vaillant 20/1 T2W CH starts and stops

[gorselands]

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. Came on with the timer OK this morning for a few hours 'till timer switched it off. Then I switched it on constant and trouble began.

I eventually called a registered CH engineer who confessed he'd never seen the model before. After 2 1/2 hours he gave up and charged me a callout fee. I knew more about it than he did. I replaced the diaphram in the stuffing valve about 2 years ago. Please has anyone any ideas.

 

[Andygasman2010]

This could be an external controls problem. Link together terminals 3 and 4 on the mains terminal strip to confirm, if it then works ok you know it's your clock/room stat.

Did you say it worked ok briefly for a few hours?

 

[gorselands]

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 water. But does not move when CH is switched on.

Do you mean terminals 3 and 4 on the Randall timer? It will not work when switched to constant

 

[Andygasman2010]

I meant 3 - 4 on the boiler terminal strip. Doesn't matter though if it's the differential valve thats not operating propperly.

This diff valve operates via pump pressure as i'm sure you know, there's no reason why it would work on one demand but not the other...

 

[gorselands]

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?

 

[gorselands]

found terminals 3 & 4, no difference.

 

[Bunnyman]

hELLO MATE PROBABLY WAX STATS GONE IN DIVERTOR VALVE IF YOU LOOK ON LHS BEHIND DHW HT EXCH YOU WILL SEE DIVERTOR VALVE.TURN POWER OFF AN PULL OFF THE ALUMINIUM COVER,UNDERNEATH ARE 2 SWITCHES.TURN POWER BACK ON SWITCH CH ON AND SEE IF VALVE RISES PS CHECK YOU HAVE NO DRIPPING TAPS AS THIS WILL STOP VALVE LIFTING AND YOU WILL GET NO CH ON THIS MODEL IF YOU HAVE DRIPPING TAP AS IT KEEPS THE WAX STAT COOL

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• Bunnyman
• 26 Mar 2010
• 499

 

[gorselands]

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.

 

[Bunnyman]

READ THE POST ABOVE MATE AN LEAVE THE DIFF VALVE ALONE THATS NOT IN YOU PROBLEM

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• Bunnyman
• 26 Mar 2010
• 499

 

[gorselands]

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?

 

[gorselands]

thanks again Bunnyman, will go and investigate again

 

[Bunnyman]

THERE IS ONLY 1 DIVERTOR ON THIS MODEL IT IS ON LHS BEHIND DHW HT EXCH U ARE PLAYING WITH A PRESSURE DIFF SWITCH AND THE VALVE AT THE BOTTOM IS A WATER VALVE THERE IS NO DIAPHRAM IN DIVERTOR ON THIS MODEL ITS CONTROLLED BY WAX STAT UR INTERPRETATIONS OF MY ABB R CORRECT

 

[gorselands]

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 anywhere of a wax stat so assume it is an integral part.

 

[Bunnyman]

That's it flow switch is old Vaillant terminology for divertor! Did u say pd valve holds on hw but goes in again on ch ??

 

[gorselands]

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