I’ve been able to find excuses for several days why I should do something else instead of sorting out the blocked flue pipe of my wood burner, which is all very well except that it had to be done at some time. Anyway, today they all ran out so after lunch I found myself back on my roof removing the slab from the top of my chimney so I could release the clip holding the top of the pipe. Then I had to go back downstairs so I could dismantle the flue length by length and take it out. Like many jobs that we dread before starting them, in fact it didn’t turn out half as bad as I thought it was going to. I thought that I’d get lots of soot falling down, but actually what did end up in my fireplace was particles of dry clinker that could be easily swept up without making much mess at all and in only half an hour or so, I had all six lengths of pipe standing on their ends outdoors.

In the process, I was able to recover the little sweeps brush that had been left up there when the cheap and nasty plastic tube posing as a rod that it was attached to had snapped but I was more interested in finding out what had been causing all of the smoking problems that I’d been experiencing as the winter wore on. And I didn’t have to wait too long, because as soon as I took out and examined the third length of tubing, the reason was obvious as the following image shows.


There was a solid plug of clinker completely blocking the upper end of the pipe. I initially gave it a couple of taps but there was no way that it would budge in either direction and no wonder, as I later found it to be about 6 inches thick. There was no way that any smoke was ever going to get past it when the stove was lit so it was hardly surprising that I had been getting so much smoke coming out into the room, because basically there was nowhere else for it to escape to!

I then compared this tube with the one above it and found there to be no comparison, as the following picture shows. The blocked pipe (length 3 above the stove) is on the right and the next length up (length 4) is shown on its left.


There was a similar story with the length below it (length 2 above the stove) which, although it got the smoke and fumes from the stove before the affected one, was surprisingly a lot cleaner, as shown below.


There’s obviously quite a bit of science involved here, but on the evidence, it appears that for my flue, the gases rising from the stove stay too hot to condense up to a height in the flue of about 3 metres. At this height, however, they do start to condense, apparently over a flue height of about a metre. This appears to be so because as the second image shows, the top of pipe length 4 is relatively clean and clear of clinker or ash of any sort.

Obviously condensation has occurred at quite a high rate in my flue for the amount of clinker produced to build up to such an extent that it could not only completely block it, but also grow to a plug of thickness about 6 inches. The question now is how to avoid the same thing happening in the future. My neighbour Jean-Claude told me some time back that he’d had the same problem and that what you had to do was seal the top of the chimney from which the top of the flue pipe emerges. This would obviously lead to more heat being retained around the pipe and have the result of moving the condensation point higher. Even so, I doubt whether it would be enough to move it right to the top of the pipe so that all of the products of combustion escape into the atmosphere, the ideal solution, so even when I’ve done it, I’ll just have to make sure that the flue is regularly swept every few weeks when its in use, to avoid too great a build-up of clinker before a blockage occurs.

I now understand why people pour Vermiculite insulation down their chimney to fill the space inside around the flue pipe, as this would keep even more heat in and raise the condensation point even higher towards the top. However, I decided in the beginning not to do that and I’m glad that I did. Imagine if I’d then had to dismantle the flue as I did today – it would all have fallen down in a dirty great heap in my fireplace and all over my lounge floor. That I can well do without 😐

2 thoughts on “Can you believe this?

  1. Bruce, that was a new flue that I installed myself only last autumn, so all that happened in just a few months between last November and February/March of this year. That was only about 16 or 17 weeks!!! OK, so this explains why it’s so important to burn dry, seasoned wood. All of the stuff I was burning had come straight out of my garden, most of it in the beginning from trees that were standing but dead but towards the end, from trees that were dormant (in winter) but that I needed to take down. Obviously they all still contained lots of liquid that just went straight up the chimney and then condensed causing the problem. I guess it’s called ‘learning the hard way’. Luckily, all my old wood has now gone so I can now buy in a couple of ‘stere’ of seasoned, dry oak for this winter which, with regular sweeping, should avoid it happening again.

  2. That’s unbelievable, Roger.
    I wonder how man seasons that accumulation covers?

    We had a wood-burning heater in Canberra for a decade and the flues here are double-skinned, with some form of insulation packed bewteen the inner and outer skins. Maybe that’s the reason.

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