Baxi banging - please help

18 Oct 2005
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United Kingdom
Hi wonder if anyone has any ideas with a problem i am having with my boiler. It is a Baxi FS 801S RS.

On startup the gas doesnt seem to be igniting quickly enough. The gas hisses immediately, when i turrn the dial, and builds up and then finally catches (up to 30sec later) with a huge bang (like explosion). The boiler then runs fine until it reaches its temp and cuts of. when it needs to ignite again the same situation huge bang.

I have had an engineer replace the gas valve, the pilot light, the thermo couple and had the meter repaired to output the correct pressure. He also cleaned out the boiler and burners etc and set all correctly. He is totally stuck with it and I dont want to fork out for a new one as its working great apart from this problem.

when the boiler is cold and I turn on it ignites Perfectly no delay no bang etc. It's only when the boiler is hot and it tries to ignite that the explosion is occuring.


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Well it may soud a bit obvious but does it spark cleanly and straight away?
Hi, thanks for your reply.

The pilot light sparks, ignites and burns fine. engineer also said that it's at correct level.

When the pilot is burning, I switch the heating button on the controller to 'ON' to fire up my pump then I turn the dial on the front of the boiler to 2 or 3 and you hear the gas valve opening and releasing gas with a hissing noise but no immediate ignite. Then after twenty to thirty seconds the gas ignites with an almighty bang, due to the amount of unburnt gas which has built up in those 30 sec (it's actually exploding when it ignites). :(

Any ideas?

Thanks in advance.
i dont do a lot of gas fires but could it not be a flue problem say a down draft or lack of a draft also is there a fan in the flue ?

also might want to check the oulet pressure on the gas valve but the engineer may have already done this
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Have him recheck the burner, paying particular attention to the inside cleanliness of the burner. Check also that the main injector is correct as stated on the data plate.

If the pilot is badly maladjusted it may not ignite any part of the burner and explosive ignition may result, but usually after only 5 seconds or so.

I'd go for the injector being partially blocked/wrong. Good luck and let us know the result (but only if I'm right :oops: )
Eireplumber, it is a boiler not a gas fire.

I have come across this problem and could not find a remedy either. I spoke to Baxi who were not much help either. It would seem the pilot assembly is fitted the wrong way round (but then I am only a heating engineer not a designer of boilers).

My reasoning for explosive ignition is that there is significant amount of gas in the chamber before the gas reaches the pilot assembly. With the pilot pointing towards the end of the burner bar, I suspect a lot of gas would have to flow before the pilot sees the gas mixture. This leads to explosive ignition. I would suggest a replacement burner but there is no gurantee that will fix it.

Other 801 owners could help by posting if their boiler is operating correctly
Have you (or your heating engineer) checked that the air intake in the flue is clear and unobstructed ?. Also I know he cleaned the burners but did he actually remove the gauze from inside and clean it?
Hi guys thanks for all the suggestions so far.

I have a quick update which may help pinpoint the problem.

When the boiler is cold and has not been turned on all day I fire it up and it ignites perfectly with no delay, bang or problems whatsoever. The problem only occurs whent the boiler is hot. Also on inspection the pilot light appears an orangey glow.

When I remove the pilot assembly the draught which is coming through the chamber appears a lot. It is sufficient to blow the pilot light out sometimes.

Could the problem be heat related or due to the mixture.

thanks in advance.
Excess air flow bothering the pilot shouldn't be a problem. Hopefully nobody's running it with the case off...? (It IS a common problem where there's a fan!!)

Burners sometimes need to be removed and cleaned thoroughly from the inside, which can be nearly impossible - this is what others are saying in other words. But did the engineer take them both out?

Chreck the Burner Air Guide is in the right place
When he changed pilot assembly did he renew the injector or use old one, have seen same symptons caused by hole in injector not being large enough due to very minute deposits on it causing it to close slightly as it gets hotter. New injector req or clean it out with a injector tool which is a very very fine piece of metal that goes right through injector hole ( not a needle).

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