Chimney fire mystery

[aesmith]

Hi,

We have a small wood burning stove in the living room. Last week shortly after lighting the fire normally, we had what appeared to be a chimney fire. There was a hoarse roaring noise, unrelated to the fire in the stove, and it only died down slowly over two or three minutes when I closed the stove vents fully.

The stove was installed by a previous owner so I decided to disconnect the stove altogether and investigate. As far as I can see it's a flexible liner, and the inside surface looks pretty clean with a light grey appearance, certainly no significant soot build up in the first 10 feet or so that I could see.

I'm a bit puzzled. We get a lot of twigs dropped down the chimney by jackdaws, and originally I thought this must be what had caught fire in there, but now I can see the liner I can't really see where they could get caught. There were none visible when the fire was lit, as I always check.

Any ideas as to possible causes? My brushes are too large for the liner so I can't sweep the flue yet, but will do so as soon as possible. There my be soot deposits further up, but surely those are unlikely to get hot enough to catch fire.

Also, what are the chances that the liner has been damaged, any suggestions about how to check?

Thanks,

Tony S

 

[Burnerman]

Birds will drop twigs down until the flue bridges, which can result in a huge plug of debris.....often enough the fire draws poorly though! Small diameter wire sweeping brushes are available from Toolstation, and a stainless wire rose is a gòod move too. I'd like to look down from the top for damage but thats unlikely by the sound of it t.
John

 

[aesmith]

Cheers. I generally check the flue's clear at the stove before lighting, but of course there's no way of checking all the way up. I was surprised that twigs could catch in the liner though. I was expecting to see a ledge about the lintel where stuff could sit but that's not the case. Will check from the top, and sweep as soon as I've got a longer ladder and smaller brush. My sweeping gear dates from a previous house with a 9" clay lined flue, and it would be much too tight a fit in the liner.

Also fit bird guard while I'm up there!

 

[muggles]

Sweeping the chimney would be a good start, there is guidance on Approved Document J of the Building Regulations on how to smoke test a chimney to check for integrity

 

[aesmith]

Thanks. Would a smoke test show up a fault in the liner? As far as I can see the space between the liner and the existing chimney is filled with vermiculite, so I'm not sure any smoke leakage into that space would be visible anywhere either inside or outside the house.

 

[aesmith]

By the way, what sort of temperature would the metal pipework on a wood stove reach during normal operation? I need to reseal the joints when I refit the stove, and wondered whether high-temp silicone could be used instead of fire cement.

One thing this episode clearly showed was that the stove's not as airtight as it should be, so I'm replacing door and glass seals. There's also a poorly sealed joint between the stove and the first section of pipe, not relevant to control of the stove, but also feeding air into the chimney just when I really wanted to shut it right off.

Thanks, Tony S

 

[muggles]

High temp silicone will catch fire, use fire cement or this

 

[aesmith]

Cheers. Don't want anything else catching fire in the wrong place.

 

[aesmith]

That's it back together finally, I wanted to sweep the chimney but didn't see the point until I could fit a mesh screen at the top to keep further birds and their nest materials from getting in. Finally able to deal with the chimney today. Even with the long ladder I didn't think I'd feel safe sweeping from the top, but with the stove disconnected and moved aside for the initial inspection, we had clear access from the bottom.

That chimney was as clean as a whistle! Absolutely nothing was swept from the first four metres or so, and only a few bits of hard ash from the top couple of metres. Less that a cup full of material in all. That's the first time I've swept a flue with a metal liner, is it normal to find virtually nothing deposited on them? I'm wondering how often it will need to be swept in future, give that it's not easy access I don't want to do it unnecessarily.

Cheers, Tony S