CH only works when HW is on

23 Feb 2006
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United Kingdom

Can someone please help me diagnose the following fault.

When my CH and HW both come on in the evening, my water and radiators heat up OK. However as soon the HW tank stat reaches temperature and stops demanding HW, my CH also stops. The CH will stay off regardless of the position of the room stat (which still clicks when turned pass the current room temp). If I increase the temperature of the tank stat, then the pump kicks in again and my HW and CH both start to work.

I think I have a Potterton boiler and programmer. This is a gravity fed non-combi system.

I'm thinking this could be the programmer that is faulty?
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Is this definately a gravity h/w pumped c/hs?

If so, unless your system has been upgraded to a 'C' plan you dont have cyc stats on gravity systems.

is there 2 pipes of 28mm going into your cylinder on one side with a silver box (or 'box' nonetheless) on the bottom of the two pipes.

I suspect a fully pumped system with a 3 port valve, with the valve not moving to 'heating only' when cyl stat no longer calls for heat.


Thanks for your reply. I may not have been using the correct terminology when I stated this was a gravity fed system. I know its not a combi boiler and I have a hot water tank that is fed via gravity from a cold water tank in the loft.

If my 3 port valve was stuck in the HW & CH position and the system was still demanding CH but not HW, wouldn't the pump continue to operate even though the valve was in the incorrect position, meaning that I'd still get HW and CH?

I had the same problem this week - rads went off as soon as water did.

Quick remedy was to manually slide the switch on the mid-position valve in the airing cupboard see here for photo

I then realised I had a British Gas service contract :oops: :oops:

New mid-position valve fitted 3 hours ago :p :p

Check the switch

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Your 3 port valve works in two stages. It obviously moves to the mid position and then this where your problem starts. It either does not move on the CH positon or if it does move to the CH position then the microswitch that feeds power to the boiler maybe faulty.
Power to the boiler when using HW or HW & CH shared does not go through this microswitch (it comes from cylinder stat). put system on HW & CH shared where valve will be in the middle, then turn cylinder stat down and listen if you can hear valve move to CH only position. It is easier if valve head can be removed as you watch if the quadrant responds to the various demands.
My valve did move correctly, but the orange output wire did not become live due to faulty microswitch. With actuator head off suggest you try spindle for stiffness. Also try search on 3 port valves.
:rolleyes: :rolleyes: :rolleyes:

I've just tried pulling the manual override switch. The HW LED goes out whilst I'm holding the switch, but as soon as I release it it comes back on again. So I'm stuck with both CH and HW LED's on.

The valve is a Potterton MSV322. I guess this meens it needs replacing? Anyone got any ideas if I'll just need to change the motor or the valve itself?

I took off the valve head and tried what you suggested. When I turned down the cylinder stat the quadrant didn't move, but stayed in the mid position. Also if I turned off the HW from the programmer, the HW LED went out on top of the valve, but again the quadrant didn't move.

I tried turning the spindle by hand and it turned freely.

So I guess its safe to assume now I need a replacement valve head?
Yes or you could just replace the synchronous motor for about £15. My new head cost £38. But at least you know what causing the problem
:rolleyes: :rolleyes: :rolleyes:
I fitted the new actuator (Myson ACT322) last night. 10 minute job - CH and HW now working correctly. Thanks :D

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