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Valliant Combi VCW Sine 18 T3 overheating

Postby carlosg » Wed Sep 12, 2007 5:32 pm

The boiler as the following problem hope an expert can help me.

I turn the room thermostat to click (23degrees) & the boiler fires up. The temp dial on the boiler hovers around 60degrees for aprox 10Mins, flicking between hi/low flame. The flame then stays on high all the way from 60-88 degrees then the boiler switches back to a low flame & the boiler temp indicator keeps falling all the way back down to 60ish. During this falling temp period the flame doesn't go high again not even once. Only when i move the room thermostat to a higher temp it kicks back to hi flame but this time the boiler temp indicator rises to 100+ degrees – I then manually switch the boiler off to be safe.

I managed to get an identical boiler off ebay to use as a spare & have since swapped the NTC resistor but still overheating.

I've been told that it could be the room thermostat (must admit it's quite old) or the swapped ntc could also be faulty (i think small chance having exactly same ntc fault).

Anyone any ideas? I know it's a long read but am desperate to get this sorted so have put as much info in as possible.
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Simply Build It

Postby humperdink » Tue Sep 25, 2007 8:24 pm

i would never recommend using a second hand boiler for anything - infact corgi will only allow it if it has been checked over - and certainly using second hand parts is not allowed
i am pretty sure that your model of boiler is about 20 years old which means it could have reached the end of its useful life(and done pretty well)

have you ever had to manually reset the overheat stat on the boiler?
do your radiators heat up?

the boiler does have an anti-cycle function which stops it relighting after a certain period of it reaching temperature, vaillants are also normally dependant on a lot of hydraulic compenents that can easily choke on older systems

if your radiators are not heating if may be the diverter valve sticking

hope this helps thanks
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Postby htg engineer » Wed Sep 26, 2007 9:13 pm

Why is using second hand parts not allowed ? Who states this because CORGI and Manufacturers definately don't.

CORGI registered engineers will carry out tests on boilers and parts. If the parts pass the tests then whats the problem ?

Are you saying CORGI will personally check the second hand boiler ?
or a CORGI registered engineer ?

It's up to the engineer to decide whether the boiler is safe to be re-used, he's putting his name to it to say it's safe.

Second hand or brand new - if working and fitted correctly they do the same job.
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Postby carlosg » Mon Nov 19, 2007 11:28 am

Hi Guys thanks for the replies.
The boiler works fine with HW & CW only issue is occasionally it trips & requires a manual reset. I've noticed it trips out more often when its a very cold morning/night.
My room stat (newly fitted) is right next to the front door/hall way & the hallway is much colder than the rest of the house. This makes me think it could possibly be the location of the stat & not a boiler prob but i'm not 100% sure how the boiler should handle a situation where the stat is constantly calling for more heat. Should it cool down then start up again or should it keep heating to try & reach the stat temp?
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