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Photo for matt1e. I'll identify the wires tonight, no time this morning. Not too good quality I'm afraid as it's all behind the pipework and a bit difficult to access.

I can't go back to my son's until Saturday week so probably my best bet is to start all over again using D_Hailsham's fault finding info. Perhaps coming to it fresh will help! Thank you all for your input so far. I'll report back in ten days or so.

No power from the CH on terminal and I've checked the wiring for continuity from the on terminal through to the valve. It occurs to me that when my son told me his CH wasn't working I suggested that he gave the valve a bang in case it had stuck fron not being used all summer. This he did and...

I've worked my way through the electrical system and realised that there is no power from the CH 'on' wire from the programmer. I've linked it out so that 240v is available to the room stat and CH starts with no problem, whether HW is called for or not. I think I'm going to bring my identical...

I'll do that in the morning when I get to my son's house. Thank you.

The connector box is a cat's cradle of wires which seem to have been thrown in by someone who wanted to get the job done as soon as possible! CH won't initially start unless HW is on, however it then starts when HW is switched off.

Thanks, I'll check that out.

Correct, the pump doesn't start and the boiler doesn't light - the room and tank stats were on max for all the tests I did. I'll check the voltage at white tomorrow morning. If no volts what's next please?

Thank you for that comprehensive description of the system. I've checked the wiring using a wiring diagram a number of times but perhaps a systematic work through using your description wouldn't go amiss. The valve is only a few days old so I'm not sure that the contacts would be burned yet...

I changed the cylinder stat 'just in case' - it made no difference. I've done a continuity test from the grey wire to HW off and it's ok. The programmer is the same as mine at home, I wonder if it's worth swapping them to see if it makes any difference? Grasping at straws here! Test 3...

I returned to my son's this morning - the results - 1,2,3 test done, nothing happens, 240v on grey, wire continuity checked back to stat and ok. 4.5.6 test done HW stops heating. With both CH & HW stats on high I get the following - All off - 240v G 0v O CH switched on - won't...

The small gear wheel on the end of the motor sheared off. B&Q were the only people open at the time and didn't stock a replacement power head so I bought what they had. On reflection I should have waited until the next day!

Thank you, I'll check that out tomorrow.

Correct, I gave the wrong info in my first post. I've checked the wiring a number of times but intend to do so again tomorrow - the system is at my son's home a few miles away. I can get CH when the hot water is heating but not by itself.

Ah, but I still have the problem of the CH not firing up - it was just identity of the valve that I've sorted out!

Thanks for the reply. You are quite right, I've looked again and it's the MA1 - it has five wires. Now I feel stupid!

I've just replaced a pretty old Honeywell V4073A1039 three position diverter valve with a Drayton ZA3/679-3. Now the central heating won't start, although the hot water is fine. I've checked my new valve wiring a number of times and can find nothing wrong, having just swapped colours like for...

Thanks for the suggestion - seemed a good one but unfortunately didn't make any difference! I've asked for a replacement spout but they have refused, probably because they have already replaced one that had the centre pipe support rose off centre - this gave fairly similar flow characteristics...