Explosive Baxi Boston2 RS boiler startup when cold

[Casino]

Hi, I recently carried out a service on a Baxi Boston2 RS boiler.
although old, everything works well. Gas rate, burner pressure, flame picture, heat input and gas analysis. all good.
On leaving, the customer happened to mention that sometimes in the morning when it first fires up, it makes quite an explosive bang. I had it on and off several times but couldn't emulate this. I don't know how much of a bang it is, or if the customer is exaggerating.
Has anyone come across the same on this or another old pilot flame lit boiler?
cheers

 

[mylesy]

the gas valve should have a slow open to allow the gas to come in gently. what will happen is it will just come in from closed to open immediatly. and it makes a bit of a noise when this happens.

 

[Casino]

I considered that but it opened smoothly and quietly when I tested it from warm. it only seems to happen first thing in the morning when cold. do you think the cold metal temperature in the morning makes a difference?
cheers

 

[TBServices]

Could be spark electrodes dirty/bent. It's pretty common on ideal isars. Dunno about baxi's


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[SimonJohns]

Tbservices. This is a permanent pilot burner

I would suggest checking burner for dust deposits, small cracks. Good pilot flame. Also injector and gas rate.

 

[Roger Welton]

Not sure about this particular model but have had the internal baffles fail on the burner bars, causing similar issues in the past.

Also agree about basic stuff, is clean or full of fluff, burner pressure, pilot flame, seals on balanced flue, etc.

 

[TBServices]

Tbservices. This is a permanent pilot burner

I would suggest checking burner for dust deposits, small cracks. Good pilot flame. Also injector and gas rate.

Haha. Oooops


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[mylesy]

if its in the morning after its been off. maybe a small gas build up over night. but then again if its a pilot it would burn it already?? so if everything is ok during the day it has to be if the boiler has been off for a while??? have you witnesssed it yourself? a bang would come from a sudden large amount of gas being ignighted. now whether thats from a build up then lit (automatic ignition usually) or a gas valve that opens suddenly or pilot not lighting gas in the right place because of a blocked burner. i would examine these things.

 

[Casino]

cheers guys, I will take a further look as doing other work on site next week.:smiley2:

 

[diamondgas]

Could just be an ageing gas valve getting a bit slow to lift when cold but a lot more fluid when warmed up

 

[Casino]

Could just be an ageing gas valve getting a bit slow to lift when cold but a lot more fluid when warmed up

yes sounds most likely as it was smooth when i tested it.

 

[southcoastboile]

Could also check for leak on nut on pilot connection?

 

[jtsplumbing]

May have a small let-by on gas valve, or it's a bit sticky, Standerd Honeywell valve , just replace it

 

[Syty]

I would check SID if there is one otherwise Gas valve could be letting by, so gas is building up in the chamber when not in use.

 

[Deleted member 25704]

Haha. Oooops


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If it is a permanent pilot then TB has good advice.

Remove the pilot injector and carefully clean it (boil this injector in water for a few minutes, dry, and check injector is clear by holding up to light).

If the pilot is blocked this will not often show when the burner is off, and the pilot is observed to be burning. But, when there is a demand for gas at the burner, the gas pressure to the pilot drops slightly, and if the pilot is slightly blocked, then the flame diminishes at the exact time that gas is released to the burner. The blockage at the pilot injector seems fine until the main burner lights, we don't notice diminishment of the length of pilot flame easily, when the main burner is lit.

So we have a volume of gas, and a diminished pilot flame, which may result in explosive ignition.

If the boiler is natural draught balanced flue or open flue, then the cold air dropping into the comustion chamber from the flue, will be heavy compared to the gas realeased, so this turbulence on the first fire in the morning, with a diminished pilot, adds up to a likely diagnosis for explosive ignition.

But just as Aristotle said, "there is always something we don't know".