Creosote buildup: by smoke, fumes, and water vapor?

[Cinnaman]

Hi,

I have a masonry fireplace, no insert, no stove. My chimney is on the exterior of the house and has to be warmed (with fire starter gel in an aluminum tray) prior to starting a fire to establish a good draft. Once the draft is started I don't have any problems. I have the chimney cleaned once a year after the burn season. Last year the sweep told me the creosote buildup was really bad and I should burn only seasoned wood and have it cleaned twice a year. It's possible the wood I used wasn't the best since it came with the house we bought 2 years ago and I don't know how long it was there (bottom layers were rotted), but my guess is the chimney cooling at the top is more of a factor than the wood since I know it is a very cold chimney to start.

Since I used all of the wood from the previous homeowner last year, I bought a cord of oak this past summer (I burn less than a cord per year). I get an occasional log that hisses during the fire and try to throw in some small pieces to keep the temp up. At the end of the night when the fire is reduced to small flames, coals, and no smoke I'm assuming the top of my chimney must be cooling very fast. The coals must continue to burn for a few hours after we close the glass doors and go to bed since there is nothing but ash left in the morning.

After my long winded story, my concern of course is creosote and chimney fires. I will have the chimney cleaned twice a year (once in the middle of the season and again at the end).

Question: Am I getting creosote buildup during the coal burn-down over night with the fumes? Or is creosote buildup greatest with smoke and water vapor during the beginning and middle of the fire? Should I do anything differently?

Thanks for your help,

Dave

 

[Cinnaman]

Dylan,

Any smokeless suggestions other than fire gel to warm the chimney to get the draft going in the right direction? I've tried the newspaper in the flue trick and more times than not I get ashes and smoke blown into the room.

Dave

 

[stangds]

how about a propane torch? I'm not sure how much heat it takes to get the draft moving, though....

 

[jabush]

How bout a hair drier??

 

[johnsopi]

I have heat gun that hoter then a hair drier. They use them for shrink wrapping.

 

[Todd]

By the time your fire is down to hot coals, there is little creosote producing beasties. Most of the creosote will form at the beginning of the fire as the moisture is evaporated from the wood.

 

[stoveguy2esw]

hi dave,
im going to indulge myself a bit here with a longer than normal post. what i am seeing reading your post and this string is that the symptom is obviously a chimney that appears to need to be pre-heated in order to draw. a "cold chimney" is not usually the fault of the chimney itself (if properly built) the problem in most cases with well constructed but cold chimneys is usually negative pressure within the house itself. lets look at the whole picture. the house is insulated, sealed up, as it is cold outside and we dont want the cold air in (obviously) defeats the idea of heating right? now, a chimney is designed to remove smoke from your house, thats its job. as it does its job it is also removing air from your house as the air the fire uses is coming right in through the front of the fireplace. here is where it gets tricky,the air that is being pulled out of your house by the chimney needs to be replaced by air from outside. this occurs through leaks in windows and door and other little places that are not sealed up. now think back to the fact that the house is sealed up as well as can be in order to keep heat inside.think about it this way, say you have a bottle of pop, you drink this bottle of pop through a straw( easy right) , now imagine drinking that bottle of pop with a straw which passes through a cork with no leaks. you would suck the straw flat trying to pull the pop up the straw, the reason, because no air is going into the bottle to replace the pop that is being pulled out. this is negative pressure. what eventually happens is the house itself being well constructed and sealed up is not releasing the air up the chimney as fast as it needs to in order to get the smoke out. this causes the smoke from your paper to come back into the room instead of going up the flue. now , as the chimney heats up, the pull in the chimney gets stronger, and becomes powerful enough to pull the air into the house, through the fire and out. now, while the hot air is rising in the chimney, it is carrying smoke, which is basically unburned wood. these particles have weight and as they leave the fire and go up the flue they become harder and harder for the chimney (which is not as hot at the top as it is at the bottom, therefore not as powerful)to pull out. they eventually adhere to the sides of the flue and become the creosote that the sweep cleans out. the crucial time is in the early stages of the fire, when the flue is not heated all the way up and the wood is still containing moisture. the smoke at this time is at its heaviest and wettest (steam from the wood cooking out) and this is when the creosote is formed. creosote is less likely to form late in the fire as the wood is burned down to coals and is literally devoid of moisture, also the late stages of the wood burning is releasing mostly gasses and very little solids. also the chimney is warm ( a well built chimney will not cool quickly the brick structure will have absorbed heat from the fire and will cool slowly).

ok, with me so far? here is your method to help get the fire going faster and without the "smoke spillage" you have run into with paper in the past. select the closest window to the fireplace and open it about an inch say , 5 minutes before you get ready to light a fire. when you open that window you will likely feel air rushing in , that is because of the negative pressure inside the house. at the same time , the chimney will start to pull even though it has not been preheated. this cold draw (or standby draw) should be sufficient to pull the beginning warmth up the flue and pick up speed rapidly. using the gel actually should not cause measurable condensation. anyway, after the window has been cracked for a few minutes, light a match and hold it at the top of the fireplace opening , you should see the flame lean into the fireplace. this would likely not be happening or if so , not as strongly as before. once that airflow is established , ligt your fire. you shold see the smoke wanting to go up the flue instead of coming back out into the room. after lighting your fire, leave the window cracked until the fire is well established. then close the window. this allows the house to be equal pressure to the outside and the flue will pick up the draft a lot faster and stronger, also the wood will be able to steam out before the house pressure starts to drop, this allows the chimney to get the heavy stuff out. reducing your creosote in the flue.

i know this was long winded, its easier for me to explain this talking rather than typing. but preheating the flue to get draw cures the symptom, but does not cure the problem. i hope you find this helpful.

mike

 

[johnsopi]

Very good post. I understand draft better now.

 

[Cinnaman]

Mike,

Wow! Thank you for taking the time to write such a clear and detailed response! I follow your thoughts regarding the pressure differences and it makes perfect sense. A few times, before I started using the gel, I filled up the room with smoke and opened a window about 5 feet away from the fireplace to vent the smoke out and saw the whoosh of smoke go into the fireplace and up the chimney......your pressure explanation. I will try to establish a draft with the window opened and no gel starter (not fond of using and storing something so flammable). Is there a rule of thumb regarding temperature differences between the outside and inside of the house to establish a good draft and when I should go right to the starter gel? It's a little embarrassing to start a fire with guests in the room and smoke them out (maybe a good thing depending on the guests).

I also feel much better about the coals burning down overnight and not contributing much to my creosote problem.


Thanks again for an awesome reply.

Dave

 

[elkimmeg]

Mike great explaination But i would like to add to it
consider you fire place opening usuall in the lower part of your wall and living space right
hold that thought a monent.
every home has a point where ther is a neutral pressure plain too story home it is usually at ceiling level of the first floor. that means all pressure below the neutral plane
is negative and all pressure above it is possitive. Higher positive pressure gravitated towards negative to neutralize it thus opening the window acted as a pressure relief valve
Every leak in the top floor adds to the positive pressure difference. Meaning your chimney not drafting could be your leaking attic pull down stairs ot every ceiling light fixture bath fan all penetrations above the nuetral pressure plane. Now imagine your fireplace is in the basement the lower you go the greater your negative pressure is. The more leaks above it influences the situation.

That one part of the solution Verticle length is another piece.
Example same caliper pistol and rifel is fired, the bulit leaving the rifel leaves with higher velosity. why barrel lenght? Your chimney and its draft is governed by verticle length
there are situations where the added verticle length, it is better for basement locations than first or second floor locations. The verticle length may over come the negative pressure
Remember once things are put in motion they tend to stay in motion. Once the draft is established it tends to stay in motion. What also comes into play, is other appliances in the basement needing combustion air, this tilts the scale in favor of not locating a wood stove or fireplace in the basement

The final part of your draft issue is the fireplace and chimney its self The opening has to be sized correctly to the flue area an over sized flue is just as bad as an undersized one
it takes more heat to warm up the larger area of the overside flue and weakens the draft the undersized one does not have the vollume capacity to properly draft the openings size.
also taken into consideration is the veryticle length in the sizing a smallyr flue but longer may draft better than short wider one,

Finally the firebox, throat, and smoke shelve construction might be the problem. The fire box should have curbature to direct the smoke into the throat. The damper location might be positioned too far forward or too far back. Yours is not designed right is not the only poorly constructed or mpooorly designed on in existance truth being this is more common than John q publis would ever guess.
So why are fireplaces located on outside walls? Simply put, for convience, not taking away from living space, Truth the worst possible location for draft. 3 wall exposed to cold out side is not condusive to heating a flue for drafting. Very condusive for cresote buildup. Fact exterior fireplaces and chimneys are a negative heat loss area even when the damper is closed.
those bricks suck out your heat. Even worst when you run your fireplace, it pull much more heated air out than heat radiated forward

As mike suggested opening a window is a stop gap measure nuetralizing pressure ,but the problem could be many other things, including a leaky pull down stairs