Vaillant - heating goes off. Why?

[Searcher2]

I have a 9 year old Vaillant TURBOmax Plus which developed a problem a couple of years ago. When the central heating is on sometimes the radiators go cold. When I go to the boiler it is still churning away as though it is working (fan/pump going) but the heating indicator light has gone out. This is resolved by switching the boiler off and back on again. It doesn't happen very often and doesn't happen at all in summer when I use the boiler only for hot water.

When it first happened I rang Vaillant to try and find out what would cause it but they didn't seem very sure of themselves and were more interested in getting someone out at a cost of over £200. They did say it seemed like the diverter valve. I got a local plumber out who said it wasn't the diverter valve but was.... embarrassingly I can't remember but it was a three letter abbreviation and there are 2 of them in the boiler. He changed one and it had the same issue. He swapped it over the the other one and it seemed to be okay but the issue eventually returned. Because the problem is intermittent I don't actually know if he actually fixed it or if it took a long time to show itself again. The problem seems to be heightened when all the radiators are on (i.e. under more pressure).

My sister has been given the same boiler by a plumber for spares (one he took out of a house) so I have access to most bits. Does anyone know what is likely to be causing this issue? What is the three letter abbreviation that the plumber replaced?

Thanks in advance!

EDIT : Looking at the manual - when the fault occurs the Yellow indicator goes out (so the main burner is not ignited) but the boiler itself is still pumping so presumably 'knows' it needs to be producing heat.

 

[Agile]

It sounds as if an NTC sensor is intermittent.

There are two and one is in inside the combustion chamber which a DIYer should not be opening.

They can be tested by measuring the resistance ( or the voltage across them if you know how to interpret that ! ).

Tony

 

[Searcher2]

Thanks for that! Sounds reassuring that the visiting plumber was correct - just a bit odd that the problem came back quite quickly once the NTC was replaced.

 

[mickyg]

i doubt its an ntc they are very reliable on these boilers and if you've already replaced it your in the wrong area. What you need to do when you think the boiler should be firing press the "i" button to see what the boiler is doing or thinks its doing. You should get an S code which you can find in the manual or repost it here and we'll tell you what it means

 

[Searcher2]

I will report back when it happens again - it's not that often.

However, I've looked at the manual now that I know what the NTC is called and there aren't 2 of them on my model (Turbomax Plus 837e) - there are 3. I know the plumber tried the replacement NTC in 2 of them so maybe it's the one he didn't try? There's 2 up near the main heat exchanger and 1 near the DHW heat exchanger.

 

[mickyg]

it wont be the ntc, trust me

 

[Bunnyman]

I'm with Micky ntcs don't fail often on these possibly aps at a guess ?

 

[Agile]

The third NTC is used for the DHE preheat and not relevant to your problem.

I agree that those NTCs are very reliable.

However, the APS as suggested is equally used on DHW as CH and if that was the cause would be expected to fault on DHW as well as CH.

A DIYer should not be fiddling with the APS as its a safety device associated with the combustion.

There are many possible causes of boiler faults. The NTC is an obvious suspect if its only CH is affected. But the PCB is also involved.

The only place to diagnose is in front of a boiler using test equipment.

Tony

 

[Bunnyman]

I agree u need to get your hands on it to repair it tony but, the black aps usually fail in the winter when the airs a bit heavier! But as I said its an educated guess

 

[Searcher2]

Thanks for your replies.

As it doesn't happen very frequently I am having to go off memory.

I strongly suspect that when the problem occurs the hot water doesn't work either - but the problem ONLY has occurred when the central heating is on and generally that's when the majority of the radiators in the house are on (doing more work). I can hear the boiler fan but the radiators are stone cold. Not great to come back to a cold house. The fault has never happened when just using hot water.

I thought I would try on here - and judging from the comments I guess the plumber I used didn't have the test equipment. Perhaps I'll put all the rads on full one night and see if I can get it to fail again to see what the 'i' button info is instead of just waiting for it to happen.

 

[Searcher2]

Having been on all day the heating has finally gone off (when it shouldn't have). On pressing 'i' the code is 5. 3

Hope that helps!

 

[The831Bunny]

Air pressure switch it is then

 

[Searcher2]

Brilliant! Can investigate replacing that now then.

Many thanks.

EDIT : Even better it should be on the spare boiler my sister has which isn't the 837 (828 I think) as all models seem to use the same one. Hopefully it wasn't faulty on that boiler.

 

[The831Bunny]

It is a combustion/flue safety device though, so should only be checked or changed by a Gas Safe engineer. It is a straight forward job when you know for sure, but thems the rules

 

[Searcher2]

Arrgh. Changed the APS for the one on the 'spare' Vaillant boiler. Worked fine until just now. Code 5.3 again. So if 5.3 definitely means a faulty APS then I guess the 'spare' is faulty too.

I have a new fan coming in a day or two as the current one is noisy. I'm not sure if a new one will help at all...