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Matt, you're an absolute star! I attacked the airing cupbaord with my multimeter and it was exactly as you described. I then swapped over the orange wire, and now it all works perfectly! Strangely, I can even get CH only without HW. I realised the tank stat goes into a separate control...

OK, I've followed your instructions! As regards the actual wiring, the pump is definitely connected to the orange valve wire. The junction box in the airing cupboard has nine connecting blocks, as per the photos I posted before. As far as I can make out, the connections to each one is as...

Yep - I've got a multimeter (and a mains tester screwdriver!). Chris

I'll check this in due course (prob not for a day or two, though). Although I think everything goes to the single box in the airing cupboard. There are the three cables for the valve,pump and tank stat, plus three cables coming out of the wall, one of which is the room stat, and the other two...

Thanks for the replies, guys. I'm pretty sure, yes. The valve moves to the correct position, but the boiler locks out immediately. So the bottom line is that without the DHW off connection there's no way it can work properly. I can't see why (or how) the plumber would have removed...

Just a quick update. The wiring diagram for the 3-port valve shows a connection going from the boiler "no hot water" terminal to the junction point of the orange wire. It says this is needed for CH only, although I can't see why. However, this is not connected on my system (see picture...

The boiler trips (needs the reset button pressing). Although it won't reset until I turn the CH off on the timer.

Yep, it appears to run even after the boiler's tripped. Chris

Hi. Plumber recently came round to flush our system as rads weren't working after not being used over the summer. He also replaced the three port valve (I wasn't there at the time!). Now, the heating will only come on if the hot water is also on, otherwise the boiler trips if you just demand...

Problematic CH system