What's this brown gunk?

I've got some brown stuff coming out of my chimney.

Here are some pictures.




When i scrape it off after it's dried, it doesn't really burn - but melts.
It's coming down the chimney from the top of the chimney.
And seeping onto my heat exchanger. There are some flat horizonatal surfaces inside the heat exchanger, which i can't easily access - so i'm a bit worried that the gunk is building up in there.

I'm overseas, so can't call an engineer or do access any hi tech shops - just need some basic advice - which would be much appreciated

there a description of the stove here : Backyard welded stove. Key issues with burning coal?

do you have a cap on your chimney?? what are you burning bit or something else?? I think you've got a combination of both those causing the goop.

I had wondered whether the coal i'm burning is related.

I've just started a seperate topic to try and find out about the fuel :
is this Bituminous coal

The chimney is vertical and have a fixture on the top which i guess is to stop rain / snow going down it. Is that what you mean by a cap?

Does that mean i should remove this and the brown gunk will stop?

You can't just guess it is stopping the rain & snow. Somehow you need to get up there & make sure. I saw the old post from the mines?? you got your burnable STUFF??? bit, lignite??? I'm going back to the link you've got in this post. I think some people told you how to test it for being bit???

freetown fred wrote:You can't just guess it is stopping the rain & snow. Somehow you need to get up there & make sure. I saw the old post from the mines?? you got your burnable STUFF??? bit, lignite??? I'm going back to the link you've got in this post. I think some people told you how to test it for being bit???

If the rain / snow is getting in - would that contribute to the brown gunk?

Can't guarantee anything, but I always work on the obvious process of elimination concept.Being we're not sure what you ARE burning, if moisture is hitting it, yes, it may well be contributing to the goop mess.

With anthracite the general recommendation is the flue pipe should have the male end going towards the thimble. This is small preventive measure to keep gases inside the flue pipe.

With a wood stove it's the opposite so the creosote that leaks down the chimney doesn't end up on the outside of the pipe like the images you have posted. Looking at your pipes you have them set up like you would for anthracite. If it's leaking from a section near the top where the male end is going into female end If you switch the pipes around you can prevent it from leaking however that of course doesn't solve any problems for what it's doing inside.

While on the topic I don't see any screws holding the pipe together, unless that is weld that I see get some screws for it. 3 per joint.

I've also seen the brown gunk coming out of the bottom of my T outside. I've concluded that its condensation from moisture in the coal condensing on the inside of the pipe and running down carrying fly ash with it. BUT since I've installed the Baro Damper it seems to be a lot less to nearly gone. I'm guessing the extra air from the baro is slowing the condensation process and the moisture is being carried out the chimney. Just a hunch

Could this be a form of coal tar? Depending on the type of coal being burned.

It is highly unlikely that's from the moisture in the coal itself; if the coal isn't sopping wet when loaded into the stove it's from rain/snow entering the stack. A barometric damper will lower the dewpoint of high moisture stack gasses and prevent condensation, but I doubt that's what this is. Regardless, it is NOT creosote or "coal tar" it is simply soot mixing with water and dripping out of the pipe as a brown mixture. On any solid fuel appliance, wood, coal, anthracite; wherever there are any solids in the flue it's best to have the male ends of the pipes pointing DOWN towards the stove so that anything that gets into the pipes (moisture, soot, fly ash) will fall back into the stove instead of falling or seeping down the outside of the pipe. Even a high moisture coal shouldn't see stack condensation from the fuel itself outside of an exposed, external flue pipe and extreme sub-zero temps. There's just not enough moisture and hydrogen in coals of any variety to make this a problem for all but the most extreme scenarios.

Berlin, I don't know what else it could be other than moisture in the coal. I have a cap on top and I sealed every section with hi temp silicone so I don't think any precipitation can get in. But, as mentioned earlier I did see a substantial decrease in the gunk since the barometric was installed. Any other suggestions?

do you have single wall pipe on the exterior of the home? if so, wrap with fiberglass or other non-flammable insulation and then wrap some thin gauge metal etc around it to keep it from getting wet.

Of course the best solution is simply reverse your flue pipes so that the male end points down as it should. Even if it's condensation from the flue gasses, it's not creosote so there's no real issue here other than keeping it out of the house. Either increase the temp/insulate the pipe so that condensation doesn't occur, or, better yet, reverse the pipe so that it just runs back into the stove where it will be vaporized and become a non-issue.

on edit; i remembered that lightning is not the O.P. : (btw, what coal are you burning?)

stainless pipe, even well insulated double wall etc. has little thermal mass. This can tend to create condensation during certain parts of the fuel burn cycle. As unlikely as this is with any coal; flue condensation is a result of low stack temps and a high dewpoint. If you have condensate mixed with soot running out of the flue you can either: increase temp of exhaust gasses, lower dewpoint via baro etc., or make the pipes face down so that nothing leaks out.

Did you burn wood with that chimney before going to coal ?

I have the same gunk coming down my pipe also.It's been so rainy and warm this season that I figured it's just water and flyash mixture.I also have a cap on my pipe and you can see the flyash mixture coming off of it.

Mike

Hey guys - thanks. That's really helpful.

The consensus of opinion seems to be that it's fly ash and water. I do think the coal has quite a lot of water in it. so this would make sense. I could probably improve the cap on the top of the chimney, but the gunk is happening, even when there is no rain or snow. There's more gunk on the cap - so this would make sense too. I had to have the chimney over a neighbours garden, so I'll have to check that it's not dripping into there yard

The flue pipes aren't so well made that I can absolutely choose which way to orient them - especially at the top of the heat exchanger.

I can live with it coming out of the pipes are bit, I was more worried it case it was an indication of something bad. So i'll probably store this thought up until next autumn when i reinstall the stove.