

I finally figured out that it was not working as the boiler did not need to heat up any hot water (i.e. the HW tank was full and to temperature). When I upped the HW tank thermostat by a few degrees the boiler fired up and now I am lovely and warm again!!!

Anyone know or have any ideas what might be wrong?

Thanks,

Mark

 

[MANDATE]

Yes! Although you have not described your system fully, I would guess you have a 3 port mid position valve.
It seems to be a common problem, when you cant get CH only but you do get some heating along with HW.
It means the valve moves to mid point and works ok, that is until it tries to go to CH only, or you select CH only.
It means that the valve does not go beyond mid point due to a failed motor and/or stiff spindle. On the other hand it may well go to the CH only position and if the boiler does not light, this will be due to a failed microswitch.
There's plenty on here about 3 port mid position valves

 

[mjr1972]

Thanks for the reply. I believe I do have a 3 port valve.

Is there an easy way to check whether the spindle is "broken" or the motor (a Honeywell unit)? There is a slider on the rear side of the Honeywell unit which I wondered if it would manually move the spindle?

If I can decide which part (or both) need replacing then I can decide whether I should do it myself or call in someone. The Honeywell unit says 9/9/2003 on it which I presume is when it was fitted.

Mark

 

[MANDATE]

The spindle won't be 'broken' as broken in half. It may well be stiffish which does not help. You should be able to remove the actuator head off the valve spindle. Then you can check the spindle for stiffness,
While the head is off you can also try it in the different positions to see if it goes to CH only. If it does (microswitch fault) if it does not (motor fault).
That is assuming it did once work and the wiring is ok.
The lever on the side will move the spindle, but only from the HW position to just past mid point. The lever turns the quadrant/motor against the return spring and its purpose is to use the latch to keep valve in mid position while refilling a drained system,(prevents air locks)
I've had same problem twice. faulty microswitch 1st time then motor 2nd time.
I replaced the whole head both times (about £40 ish)