Hot water but sporadic CH - no idea how to proceed

[xpiranhax]

Hi all,

I've recently bought a new house and have a problem with the CH that I can't work out.

HW works fine - it heats up, it gets used and it heats up again.

However, the radiators will only heat up when the HW is being heated. If I turn the HW off, then nothing happens with the CH. Also, once the HW has heated, the radiators stop heating.

The pump seems to be working fine as HW is ok and the radiators do heat up, but only temporarily.

Does anyone have an idea what could be causing this? Could it be a faulty room stat?

Thanks in advance to anyone who can help.

 

[MANDATE]

My money says you have a 3 port mid positon valve.
The fault is probabley a duff micro switch in the motorhead or a duff motor. Everything is ok when both HW and CH is called for, because the boiler/pump is getting the power from the HW side, but as soon as HW is satisfied and the demand is for CH only, the valve either does not move the CH position or if it does the microswitch does not operate and no power to the boiler. You can usually remove the motorhead, you can check the valve spindle for stiffness, you can observe the valve moving when CH is selected and you can check for voltage on the orange wire with HW off of course.
A new motor I believe is around £15. A microswitch £1 ( can you fit it, probablely too difficult) A new motor head about £40 ( can you fit it ? Yes!)

 

[xpiranhax]

Thanks very much for the advice.

If I move the manual lever on the side of the valve it works fine and can get independent HW and CH (or both at once). Does this mean it is the motor that needs replacing?

Is it necessary to replace or can I leave it as is?

Thanks again for any help

 

[MANDATE]

With HW off and CH on you have moved the valve to the CH position and as it has functioned ok it proves the microswitch is operating, which means the valve is only reaching the correct positon manually. This may be due to a stiff spindle. Don't forget the motor has to drive the valve quadrant against the spring so a stiffish spindle will add to the problem.
Remove the head and test spindle between finger and thumb (should turn very easily) WD 40 helps. If spindle is free and problem still there then it can only be the motor.
You can leave it as it is for me, but by asking for help you're obviously not willing to live with it. What hold the lever against the spring until the house warms up, I don't think so.