Baxi 100 HE overheats and melts heat exchanger - an explanation

[buckley plumb]

I thought it was the gas valve to start with. I was lucky to see the ceramic move as the thing overheated. It was a one in a million chance that I saw the movement. It took me a couple of months of contemplation after that before I realised the implications and could formulated a coherent explanation.

The difficulty is that after a boiler has overheated and switched off, the glued-in ceramic cools off, solidifies and looks perfectly OK. So even in a bad overheating incident there would be no reason to suspect the burner. I have not had so much experience with the older burner with the cloth-like, or fibrous element, but the one I have seen had very poor seals around the edge where it was bedded into the pressed steel burner base.

In my opinion this is manufacturing and design fault. After a fair bit of investigation I realised there is no national authoritative body to report issues like this. The Gas Safe Register can't help, and as many of their staff where employed by the large boiler manufacturers, one could be forgiven to think they are there to suppress any bad press about their old employers products. The Fire Brigade can't help and the HSE want someone to die before they will get out of bed. After reporting it to my MP it did get as far as the Baxi's CEO but off course he said it is fine, and my MP believed him!

I would love to get hold of a set of pictures of melted heat exchangers from this range of boilers so that I could re-start my campaign to force Baxi to start a national repair process. There was a picture in the September Gas Engineer but despite numerous attempts at contacting the contributor, he would not respond.

I would be interested to know how you get on. Could you provide the boiler CG number please?

wish i had seen this post earlier - but did get there in the end put my photos on the thread baxi 100he combi

 

[mutley racers]

Hey Buckley where the pics

 

[TheGasEngineer]

The only way of stopping thermal runaway of those BAXI fireboxes would be by restricting the gas flow by gas isolator/service cock (partially closed) before the boiler (or fitting a suitable in-line orifice), so it can't maintain 1:1 air/gas ratio on high outputs (above rated power)...

I would certainly not recommend running the boiler with a defective burner. A fix like this could result in boiler meltdown just as much as with the fault left unaddressed.

...As to me, this is major design flaw: why the governor allows more gas in than the rated boiler input? ...

The regulator is a 1:1. This means that it adjusts the gas flow to match the air flow through the fan. The effect is that the fan speed controls the flow of combustion mixture flow. The heat input is therefore controlled directly by adjusting the fan speed. If the burner ceramic face lifts and creates a large gap, the fan is then allowed to over-speed due to lack of back-pressure. The gas flow rate follows, and as said before, furnace-like quantities of combustion mixture enter the combustion chamber. No wonder it overheats and melts...

It you read my report you will see that there is every chance that no one will ever realise that the boiler has overheated because of the way in which the fault shows itself. I would imagine Baxi realise this and don't admit to the fault and steer clear of discussing it. That's why I suspect that no insurance claims have been made.

 

[buckley plumb]

Hey Buckley where the pics

on the one i posted but will put them here as well

 

[mutley racers]

Blimey. Thanks Buckley

 

[buckley plumb]

as this post states only noticed the burner panel lifting on full hw max gas rate as stated by thread starter one to watch for - this customer was lucky from what the thread says the heat ex could melt .

 

[TheGasEngineer]

These are brilliant photos. Thanks to Buckley Plumb. I wish we could find more examples and then I would be willing to go back to my MP. In fact I may just do that anyway. I know there are other boilers out there that are much worse. This is not too different from the one I encountered in 2010 which finally made me think about what was happening. I mean, would you look at that burner and think there was anything wrong with it? Probably not. Clearly a really hot flame has come from, presumably, the burner. But then there is the dilemma of how could that happen? Once it is explained that the glue of the ceramic gives way and allows the ceramic to lift and create a big gap around its edge, then the downwards flame becomes a realistic proposition.

The Sherlock Holmes - type mystery is then put in place by the ceramic settling back into the burner base making it look perfectly OK. And once it has cooled the ceramic will be secure and not give any clues that it is faulty. The one I saw in 2008 had a totally melted heat exchanger but the burner looked just the same as above. I didn't take a photo ... aarrrgggh!

I actually think this boiler would work perfectly well if this fault did not exist. Ok, the trap would be a pain; but there would be none of the lock-out or overheating issues. I have replaced burners on a couple of these and they are running fine. I have a note when the third anniversary is due, and the customers are happy to pay for new burners as I have recommended.

Actually I think BPs customer was lucky because this boiler could easily kill. It could start a fire as easily as can be. There may already have been incidents. A full-blown fire would destroy the boiler though and effectively destroy the evidence that shows how the fire started. Baxi are fully aware of this issue now because I have told them. They know however that any fire would destroy the evidence that would otherwise condemn the boiler. The only way we can make the authorities take notice is to find more pictures and installers who would be willing to tell their story.

 

[Dotty]

Right, now we've non verified gsr's joining in on this. I've been uneasy about where this thread was and letting it ride but enough's enough.

Moving to the private forum.

 

[arran197]

There's a few of these on my patch, I've changed 3 burners so far I think. I'm just waiting for the next one to go.

 

[SafeGasInstall]

I have re system boiler at the new house , boiler does not responded to thermostat knob and only cuts out on the high limit ..... thermistor reads 2.6 k ohms
high limit stats are on 0.5 ohms .... Burner pressure is 18.5 mbar
any one experienced this ?