I've never seen a zero-clearance factory-built wood-burning fireplace that was designed to function like a wood-burning stove. When wood burns slowly due to a reduced air supply, it doesn't burn completely and that creates more smoke and a cooler chimney preventing a strong up-draft. It is important to allow the fire to burn hot for the first hour to dry out even dry hardwood and warm the chimney. Green/soft wood with restricted air flow without the catalytic element found in efficient wood burning stoves can add to the smoke and cool the fire resulting in creosote deposits in the chimney and soot on the roof when the smoke hits cooler air or a downdraft at the top of the chimney. This is a difficult system to keep in balance; even the design of the roof is important.

IMO, if you want to heat efficiently with wood you should use a cast iron wood-burning stove designed for that purpose. I don't see how such different devices can be combined. How would you even size the chimney? But I learned what I know about burning wood in the 70's before smoke was a pollutant and efficient stoves were more popular than wood wasting fireplaces.

Renovator, I'm embarrassed to admit I dint comleyely understand your comment. The fireplace store installed this--we didn't jerry-rig it or anything.

So the zero-clearance fireplaces are actually in all technical terms, wood stoves, that are designed to be in close proximity to flammable objects (ie. walls), which allows them to be inserted into a chimney cove much like a fireplace. But in reality, the design in such that they have a fresh-air intake that draws cool air from outside, feeds through the fire, often INCLUDING a catalytic converter if you have a high efficiency one (which most do now of days), which re-burns to reduce creosote build-up and then expels the smoke and soot up a stove-pipe just like what is used for wood stoves...only they're usually not as visually attractive because they are tucked inside a false "chimney" chase.

There are some reasons why soot builds up on the exterior of your chimney, and some things you should consider that could be causes:

1) As Renovator says, down-draft can cause excessive soot build-up, along with starved cool fires. You need to consider if you burn your fire nice and hot (blue flames) or if you tend to have cool burns (which are often "showier" fires).

This also applies for the insulation of your chimney. If you are in a cold climate, it is usually recommended that unless you have a catalytic converter (and even then really), to insulate your chase to the TOP, right to the point of the chimney pan. This keeps the chimney pipe warm all the way to the top, reducing the cold down-draft and the settling of soot within your chimney pipe. It is possible that your chimney chase is not insulated the entire length, and that it is cooling in the last couple of feet, causing the creosote to settle out shortly after it leaves the chimney.

2) What type of wood do you burn? Different wood has different levels of creosote that they release. Are you burning sappy woods? Are you burning woods like poplar that break down very quickly in fires, meaning you're burning twice as much fuel?

3) Is your wood dry? This is a HUGE one, as wet wood increases your creosote. Just because it's dry to touch, does not mean it's dry wood. What are your storage conditions? What is the humidity where it is kept? Does it acclimate in your home before burning? How long ago was it cut? Is there any rotting that could retain excess moisture?

4) How high above the top of your chimney pan is your chimney cap? Most designers will extend the chimney pipe 1 to 2' above the chimney pan, so that the soot will travel away from the pan before landing. If your cap is really close to the pan, this increases the likelihood that the soot is depositing on your chimney instead of being carried away.

5) Are you in the open or in a sheltered space? Winds can help carry away soot as it leaves the chimney, and a well sheltered chimney can cause the soot to fall shortly after it leaves. Are you finding soot predominately in the direction of the prevailing wind?

6) Have you cleaned your chimney pipe (or had it cleaned) in the last year and a half? It is important, irrespective of fireplace/woodstove type, to have your chimney cleaned on an annual basis to remove creosote build-up. Just as the creosote can build-up on the glass of your fireplace doors, it builds up within the walls of the chimney pipe. This increases your risk of chimney fires.

Many chimney cleaning companies will also provide a service to clean your brick/stone chase. There are also products available commercially so that you can do this yourself:
(for example: http://www.fireplacemall.com/Chimney_and_Flashing_Repair/Chimney_Cleaner/chimney_cleaner.html)

7) Do you ever "douse" your fire in water? Remember, always damper down to extinguish your fire. Water increases creosote.

Overall, how and what and where you're burning will have a huge impact on the soot staining of the stone on the exterior of your chimney. Keep in mind that light colors, porous stones and more textured faces will increase the visibility of the creosote buildup.

We used wood as the primary or supplementary heat source for over 60 years. The short story is that except for gas fireplaces, there will be soot stains at the top of the chimney -- period.

Thanks, all. The fireplace people are going to come have a look and clean the chimney.
The fireplace above this one has never been used, but the flu damper is about 2 inches too narrow for the flue. Smoke from the basement chimney comes out of this area. Is the flue damper supposed to be flush with all four sides of the opening? If so, how can this be fixed? Thanks.

Also, I know people with old wood burning fireplaces and their chimneys are painted white (i'm talking the chimney outside, by the roof) and even they don't have soot stains. In fact, I don't know anyone with soot stains unless I do a google image search online. Seems I'd see more in person if it were normal.

This post was edited by three3apples on Fri, Dec 19, 14 at 20:52

"The short story is that except for gas fireplaces, there will be soot stains at the top of the chimney -- period."

That is our experience also.

Sounds to me as if you are either not burning hot enough or you're burning unseasoned wood, or both.

I heat with wood exclusively and I have no soot buildup and my annual cleaning always gets me no more than a quarter cup of debris.
This is because I learned how to burn hot and we allow our wood to fully season over the space of two years from dropping the tree to burning the wood.