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Just an update. Put new diaphragm in diverter and checked pipes. Flow valve seems to be operating micro switches. Another engineer came out and said pipes were getting too hot. Definitely blocked/scaled heat exchanger. They are still available @£200.00. I have taken it out and flow seems fine...

Thanks The 831 Bunny. 1,2 & 3 seem ok, so is looking like the replacement flow switch. Will probably try to replace as I have two boilers side by side running 21 rads; big old house!! They flue up the chimmney - now illegal and the cost of replacing the two with pipework would be prohibitive...

No dripping taps. Removed multiplug from flow switch before removing Aluminium cover. Put it back again and now nothing is working HW or CH. so can't test at moment. Have started looking for old hot water bottle and puncture repair kit!

yes, the HW works ok only prob with CH

sorry Bunnyman just realised we were on to page 2. Pin only goes back in on CH when flames die, It does not go in and cause flames to die.

No, pin in diff/stuffing holds on both but goes back in as soon as flames die down.

sorry, confusing diverter with differential. But cant find it in the manual. Could it possibly be referred to as the FLOW switch which has a multiplug at the front and contains micro switches? This sit just above and behind the Domestic Heat exchanger (about the size of a thermos). No mention...

thanks again Bunnyman, will go and investigate again

Thanks Bunnyman. Not sure of abbreviations LHS =left hand side? DHW = direct hot water? Heat exchanger the divertor valve goes into the stuffing box, and has rubber diaphram. Is there another one?

waited for boiler to flare up again. Diff valve working fine on both CH and HW. Flames still cutting out after a few seconds and pin in diff valve goes back in.

found terminals 3 & 4, no difference.

Thanks I'm not that sure of what I am doing. The diff valves have 2 pressure pipes going to it. Is one for water and one for CH and could one be blocked? So works when hot water tap on but not when CH comes on? Can't see a terminal block, is it in the PCB?

Thanks for the reply. Yes it was working this morning but had problems yesterday. Been away 10 days left pilot on. Thought it was the pin seized on the differential valve so sprayed with wd40 initially seemed to do the trick but when it failed again checked it properly, works ok on hot...

First post so hope this is ok. Vaillant VWC 20/1 T3W combi. I have maintained this old boiler for a number of years but am stuck. Hot water works fine. CH - boiler fires for 3 secs then goes off. This repeats every 20 mins or so. Stripped differential valve, all ok. Pump working OK...