vaillant thermocompact cutting out

[topkiddermarc]

Hi, Calling all fountains of knowledge who understand and appreciate our old (usually) reliable workhorses.
I have a vaillant thermocompact vc gb 142 eh. gc41 044 09 that is at least 20 yrs old. (no fancy codes to give fault clues)!
the boiler runs perfectly fine for around 1-2 hours and then cuts out. while running (3 way valve in split position), I can turn hot water demand off and 3-way valve does what it should and move to heating only. it works fine the other way round.
Pressure is all ok and steady at 1.5 (no spikes or drops).
boiler been stripped and checked yesterday and and told gas valve/exchanger etc all ok and must be a wiring problem?
Electrician to it today and all checked and tested ok.
everyone involved seems to think its an overheat problem??
Pump ok, rads warm up when running, hot water storage warms up ok, then just cuts out.
I'm not in the 'just change the boiler' camp as I believe the fault is probably a niggling minor sensor type????

 

[Agile]

Who "stripped" the boiler ?

Who decided its "all OK" ? I don't think that it is!

Why have you now called an electrician? They deal with power wiring and not the electronics which operate boilers!

They are old but perfectly repairable but do need a competent boiler engineer who specialises in repairs!

Tony

 

[topkiddermarc]

Hi Tony, Thanks for the reply.
The Guy who stripped it is a gas/boiler engineer who works for a 'large company' and has repaired it a couple times previously only at 'weekends',(if you know what I mean)!
House was rewired in jan/feb and new stats etc fitted following a house fire so the guy who rewired house called back yesterday and double checked stats/power wiring.
not at all worried about the unit being old, and deffo don't want/need anyone from the 'you need a replacement' brigade!!
After reading through a load of posts on this forum, the general concensus is to get 'old school' engineer who knows/understands the old boilers - which is exactly what i'm looking for on here (and/or some 'pointers' as to the fault prob). (Alot of posts are saying that vaillant tech help isn't worth contacting)?
All help gratefully received - or the tel no of local old school engineer.
Cheers
Marc

 

[Bunnyman]

What do u mean by cuts out? Does it lock out an pop the black button an red warning light?

 

[Agile]

Boiler tech help is primarily supplied to help people fit brand new boilers.

The manufacturers have no benefit helping keep old models going. Quite the reverse.

They are often not manned by older engineers as you would expect but by younger people who just read the manuals and have a crib sheet giving common faults on each model.

Very oddly perhaps some of the best have no practical background with boilers but just take to it well.

Tony

 

[topkiddermarc]

Thanks guys!
And so the plot thickens.......
ok, the boiler does cut out (red light/black button).....but this is due to the boiler working correctly I have now deduced!!
right - All controls (cylinder stat/ 3 way valve/ timers) working.
the problem seems to be the foam lagged cylinder?
I have a 30 degree difference between top and bottom!! flow and return from boiler working, so coil not blocked or airlocked. same temp difference when immersion used instead of boiler.
so, the boiler heats up the coil, volume of water starts to heat up, hot water stays to very top of cylinder (stat at bottom of cylinder not getting to temp), boiler/immersion keeps heating until boiler trips or immersion kettles over into cold header.
immersion running with a cylinder stat, so I moved this stat to top which made everything work perfectly except I'm only getting 1/3rd of a cylinder full of toasty hot water before shower/bath starts to run cold.
hairline crack in top section of coil????? (remove and pressure test)?
18" of sludge in bottom of cylinder???? (unlikely as hot water runs clean)
new cylinder = £300+ (being from Yorkshire - I could have a week in sun for that)! lol