Interesting Boiler Problem

[davidpashley]

Hi folks,

(sorry this is a bit long)

Turning in at 1am last night, I noticed it was a bit warm to say that the heating is supposed to go off at 10.30pm.

Sure enough, the radiators were on. According to the stat (CM67) it was

21C, despite the demand being set at 18C. Being too tired to worry about it, I turned the CH off at the boiler and went to bed.

We have a trusty old Worcester Heatslave Highflow combi - normally very reliable, and totally silent. Because the clockwork programmer on the front of it is a bit dodgy, we normally leave everything on continuous, and rely on the CM67 to control the heating.

This morning, woke up at 7am (after 5 hours sleep - which is sheer luxury when you have an 8-week-old baby) thinking "hmm, a bit warm to say that the heating's off".

Sure enough, the radiators were on, and temperature was 20.5C. Interesting. Turning the HW off as well as the CH did the trick.

Now if the heating had been on full blast all night we would probably have cooked. So my guess is as follows:

On a combi, every time the internal HW store cools down a bit, the boiler comes on for a quick blast to restore the temperature. In any case of HW demand, the CH valve shuts. If the CH valve is stuck open, then once the boiler attempts to reheat the water store, the radiators will be heated, and it will take much longer to restore the stored HW temp. Eventually HW temp will be restored and everything will turn off.

To test this theory I tried to run some HW when the radiators were cool. As expected the HW was running virtually cold - I think that confirms that the CH valve is stuck open.

Anyone care to agree or disagree with my diagnosis?

If I am correct, how do I fix it? Is it a DIY, or should I call in a boiler man?

TIA

David Pashley

[lee]

Do you have an external frost stat or built in frost stat on the boiler that would override the controller settings?

Lee

[John Rumm]

Did the boiler fire up in response to you turning the tap on? Did the rads start to get hot?

If the boiler fires ok when you run the tap then that is certainly possible.

Other items like the flow switch or HW temperature sensor may also be in the frame depending on the exact symptoms.

It is diyable, but it depends a bit on your confidence and skill level...

Have a look at the service and install manual for the boiler (download it from the makers web site if need be).

Work out from the diagnostic flowcharts what bit is knackerd, and then look at the procedure for changing it and see if you feel up to it.

Assuming it is the valve that has the problem it might be down to the wet bit of it getting stuck (in wich case just operating it a few times by hand may free it), or the actuator failing (either because it has failed, or the wiring to it has a problem, or possibly even the PCB has a fault).

Replacing the actuator on some boilers is very simple (on my one you unplug one lead, and it is held in place by a bayonet action - so just push and turn to get it off).

[davidpashley]

Hi John,

Thanks for your help.

John Rumm wrote:

Yes and yes.

The boiler responds normally to flow and to varying the HW stat, so I'm definitely suspecting the 3-way valve.

I've taken a look and the valve seems to be of the diverter type (i.e it's 3-way, but has a single L/N/E connection). No sign of movement when I switch things on and off, and the override lever is permanently loose.

Would anyone care to guess whether this is an actuator or valve failure? I'm tempted to try replacing the actuator.

Thanks again.

David

[Roger Mills (aka Set Square)]

Can you remove the actuator from the valve whilst leaving it electrically connected? If so, see whether it moves like it should when you run the heating and then turn on a hot tap. Also, if you can, move the valve spindle to the HW position and make sure that the HW works ok. You should be able to work out from what happens which bit is faulty.

[John Rumm]

If it is the type that pushes onto a peg that sticks out of the body of the divertor then that may indicate that the peg is stuck in the depressed position.

If above then the valve, by try Roger's diagnostic to see which is at fault.