vaillant vcw sine 18 boiler trouble

i will tell plumber all what youve said and hope it fixes plob, thanks very much for your help, thanks higgsy
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Plumber knows about pipes, heating engineers need to know about heating systems.

If the pump were defective, burner would not light.

If water at tap hot, I would say controls and electronics are good.

If your guy is thinking of changing the PCB, it is because he is guessing- get someone in who can diagnose the fault

This boiler always fires up in HW mode when a demand is made. Pump runs and allows burners to fire. Burner goes to high rate and a short while later, downsize if demand is for heating. Left hand 22mm pipe at this stage is cold.

It very soon starts heating up and burner goes to high rate. This is because the wax capsule now allows water to flow out of the diverter to the flow pipe instead of water circulating within the boiler.

For HW, burner stays at high rate to modulate during demand.
dhw is fine turn tap on slow, burner modulates,water hot from tap turn ch on from cold burner fires and dont stop till temp reaches 60 thencuts off temp drops to bout 55 then fires back up burns till 60 again drops to 55 then cuts off just keeps on doin this rads only get luke warm dhw works fine seems to be just ch with prob as water for ch gets hotter flow valve rises just touching micro switch when water gets to 60 rises more and pushes micro switch burner cuts out after few seconds flow switch drops relasing micro switch burner starts up, turn dhw on while ch on flow switch drops pressing on rhs micro switch dhw runs hot burner modulates turn off dhw flow switch rises and starts over again cleaned everything still no joy,i tightend 7 ml loking nut on top of flow switch ch got up to 70 then cuts out drops to bout 65 then comes back in lossend it back off droped to 55 burner starts back got to 60 same again. when i turn on ch burner fires up dont modulate just burns til reaches 60, its a new flow switch wonder if it is faulty?

Is definitely a problem on the heating temperature control circuit as you rightly say the dhw is OK. Given that the flow switch is 'new' (it is new not reconditioned or 2nd hand?) then the microswitches on it should be fine.

You are looking for a poor connection from the LHS microswitch to the PCB edge connector (check multi-plug and loose edge connector at PCB) AND/OR a duff microswitch on the DC Solenoid. These all operate on low voltage so slight corrosion/dirty contacts can cause problems. Spray contact cleaner on the multi-plug and the PCB edge connector. Pull apart and reconnect these a few times to let the contact cleaner work in. Also change the DC Solenoid microswitch (as mentioned before make a note of the colours).

If after doing this, and the information you have given us is correct, then the only thing left is the Electronic regulator PCB.

Cheers Bunny ;)
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Hi bunny,today ihave cleaned multi plug contacts no joy changed multi plug on on old flow switch to new one no joy taken off and cleaned contacts on multi switch to celanoid (note they were black) cleaned with fine sand paper no joy
have taken off pcb edge connector pushed off and on few times checked pins in connector no joy will try conntact cleaner like you say and check micro switch on cellanoid can i use a micro switch off old flow switch on cellanoid to test thanks higgsy
[Hi Bunny, it was the pcb changed it works fine again now,board was 89 quid fitted it me self not bad having to buy pcb and flow valve at 148 quid first problems ive had with boiler in 28 years cant moan at that, thanks very mutch for all your help, thanks higgsy
Hi Higgsy

Glad it all worked out for you :LOL:

Theres plenty of life left in the old girl yet! ;)

Take care


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