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British Gas UP2 failing to turn on Central Heating except when Hot Water is on

So I have noticed that it's slightly different to how I described before:

1. If I turn on just HW then the valve moves to just HW I get just HW and it heats the cylinder.
2. If I turn on just CH the valve DOES move to the CH position, but even if the the room thermostat is set above the room temperature the CH DOES NOTcome on.

Strangely - if I am in scenario 2 above - with the CH switched on and the valve in the CH position, if I turn the HW on as well then the pump kicks in and the boiler starts up - the weird thing is the valve does not move into the middle, dual CH and HW position - it stays at the CH position and the rads heat up. This does not seem to heat up the cylinder (which is fair enough as the valve position is in CH only for some reason).

In the above scenario, once the room comes up to temperature the signal goes to the controller to turn the CH off (obviously the CH light is still on in the controller as the timer is on CH) - it's just got the signal to stop as the temperature is up. When this happens the valve moves to HW only and then it heats just the cylinder.

If I then put the room thermometer back up it moves into the dual position and heats both.

So I still have the same problem of the CH not coming on by itself...

So to me the really weird thing is:

1. The CH not coming on by itself, yet the valve is moving into position and so thinks it should come on.

2. If the CH is supposed to come on and the controller has a green light and the valve is in the CH position - if I then ALSO then put the HW on, both lights on the controller are on, but the valve does not move to the dual position and stays in the CH position heating just the rads.

Then I think we can go back to my original suggestion of the microswitch in the valve being goosed.
This microswitch gives the boiler the signal to fire once the valve has moved to the CH position.

There is not a CH 'off' signal in the system, you only have a DHW off signal.
 
An electrician could have found the problem within a quarter of an hour, yet you prefer to spend days with a broken heating system to come on here for our best guesses?

Well as I said I've figured out how to put the heat on - and that's been fine as I've not been around much.

I came on here for some advice - or as you call it best guesses.

Problem is I've been burnt before by 'So called Experts' who come around and charge you a fortune to do the same best guesses and get you to replace piece by piece - which really mounts up as they charge for each call out.

I've never had one find the problem in quarter of an hour - in fact I had one who tried to rip me off to such a level it was criminal.

I thought this was a reasonable way of doing it: i.e. try to understand what's going wrong myself - and then try to make an educated guess myself based on the people on this forums advice - then just buy those parts myself and replace them, saving a fortune on people as I don't know a good heating electrician who's available immediately - especially as it was just the weekend.

I know a great electrician - problem is he's really good and so isn't available for weeks and told me he's not a heating control expert.

I thought this is what forums are all about? I have appreciated you help - but why ask me a question in your last post and then write that in your latest when I have answered your question with as much detail as I can?
 
Then I think we can go back to my original suggestion of the microswitch in the valve being goosed.
This microswitch gives the boiler the signal to fire once the valve has moved to the CH position.

There is not a CH 'off' signal in the system, you only have a DHW off signal.

Ah ok, thank you...
 

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