Central Heating switch does not fire the boiler

15 May 2014
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United Kingdom
Foreword: I've already replaced the 3 port valve head...

- Boiler: Potterton Flamingo 50s
- Controller: Potterton EP2000 > EP2
- Room stat: Old brown Honeywell > T6360B
- Honeywell Mid Position 3 Port Actuator/Valve

I moved into a 1980’s house with an old EP2000 controller, which only allowed both HW and CH to come on together, not independently. So I decided to switch to a newer EP2 controller.

The boiler clicks and fires up no problem and the pump runs when the hot water is switched on and gives hot water, but if just the Central Heating is selected, there is nothing at all from the boiler and the pump does not run.
Central heating will only work when both HW and CH are on.

From the forums, it appears the 3 port valve head is a common point of failure. The test lever sprung back to position with ease so it didn’t look like a stuck valve. When set to MAN, nothing happens, no boiler or pump reaction.

I changed the head for a new one anyway, however it still behaves the same way! :(

Anyone know have any ideas? Could it have been wired up to behave this way?
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Probably wired up that way, my parents place is the same (one day I'll get around to altering it). Hot water priority - once the cylinder is satisfied, it'll automatically switch to heating only
Think you will need to know how a three port valve operates.
It gets complicated when valve needs to move from mid position to CH position.
It relies on the valves grey wire being live. If this is not live then motor can not move valve over the second half of the range.
The grey is made live when the HW is switched off or satisfied if HW is ON
There has been many cases where the required wire from the programmers HW OFF terminal to the valves grey wire does not exist

To make sure i understand correctly, i need to make sure the wire circled in pink exists?
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Bingo! The that wire is missing :(

The question is how/is it worth doing keyhole surgery on the house to install another cable....
After alot more researching, i found this topic. //www.diynot.com/forums/plumbing/can-this-y-plan-wiring-be-fixed.300557/

Particular this post:
My fallback plan is to disconnect the CH output from the time controller, and supply the room stat directly from the live supply to the time controller. This frees up a wire to use as HW OFF, and I have enough 'spare' wires in place to connect it the rest of the way to the tank stat. This seems quite feasible in practical terms, but feels like a nasty bodge.

How horrified would you be to see a Y plan system with the room stat bypassing the time controller?
That's quite normal when using a programmable stat. See below, taken from a Honeywell guide:

So essentially i would:
1. Install a short wire from LIVE to ROOM STAT at the junction box
2. This will free up the CH OUTPUT wire...
3. Which can then be used for HW OFF instead


Sound about right?
So i had a chance this weekend to take a look at the wiring, this is how it actually looks:


There is no way to bridge Live and the room stat at the junction it seems :(

The only alternative i can think of is that near the junction is the immersion heater switch.

Can I run a line from this? I understand this is on a different ring/fuse - is it a bad idea to mix rings?

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