Potterton Suprima 50 good HW intermittant CH

[toad1960]

Hi there I did have a trawl through all the older posts before requesting help but was hoping someone might point me in the right direction.
Our heating system is about 11 years old and we have replaced the pcb board and the myson 3 way valve over the years thanks to advice here in the past.
Now we have HW no problem but intermittant CH , when you turn the thermostat , Potterton PRT100, up & down in the hall you can hear it click indicating the CH should come on but it is hit & miss as to if it does.
I have just turned what I am pretty sure is the 3 way valve ,Myson Power Extra, to manual and everything clicked into life and the CH is now working , I have slid the switch back to auto and the CH is still working.
I am torn between thinking it is the thermostat in the hall or the 3 way valve does anyone have any advice?
BTW I have bought a new thermostat on ebay as at £8 inc p&p thought it worth the risk just waiting for it to arrive now.
Many thanks for any help

 

[45yearsagasman]

I would think that your valve is seizing up.I would remove the head and check the spindle moves freely.If seized then ditch the valve for a Honeywell mid position,much better quality.

 

[toad1960]

Thanks for your reply would that be the valve in the groudfos that sits above the 3 way valve as I removed an inspection screw caught the dirty water and then using an electrical screw driver rotated the spindle in there which rotated freely?
On the plus side the heating is still working fine since I swtiched the myson box to manual and then back to auto.
Again thanks for your advice

 

[45yearsagasman]

No that screw you removed was on the Grundfos pump.I was referring to the Myson 3 port valve head which is held on by 2 screws.Remove that and check the spindle underneath the head.No water will come out.You should be able to move spindle by hand.

 

[MANDATE]

The lever basically is provided for filling purposes, 'manual' holds the valve in mid position so water can pass through both HW and CH.
Otherwise it should be on 'auto'
As already stated the valve spindle needs to be easy to turn with finger and thumb.
If it is ok and assumming there is no fault with the wiring, then the actuator head is where the fault is.
Sometimes the motor fails, meaning the valve will not move from HW to mid position.
Also there are two micro switches of a 'two way' type, so each switch has two pairs of electrical contacts.
These contacts get dirty/burnt which result in either contacts not breaking when they should or not making when they should.
There are two requirements to get CH (a) valve must move 1st to mid position, then continue to CH position. and (b) valve must supply boiler with power through 'orange' wire.
One micro switch is responsible for (a) and the second micro switch is responsible for (b)
I have replaced my actuator head twice in about 8 years.
One was due to a dirty contacts on switch so no power to boiler, the second was valve failed to move (motor or switches?)

You need to understand the power supply to feed boiler comes from the cylinder stat whenever HW is part of the demand, but when CH is the only demand it comes from the valves orange wire.
Or put another way, when both HW and CH are selected and both are in demand, the valve will take up the mid position. Of course HW will get satisfied before CH so valve moves to CH and in doing so power supply to boiler switches from cylinder stat to valve.