Tar in my wood burner chimney stack.

[michael]

I’m just getting my wood burner ready for the chilly nights ahead. My fireplace is a modern addition to the property which originally, as an old Baptist Sunday school, had virtually no fireplaces at all, just a couple in two of the small anti rooms.. The Fire place in question (the new one) is large enough to step into (with a slight ducking of the head) and is separated or sealed off from the stack above by a tight fitting metal plate which touches all four walls of the fire place. The flue from the wood burner, (about 3ft high) penetrates the metal plate in the centre through an almost exact hole discharging the smoke into the stack. The edges of the metal plate are sealed against the fire place walls with fire cement as is the flue where it penetrates the metal plate.
There is a small inspection hatch in the metal plate that allows you to look up into the stack. The construction appears to be brickwork corbelled towards the centre and then a vertical shaft about 9” or a foot square which appears to have a brown salt glazed finish.

My problem is, the interior of the stack appears to be caked with a dark brown/black tar and there is evidence of this trickling down the metal flue towards the top of the wood burner where it has broken through the fire cement seals. I generally use logs for fuel but also any other old wood to get the fire going.

My neighbour recently gave me a load of old thin chopped up planks from an old garden shed that he had recently demolished. I haven't used this wood yet as I’m sure it would have originally been treated so I’m not sure if its any good for wood burners i.e. would it just add to the tar problem? Lastly can a ‘tar caked’ chimney stack be cleaned as it’s not your standard soot that can be brushed out and vacuumed away?

 

[Flyfisher]

If the smoke from your wood burner discharges into a cold chimney the tars will condense before they exit the flue. It sounds like your stack is not warming up enough to prevent the tars condensing, which is not unusual unless you maintain a constantly roaring fire.

The ideal flue arrangement for stoves is for the flue pipe to run continuously up the stack to the pot. Any condensing tars can then only run down the flue pipe and back into the stove. Even better, the flue pipe should be insulated so that the flue remains hot - i.e. above the tar condensing temperature - right to the top of the stack. With a 9-inch flue it would be fairly easy to fill the void between the brick flue and a chimney liner with granular insulation (I think it's called Leca).

An old garden shed will probably be some sort of resinous soft wood and, as you say, probably treated with all sorts of chemicals. It will certainly burn OK but you probably don't want the combustion products condensing in your stack.

I don't know about cleaning tar residues from wood burning flues. Might be worth calling a local chimney sweep for advice.

 

[Andrew Bodenham]

Michael,

You are describing a common but rather unsafe stove installation, and I agree wholeheartedly with Flyfisher's comments.

A (relatively) large cross-section masonry / clay-lined flue like yours is not really suitable for a woodstove installation unless an additional liner is used. You are right to be concerned about tar residues, as these can ignite into a full-blown chimmney fire if allowed to build up too much. You may also find problems with condensate leaking from the bottom of the flue when the stove is first lit from a cold start.

A stainless steel liner of 150mm dia would be relatively easy to install and would make the flue safer. In my experience I have not found tar build-up to be a problem; I think this due to a combination of the flue running hotter due to insulation and the detachment of residues due to thermal expansion of the flue liner. One can hear particles falling down into the stove, especially when going from a cold start to a very hot fire. However, I do still sweep the flue periodically as a precaution.

I would seek the advice of a flue installer about your particualar situation.

 

[JoceAndChris]

My neighbour recently gave me a load of old thin chopped up planks from an old garden shed that he had recently demolished. I haven't used this wood yet as I’m sure it would have originally been treated so I’m not sure if its any good for wood burners i.e. would it just add to the tar problem? Lastly can a ‘tar caked’ chimney stack be cleaned as it’s not your standard soot that can be brushed out and vacuumed away?

I really wouldn't burn treated wood. We tried, the fumes were so awful we had to give up - that was an old telegraph pole.

 

[Penners]

Excellent advice from Flyfisher, covering all the relevant points. You really need to install a full-height insulated flue liner. This could be a single-skin liner, with the void between the liner and the chimney filled with loose insulation material - Leca or vermiculite Or it could be a self-insulated double-skin liner (a lot more expensive, but you may save a fair bit on installation costs).

Once you've got your woodburner installation sorted out, you don't need to be too fussy about the wood you burn. Just be sure that it's fairly dry, to minimise tars. Burning treated wood does tend to release noxious vapours, but so long as you keep the woodburner doors closed, these will go up the chimney.

And instead of poisoning you, they just poison the planet.

 

[michael]

One other point. This is a single story building. The flat part of the ceiling running through the main living room from gable end to gable end is 15ft above floor level and the roof ridge a further 4ft above that. The chimney stack/pot comes out quite low on the pitch of the roof so the top of the chimney pot is probably no more than 14 or 15 ft above the burner. The stack itself therefore is probably no more than 12ft in length if allowing for a two or three foot chimney pot. Don’t know if that makes any difference to the performance of the flue?

I’ve just had another look up through the inspection hatch. The salt glazed shaft (internal face of the chimney stack) is certainly no more than 9” square. Would a flue liner fit in that?

 

[Penners]

The stack itself therefore is probably no more than 12ft in length if allowing for a two or three foot chimney pot. Don’t know if that makes any difference to the performance of the flue?

I’ve just had another look up through the inspection hatch. The salt glazed shaft (internal face of the chimney stack) is certainly no more than 9” square. Would a flue liner fit in that?

As Flyfisher said, the very fact that you have visible tars is empirical proof that you have a condensation problem in the chimney - regardless of height.

As regards the fitting of a liner, you could certainly fit a single-skin one - always assuming that your stove flue pipe is a standard 150mm one. Then you can pour in insulation granules from the top.

This page of instructions is quite useful: http://www.fluesystems.com/liners/info/chimney_liner_installation.htm

 

[michael]

Thanks for all your advice.
What could I expect to pay to get the job done by a professional flue installer?

 

[MdB]

As regards the fitting of a liner, you could certainly fit a single-skin one ...

Don't stove liners have to be double skinned? I think somebody told me this sometime in the past.

 

[Flyfisher]

Regarding the short flue height, as long as you have adequate 'draw' (and I'd be surprised if you don't with a stove) then this should work in your favour as the flue gases should have less time to cool inside the flue. But this is a rather academic point given that you clearly do have condensation.

The good news is that the shorter the flue, the cheaper the flue liner!

The existing 9-inch square flue should easily accomodate a suitable liner. You'll need to check the requirements of your stove but expect a suitable liner to be around 6 inches in diameter, so there should be plenty of room. Penners' suggestion of a double-skin/insulated liner is a good one if you decide to insulate - I'd forgotten about that option.

Installation costs can vary a lot so get a number of quotes. Some installers may want scaffolding or cherry-pickers, others may just use a roof ladder. You can also DIY if you feel confident enough - it's not a difficult job as such, but there is the complication of height and access. The liners can be 'hung' from the top of the chimney pot so it's not always necessary to remove and replace the pot, which can be tricky. However, if the stack needs any maintenance such as re-pointing or re-flaunching around the pots then this is a good time to do it - but it's not the sort of job to be tacked from a ladder.

Finally, however the work is done it should be certified. This can be done by Building Control (at your local council) or by a HETAS-registered installer. If you don't DIY then a HETAS-registered installer is probably the best solution.

 

[Ushy M]

As regards the fitting of a liner, you could certainly fit a single-skin one ...

Don't stove liners have to be double skinned? I think somebody told me this sometime in the past.

I believe this is the case for solid fuel stoves. The single-skinned ones are for gas and oil appliances. We were chuffed to find there was a liner already in the flue we wanted to put the woodburning stove in, but found it was single-skin and left over from where the previous owners had their Rayburn. Apparently it's to do with the comparative temperature of the flue gases which would burn through a single-skin liner in fairly short order.

 

[skier-hughes]

Without checking, isn't it the quality of the steel that is different for the two fire options rather than single or twin skin?