Explosive Ignition

[ceecjc1]

Guys,

My boiler got serviced this morning and is apparently in perfect working order, but I'm wondering if it's suffering from intermittent exposive ignition. I shared the symptoms with the Gas Safe engineer who carried out the service but he didn't seem to think there was anything serious wrong. He's the expert and I'm not, but I thought a second opinion might be useful.

The boiler lights very smoothly, as the engineer remarked. I'm generally not around to hear the boiler but on three occasions during lighting I have heard it make a "whoosh" noise and then have to reattempt to light. Always on the second attempt it lights smoothly. What's worrying is that the whoosh seems to be accompanied by a bang from the outside exit of the flue. Unfortunately the flue exit is 5m away from the boiler so it's hard to pin down the bang with the whoosh, if that makes sense.

Does this sound like explosive ignition and is it something I should take urgently seriously? The engineer this morning didn't seem to think a bang from the flue was very likely, and he seemed like he'd been around for a while and in any case would know better than I. He suggested it might be condensate sitting in the flue, though why that would require a relight I don't know.

Thanks,
Chris

 

[gaswizzard]

What boiler?

 

[Dan Robinson]

Place yer bets...

I open with Potterton Promax HE. Possibly a 24 .

 

[ceecjc1]

Powermax 155x

 

[MC2012]

I vote ideal icos?

Matt

 

[ceecjc1]

What boiler?

It's a PowerMax 155x, from 2004.

 

[gaswizzard]

Was a flue gas analyser used? were you left with any combustion readings?

Explosive ignition could relate to a number of things, ie: ignition electrode , gas/air ratio etc etc.

 

[ceecjc1]

Was a flue gas analyser used? were you left with any combustion readings?

Explosive ignition could relate to a number of things, ie: ignition electrode , gas/air ratio etc etc.

Yes, he did the full analysis and seemed very happy and everything was in order. Good gas pressure, but I can't remember numbers. The CO reading was 10ppm, and the CO2 7.5, does that sound about right? He said it was very efficient.

 

[gaswizzard]

the CO2 7.5, does that sound about right? He said it was very efficient.

These should be set up for around 9% CO2 (60/80 ppm)

7.5% CO2 is more than likely the reason for explosive ignition (fuel lean)

 

[45yearsagasman]

Explosive ign is a concern on any boiler, but more so on these, especially if fitted with extended flues. These are joined with a silicone based glue and unless screwed together can separate. I have seen many cases like this and you should get it sorted asap. If your flue is extended, say into the roof space, was it visually checked?

 

[plumbplumb]

Sounds like a job for a digi manometer T piece setup

 

[Dan Robinson]

According to my book - <20ppm CO and 8.8 - 9.6% CO2. So I agree - likely cause is this.


The cure is a different matter though.

 

[ceecjc1]

Explosive ign is a concern on any boiler, but more so on these, especially if fitted with extended flues. These are joined with a silicone based glue and unless screwed together can separate. I have seen many cases like this and you should get it sorted asap. If your flue is extended, say into the roof space, was it visually checked?

I have inspection hatches fitted and he had a good look at the full length of the flue. I'll call them and let them know about it.

Note that it's only going bang very infrequently, so it will be very difficult to diagnose when they get here because I can't reproduce it. I have a service plan so it's not the cost I'm worried about, it's them messing around with everything they can think of while not actually fixing the problem.

EDIT: he didn't seem at all concerned about 7.5%. Seems like everyone's got different views on the subject

 

[Dan Robinson]

Only difference of opinion really is your chaps idea of what it should be and what the manufacturer's think it should be.

If I am playing Devil's advocate I would say the manual for the 155 (no x) does actually say <20ppm CO AND <9.5% - rather than giving a range.

But.... it is what it is and your chap has not left it correct.

 

[ceecjc1]

Ok, thanks Dan, everyone. Looks like I need to give them a call.

Cheers,
Chris