Vaillant Turbomax - hot water problem / overheating since CH drain. Air in Boiler?

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Hi all, I've found lots of good advice here regarding Vailliant boilers so signed up to see if anyone could offer me any advice on my current problem.

I've got a Vaillant Turbomax combi boiler (not sure of exact model). I recently drained down the whole system to fix a leaking joint in the central heating. This is now fixed, and I refilled the system using the integrated filling loop, putting up to about 1.2 bars. I then bled all the radiators in the house and topped it back up to 1.2.

The central heating now flows and works fine. Radiators get hot very quickly.

The hot water however is being tricky. It does work temporarily but then starts to get cold, while the boiler continues to burn away, with the display temp getting hotter and hotter. When I ran the hot water yesterday I saw it get to 100C before quickly turning off the tap. The flame stopped, but the temperature continued to rise up to 115C before dropping. It then takes a long time to go down - 10 mins or so.

When I looked at the boiler this morning it had a F20 error code (restarting it cleared that).

I've known it to do this temporarily before after topping up the system, but previously I've heard it make a massive glugging noise like water is replacing air throughout the boiler, and then it has started working again. This time (the first time I've done a full drain and refill) it's not made the glugging noise and I wonder if perhaps there's air in the system - I wondered if perhaps the temperature sensor is in air instead of water which could be why it's not matching the hot water temperature and then taking so long to cool? (Just a guess!)

Any advice you can give would be very much appreciated.

Many thanks,
James
 
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It sounds as if the temperature controlling sensor is not operating correctly.

However, this is likely to need a gas registered engineer to test and replace it if required.

I very much doubt that there is any problem caused by air in the boiler though as you say the rads heat up quickly.

Tony
 
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Thanks Tony, do you have any thoughts about what the glugging water sound could have been that seemed to sort it previously? It has definitely had this problem each time I've topped it up (have been sprucing up house and so radiators have been coming off one at a time over the last year), it's only this time that it hasn't fixed itself.

Alternatively does it make sense low pressure + topping up could have caused the temperature control sensor to go wrong?
Or do you think it's probably just chance that the problems came at this time?

Many thanks!
 
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Could be air trapped, is unlikely with a turboMAX, but then I don't know why they brought out a diverter valve with a hole in it. Could be to stop it airlocking, someone here with more repair experience might know the reason.

Knowing this forum there's also every chance someone who doesn't know will answer the question too.:cautious:

Try turning the gas off to the boiler and running the hot water. The boiler will have a few goes at lighting before it locks out, won't overheat cause there's no gas.

Simple reset once gas is back on, just press the flame button with a cross through it.

Other than that it is a coincidence.
 
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Hi Simon, thanks for your help! What am I looking for when running with the gas off? In case it's equivalent: I have run quite a bit with the water temp turned down to minimum, which so far hasn't caused the display temperature to go over 70C (which I'm assuming is fine).

It's also worth saying (should have thought before), that we've had a couple of problems with grit causing blockages in the house - when I first moved in, the (integrated) filling loop was blocked, and then recently the flexible piping to the kitchen tap got blocked with some grit. It's possible that the refill introduced grit into the system?
 
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Well, with the benefit of that last nugget of information I would say that an airlock is not the cause.

Regularly draining and refilling a system will promote massive corrosion and is to be avoided. Believe it or not, the best systems are the ones with 30 yr old water in them.
 
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I have added anti-corrosion stuff into the system, but I can absolutely understand that the changes will cause corrosion.

Simon, can you just confirm - is an airlock not the cause because it's run with the water temp on low without overheating, or because it's almost certainly grit in the system!? Thanks
 
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I think that this 70 C that you are looking at will be the boiler system water and not the hot water itself.

That implies that either the water is not circulating properly or the plate HE is badly blocked.

Its always possible the pump impeller is blocked with dirt.

But that would be partially diagnosed if the flow and return temperatures on heating were known.

Tony
 
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I think that this 70 C that you are looking at will be the boiler system water and not the hot water itself.
Thanks Tony, that makes sense.

I spoke to the plumber that I usually use this morning, but although he's gas registered and all that, he didn't seem very confident in diagnosing the problem based on what I told him. I'll say "blocked plate HE / pump impeller" and see if they're something he'd be confident checking.

Otherwise, if anyone can recommend someone who can come out to North London, (Tottenham, N17) I'd be very grateful - feel free to message me! Thanks
 
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By aware that many "plumbers" are not very experienced with boiler repairs.

I would say that you need a boiler engineer who specialises in repairs. Does not sound a very difficult fault(s) to locate though.

If you give your postcode its possible someone from this forum may be close to you.

Oops, just seen you have put your location. That's an area that I cover and if you wanted to discuss that you would need to contact me directly from the details within my profile here.

Tony
 
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My first thought was "blocked plate heat exchanger". If it works fine on heating I'd be doubting a pump fault.
 
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I can confirm that the plate heat exchanger was the problem - it was absolutely full of debris - Tony (Agile) came round and cleaned it out and the hot water is now working perfectly! Thanks Tony, as well as everyone else for all your help.

PS. I will be much more careful when draining the system in the future as it's clear this causes the gunk to move around the system. I'll also be installing a filter to help reduce this problem in general.
 
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I can confirm that the plate heat exchanger was the problem - it was absolutely full of debris - Tony (Agile) came round and cleaned it out and the hot water is now working perfectly! Thanks Tony, as well as everyone else for all your help.

PS. I will be much more careful when draining the system in the future as it's clear this causes the gunk to move around the system. I'll also be installing a filter to help reduce this problem in general.
£84 well spent(y)
 
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