No CH (only comes on when DHW is on)

[Chap]

Potterton Prima F (gas)

The DHW functions as it should.

The CH thermostat was turned down for the summer; when turned up to activate the CH, the boiler will only fire if the DHW is on/coming on at same time as the heating.

Where the stat is turned up but the hot water is not on, the boiler doesn't try to fire. However the CH light iluminates on the 3way valve and it hums quietly.

Any help greatly apreciated.. before it starts to get too cold!

 

[twgas]

3 port valve prob, could be jammed or need motor - does the lever (man/auto) move fairly easily when pushed, if it does it is likely to be motor, if not the valve body may well be seized.

 

[MANDATE]

Experienced this problem twice in 3 years.
First time, the actuator functioned correctly but boiler failed due to faulty micro switch in actuator head. Second time motor failed to turn actuator to the CH only position. Have you tried to remove the actuator head, if you can do this, you can check the valve spindle for stiffness. You can also try CH only with the head off the spindle. You will be able to see if the quadrant moves across to the correct position by observing the 'D' shaped hole on the underside.

 

[Chap]

Excellent.. most helpfull!

Took the actuator off and the valve spindle moves freely enough. Set the heating to come on and the actuator coupling does not rotate far enough.

Off to BnQ to see what they have.

Taa very much

As a matter of interest, is there anything serviceable when you undo the torx head bolts on the under side?

 

[MANDATE]

don't know what the torx head screws are securing, but there's nothing really servicable besides the motor.
I did strip one down, just to prove the fault was the micro switch.
This was on a potterton 322. I managed to remove the side off the offending switch and found the contacts dirty and slightly burned from arcing. Cleaned them all, reassembled and tested. It is kept as a spare.