Creosote leaking from a chimney (C) Daniel FriedmanChimney Creosote FAQs

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Chimney Creosote Questions & Answers:

Frequently-asked questions (and answers) about the nature of, cause, cure, prevention, chimney creosote from wood-burning and some other heating equipment.

This article series describes the formation of creosote in wood-burning heating appliances such as woodstoves and fireplaces and gives advice on reducing the fire risk. Creosote deposits accumulate in all types of chimneys - masonry, or metal, where wood burning appliances are vented.

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Chimney Creosote Cause, Dangers, Prevention FAQs

Chimney Creosote Hazards (C) Carson Dunlop Associates

Question: causes of oily liquid leaking out of wood heater

(June 30, 2015) A. Weido said:

Lately I noticed this nasty yellow liquid dripping down the chimney, all around it pretty much, and it's not a lot. Maybe a few ounces make it to the floor of the basement, and look really yellow.

The liquid coming down the chimney is almost gummy, it's like sap almost, and in a few spots some has dried into a gummy trail just like sap along the exposed outside of the chimney in the basement. What could this be?

[Click to enlarge any image] At left, sketch of a masonry chimney showing its components at a fireplace, courtesy of Carson Dunlop Associates, a Toronto home inspection and education company.

2017/02/15 Jon said:

I have a wood furnace. I loaded it up and titned it down before leaving the other night so it would slow burn through the night.

When i got back there was a black oily liquid everuwhere in the ground around it, on the back, dripping from the lower part of the dmoke exhause ventbright bu wjere you close and open the vent and some on theninside of the door.

Could this be from the kind of wood that was burned or something else... Also a different question, is it ok to leave the ash door open and close everything else (the smoke exhaust valve and the smoke bypass rod) really get it going and staying going if i sm around it and want itbto really stay warm?

This question was posted originally at BACKDRAFTING HEATING EQUIPMENT

Reply: probably creosote from burning wood at a low temperature - also watch for beehives


Sounds like dissolving creosote or chimney deposits. I've also seen such mess oozing out of chimneys after bees had made a honeycomb therein.


Oily liquid from a wood-burning appliance is, especially under the circumstances you describe, most-likely creosote that has condensed in the wood-furnace flue and then leaked out of the system onto the floor below.

When you run a wood heating appliance very slowly the flue is cooler and the condensation rate of creosote increases.

See if you can spot where the creosote liquid was leaking out onto the floor, as it might also be that your flue sections or chimney sections are installed upside down. Normally the female end of each section faces "up" so small amounts of condensate drain back into the heater rather than leaking out at the section joints.

Watch out: accumulation of creosote in a chimney can result in a very dangerous chimney fire if later a hot fire ignites the creosote in the flue. A certified chimney sweep can tell you the condition and safety of your chimney and flue.

Yes the type and condition of wood is a factor in creosote production; particularly, green wood that has more sap content produces more creosote.

Research on the production of creosote from burning wood:

Excerpt from Baker whose article appears in a USFPL PDF

Unburned volatiles resulting from incomplete combustion of wood can cause a large loss in efficiency. These volatiles are also a source of trouble in stoves and furnaces because of the creosote, tarry substances, and acetic they contain.

To burn these volatiles, sufficient air must be supplied around or over the fuel bed. This air must be mixed with the volatiles while they are still hotter than l,lOO°F and have not come in contact with cooling surfaces.

Theoretically, it takes six pounds of air to burn one pound of oven-dried wood, but actually more air is required because the air and volatiles are not thoroughly mixed.  (Baker 1982)

Question: chimney safety steps when changing fuels

(Nov 29, 2011) question said:

We have a hundred yearl old house. AT one time it used a wood burning stove in the kitchen.

The wood burning stove was replaced with a propane gas heater which was used for over 40 years. We have recently replace the heating system with a propane gas furnace, utilizing the same chimney.

The upper portion of the chimney had been replaced about 20 years ago.
Since we started using the furnace, we have a brown exudate which i assume is creosote leaking through the lower portion which had not been replaced.

What needs to be done to correct this? can it be corrected without replacing the whole chimney which runs from the basement through 2 stories and an attic?



Whenever you change the kind of fuel being used to burn and vent into a chimney, the chimney should be inspected for safety and if necessary cleaned. I suspect that your chimney was left with creosote deposits that should have been cleaned off. At this point, power cleaning might be needed.

If the chimney is otherwise sound and safe it can be used MAYBE: the flue that was suitable for venting a wood burning stove might not be the proper size for venting propane. Chimney size (and height and other factors) need to properly match the type of fuel and the BTU rate of the propane furnace.

You should have an expert check your chimney, but also take a look at FLUE SIZE SPECIFICATIONS ( on this page Continue reading provides an INDEX to RELATED ARTICLES that includes a live link - ).

Question: is creosote smell dangerous?

(Mar 25, 2014) Anonymous said:
I have a multi fuel stove. Just been in loft and noticed a strange smell,on further inspection ive noticed a tar like substance running down chimney breast could you tell me if this is serious.


Creosote leaking out of a flue is itself corrosive, and of course the leak out may mean air leaks in which can interfere with good draft; more serious would be enough creosote build up to block the flue (dangerous carbon monoxide gas poisoning) or to invite a chimney fire when you switch among fuels.

To be safe I'd have a professional inspect the flue and advise on cleaning needs as well as leak repairs.

Question: did snow cause a chimney fire?

(Sept 28, 2014) David Jones said:
Could snow build up on the outside cause a fire in the chimney if the log fire was just smoldering. Say it had been smoldering overnight and snow started at midnight could a fire in the chimney be started



Snow build-up would not be expected to *cause* a chimney fire but if snow build-up closed a chimney top a result might be smoke indoors as the chimney may no longer vent properly.

With some fuels there could be a more dangerous or even fatal carbon monoxide poisoning as well.

Question: fireplace makes chemical toxic odour - horrible to inhale it also stings the eyes

(Oct 2, 2016) Clare said:

I have moved into a rented old bungalow (about 100 years old at least). The previous tenant had taken the fireplace and the property had been unused and left in bad condition for 2 years.

The bungalow has been given an update including a fireplace surround created with a piece of wood for the mantlepiece and a woodburner installed. The fireplace surround is painted in what looks like black heat resistant paint.

The owner said the chimney had been cleaned about 9 months ago. The woodburner burns consistently without problem and gives off a good heat.

After about an hour there is a chemical toxic odour fills the room and all doors and windows have to be opened as it is not only horrible to inhale it also stings the eyes. So it would appear that the increase in the heat and length of time the fire has been burning is causing something to give this problem.

There is a large gap at the back of the wood mantlepiece that has been filled with fibreglass, the type you insulate lofts with. As it is open to the heat, could it be this? The carpets are new could it be that? Or could it be the creosote you talk about in you're excellent articles?

The masonry in the chimney will be old and i doubt that a liner was paid for to be put up there as things have been done on the cheap!

No smoke comes from the fire and once the heat dies down the smell does too. Any advice would be gladly taken. Hope you can help. Thanks Clare

Forgot to say that there is a regular sound when the fire is lit or unlit of falling bits down the chimney into the woodburner but not much evidence. - See more at:

Reply: stop using fireplace, ask for a safety inspection

Clare I cannot of course inspect by e-text, but what you describe sounds unsafe, including risk of a fatal building or chimney fire. I would not use the chimney nore wood stove until the system and chimney have been inspected by a certified chimney sweep or other qualified building official.

(Oct 11, 2016) Clare said:
Hi, thanks for your reply. Awaiting the installer coming back from his hols! won't attempt to use till he has been.


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