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Yes that should do the trick.

As stem said earlier, it's likely all you need now is a wire from the Tado HW OFF to the terminal where the 3-port valve Grey wire is connected in the wiring centre.

Has the heating suddenly started misbehaving or has it been gradually getting worse? Do you still get plenty of hot water or has that slowed down as well? Some pictures of various parts of the system (boiler, pump and motorised valves ) might help us.

I think the OP has gone away. Untitled by jackthom posted 7 Nov 2011 at 5:08 PM

S7 just means the pump is still running for a few extra minutes after the boiler has finished firing to carry off the heat from the boiler. Does it get to 70 in a very short time and what sort of motorised valves are in the system? Some of these Untitled by jackthom posted 4 Nov 2011 at 6:38...

Does the boiler fire up at all and the temperature display start to rise?

Should be ok with a Nest but need a bit more info about the system. Is there a 3 port valve ( looks likely from the wiring ) and do you have a room thermostat? Some pictures of the room thermostat wiring and the internals of the main wiring centre would be useful.

Yes I would check in particular that the grey and orange wires from the valve have a good electrical connection in the wiring centre. Either one of these could be the source of your problem.

As you were happy to wire in the 3 port valve I would recommmend getting hold of a cheap multimeter, Screwfix do a reasonable one for £10. Then come back for advice on which voltages to check when heating only should be on.

That's ok if the cylinder is a modern rapid recovery one which can make good use of a fair proportion of the heat output from the boiler. If not the usual fix is to balance the flow between the cylinder and the heating by having a partially closed gate valve in the hot water circuit.

The flex that carries the wires connected to 3, 4 and Earth.....can you show us a picture of it, plus at the other end where it is connected into the wiring centre. A pic of the wiring at the back of the room thermostat would also be useful.

That does look like the easiest option and just means connecting the red and yellow together and isolating the blue safely on it's own. Edit: beaten to it by stem. :)

Can you post some pictures of both the thermostat wiring and the wiring centre, if there is one. If there is it would help a lot if you were able to identify the wires there that come from the existing room thermostat.

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The poor OP only made 2 posts here and has likely been scarred for life.

How far is the very hot flow water getting? You mentioned a bypass, is it possible the majority of it is going that way and not through the radiators? Sounds like the boiler is producing some nice hot water but there's either not enough flow, a short circuit through the bypass, or both.

I would first try turning off the 15mm fed radiators then see whether any or all of the microbore ones dramatically improve. As ianmcd says it may just be a case of balancing so the microbore radiators get their fair share of the flow.

The 2 wires go to terminals 4 and 5, it doesn't matter which way round.

My own Vaillant is nearing 10 years old and still going strong. I'd tend to agree the £299 fee is not a bad option.

Yes that makes much better use of the terminals available. (y)