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Joined: 09 May 2009
Posts: 1508

PostPosted: Tue Oct 11, 11 11:20 am    Post subject: Flue question Reply with quote

I've been researching flues for multi-fuel stoves (and eventually, if life goes to plan ha ha a flue for a masonry stove). At present mostly burning smokeless fuel, but gradually moving towards wood - just starting to plant our own for coppicing. Ideally it will be rotation cropped, with the cycle including seasoning the cut wood.

Anyway, today's topic is tar and condensates.

So, I've read that if you have an insulated stainless steel flue inside your chimney, then you get far less condensate and tar.

I've read on a chimney sweeps page that tar and condensate can set solid, not shift with a brush, and these days they recommending putting a chain flail tool down the chimney and smashing it all off back to the original surface once every five years.

I'm not convinced that a stainless steel liner would stand up to that treatment.

So, any comments anyone?

ie Would a stainless steel flue survive a flail?
Would an insulated stainless steel flue need to be flailed?

Anything else?


Joined: 02 Nov 2004
Posts: 34274
Location: Hereford
PostPosted: Tue Oct 11, 11 11:27 am    Post subject: Reply with quote

No idea.

But are they suggesting a length of lumpy chain, essentially that you wiggle to bash off solids from your chimney liner?

maybe I've misunderstood, but are you going to be able to make that move with any force more than a couple of metres away? And if so, will any chimney liner really benefit from being whacked by that?

I think I'm with you on those doubts.


Joined: 09 May 2009
Posts: 1508

PostPosted: Tue Oct 11, 11 6:25 pm    Post subject: Reply with quote

The web page that mentioned the flail was one for a chimney sweep and said

"A major problem we have touched upon in the second paragraph is condensation. For many years, professional sweeps new this crust formation was still present even after brush sweeping. It can take a concrete form, that if impossible to remove conventionally. We also have encountered a hard shiny glaze deposit of a toffee like substance called creosote. In the last few years some of the professional sweeps acquired equipment which flails chains around at high speed. These chains are in turn lowered down the flue and the crust and glaze removed, right down to the bare flue liner again. It is recommended that multi fuel stoves have the chimney flailed (also known as reamed or descaled) every 5 years when using solid fuel. Open fires with high output back boilers should also be cleaned this way every five years also, to reduce the risk of serious chimney fire."


Joined: 18 Dec 2008
Posts: 80
Location: Carnforth, Lancashire
PostPosted: Fri Oct 14, 11 7:56 am    Post subject: Reply with quote

I don't think many of us would want to use a chain flail in a flexi liner - they would be used more often in clay lined flues with heavy deposits.

There is gear on the market for powersweeping flexis, but the flails are nylon line rather than chain. Even with these, there are some concerns amongst sweeps as to whether damage may be caused to liners that are past their best. In some cases though, where liners have become almost or actually completely blocked, there are not many options as a brush will not touch this sort of deposit - so it could be kill or cure.

As to whether a flexi or stainless flue would need flailing, it's very much down to what the customer is feeding the stove, and how they're running it. I've seen liners blocked in a matter of weeks where the customer has been using wet wood and trying to keep the stove in all night - something I never recommend. I've also seen boiler stoves burning dry wood absolutely running with tar when the boiler return stat has been set too low - meaning that the water in the boiler is always cool. This continually cools down the flue gases which are then much more likely to condense in the chimney. I'm not a fan of smokeless fuels in general, but it's fair to say that a dose of the stuff through the stove now and again does help to keep the deposits down.

Also - with reference to the OP - I wouldn't recommend burning wood and smokeless at the same time. Either/or is fine, but both together can result in sulphuric acid formation (sulphur compounds from the smokeless and moisture from the wood getting together) And whilst this isn't going to dissolve your stove into a big puddle, it can significantly reduce the life of the liner. You should also bear in mind that a a 904 grade liner should be used if you're burning smokeless on a regular basis, as the exhaust gases are more corrosive than those from a wood fire.


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