Don't Know What the Problem is

[BBCJ]

Can anyone help with my central heating problem.
I have a Glow Worm Micron 60FF boiler. The 3 way mid position valve has just been replaced as this was thought to be the problem but it has not solved it. Also we had been told it was a problem with sludge in the system but it has all been flushed through etc and still not solved it.

What happens is when the programmer is set to heating and water, the boiler seems to work OK with the room thermostat clicking on/off and the 3 way mid position valve is on M. Once the hot water in the hot water tank drops below temperature, the mid position valve moves across to W. When the boiler switches off after the valve has been on W, the boiler then fails to relight for sometimes more than an hour. The room thermostat clicks on calling for heat and the mid position valve moves back across to M, but the boiler fails to come on. Then suddenly, sometimes an hour or so later, the boiler relights and seems to go on and off OK .
Can anyone shed any light on this problem?

 

[D_Hailsham]

The 3 way mid position valve has just been replaced as this was thought to be the problem but it has not solved it. Also we had been told it was a problem with sludge in the system but it has all been flushed through etc and still not solved it.

Two solutions looking for a problem!

What happens is when the programmer is set to heating and water, the boiler seems to work OK with the room thermostat clicking on/off and the 3 way mid position valve is on M.

If the valve is in the M position both heating and hot water are heated.

Once the hot water in the hot water tank drops below temperature, the mid position valve moves across to W.

If the hot water is up to temperature and heating is still required, the valve should be in the H position. The valve will only be in the W position if heating is not required but hot water is.

When the boiler switches off after the valve has been on W, the boiler then fails to relight for sometimes more than an hour. The room thermostat clicks on calling for heat and the mid position valve moves back across to M, but the boiler fails to come on.

The valve should move across to H, not M, for heating only.

Then suddenly, sometimes an hour or so later, the boiler relights and seems to go on and off OK.

This could be because the hot water needs heating.

Your valve does not appear to be going across to the Heating only position. As you have already replaced the valve, this can be discounted - unless you are unlucky enough to have a dud.

This symptom will occur if there is not 240v on the valve grey wire when Hot Water is not required. There are two connections for this: a lead from the HW OFF terminal of the programmer to the grey wire; a lead from the HW thermostat "satisfied" terminal to the grey wire.

 

[Slugbabydotcom]

Once the hot water in the hot water tank drops below temperature, the mid position valve moves across to W

Do you mean reaches temperature?

Your cyl stat switches between common and 1 when theres a call for hot water.
When the temp is reached in the cylinder this switch moves the other way so that common and 2 are connected
[Sometimes 1 is common, 2 is on and 3 is off respectively.] Whatever theres usually a diagram inside the stat.
You, I am assuming a capable person, need to find out if the stat is switching properly and if it is that the current is getting to what is usually a grey wire on most diverter valves.
You need to do this when the hot water has reached the set temperature.