I live in NW Washington. It rains a lot here, 69 inches so far this year, so my firewood never gets real dry. Consequently, it produces a lot of smoke. Compounding the problem is the length of the chimney; the distance from the top of the stove to the top of the chimney above the roof is almost 25 feet. I clean the chimney at least once every year and sometimes twice. Opening the stove door almost always allows some smoke to escape unless the fire is really hot and often a really hot fire is not necessary. So I'm stuck with a smoky house. Is there some way (mechanical or otherwise) of creating a good draft when it's necessary to open the stove door?
Season firewood before you burn it, most easily done by stacking it (after splitting) on wood pallets (off the ground) under a rainproof roof, leaving the sides open to the wind (which dries firewood more than it dampens it.) Do not burn firewood until it is wholly dry to the touch and has developed drying cracks at both ends of each log.
You need a positive draught up he chimney. This is normally regulated by a trapdoor in the stove below the level of the fire. If your stove lacks this and you do not want to replace it, you could try extending the chimney a couple of feet higher. Be warned that genuinely airtight houses can inhibit the draught up the chimney.
Generally the problem is that now houses are insulated and sealed so tightly you don't have enough air in leakage to get a good draft established. To check this hypothesis, open a window in the room w/ the stove a crack and see if the issue isn't resolve...if so, then you have to decide which is the lesser evil... :)
I had a similar problem about 30 years ago. A hot fire worked, so I knew adequate air was getting into the house.
The problem was that when the fire wasn't pulling much air, cold air would flow down into the 8" flue. A balloon needs hot air to lift off, and a chimney needs a "stack" of hot air to draw.
In a couple of minutes I bent some aluminum flashing into a sort of dunce cap for the flue, reducing it from an 8" square (64 sq in) to a 3" circle (7 sq in). Kind of like a hose nozzle.
That made the "muzzle velocity" 9 times faster, to keep cold air from sneaking in. It was the start of a windy, rainy weekend, but that jury rig stayed in place. There was plenty of draft with the stove door open and a big fire, and there was still a draft after letting it burn low for hours. Keeping the chimney warmer also reduced creosote and helped warm the house.
I am fairly certain that the code regarding the height of the chimney is determined, in part, by the type of roof (sloped or flat) and how far the top of the chimney is "HORIZONTALLY" from the nearest point on the roof or a wall or other obstruction.
Basically, I think that if the top of the chimney is above the top of the roof line, it is supposed to be at least 2 feet above the top of the roof line. This also assumes that there is no wall or side of a building within about 10 feet of the chimney.
For a sloped roof, where the top of the chimney is not above the top of the roof line, the HORIZONTAL distance from the top of the chimney to the roof is supposed to be at least 10 feet (again assuming that there is no additional wall or side of a building near the chimney).
There are exceptions for chimneys where the only fuel being used from the furnace and/or hot water heater below is natural gas and the chimney itself that is coming through the roof made of what they call "B-vent".
One concept is to prevent a "down draft" down into the chimney caused by wind across the roof.
And, I think that another concept is to have the chimney be such that it is somewhat insulated from the outside cold air (which is one role of B-vent) so the gases remain hot enough to continue to rise up and out of the chimney and without causing water condensation inside the chimney.
You never want a cold chimney, period. The change in draft is normal with changes in the weather. That hot column of air rising out of the chimney will rise faster in cold air than in warm air. And the hotter that column of air is the better the draft. When it's warm out the fire is laid low so the air column is cool. When the weather cools the stove is cranked up so the exhaust is hotter. Simple as that.
Decades ago, I helped my BIL build a new chimney in a house he owned. My parents lived there. They?d had a lot of trouble with smoke and being unable to keep the stove going all night.
The new chimney was no better. He?d read that the problem was cold air flowing in from the top. I said all the masonry chimneys I?d used had had a slab on top. I thought that restriction would keep cold air out. It was a proven method, but he claimed it would restrict the draft.
I thought about it. Industrial smokestacks were actually funnels. The cross-section at the top was typically 1/3 that at the base. Ship smokestacks were called funnels for good reason. The stacks on the Titanic looked like cylinders 25 feet wide, but in fact, the flues were only 30 inches in diameter at the top. That meant the cross section at the top was about 1% of what it appeared to be. A funnel could keep cold air out while presenting very little restriction.
I said a metal funnel on top might work. He said it wouldn?t. I tested it on a cold, rainy, windy Friday afternoon in November. The fire was easier to start than before, and there was plenty of draft when I opened the door with a roaring fire. I continued the test for three nights. No smoke ever came out, and fires would burn all night.
He wouldn?t have it because he?d said it wouldn?t work. He bought a cap that looked like a bird feeder, a steel frame that supported a steel roof about 6 inches above the flue. It probably would have worked better if the gap had been only 2 inches, as seems typical of slab covers. Even so, it was an improvement over no cover. By keeping the flue gases from shooting straight up, it reduced the downflow of cold air into the flue.
You?ve gotten me interested. I see stoves are tested to find the minimum size flues they need. The chimney could be the size of the stove collar, but a rectangular flue needs a bigger cross section because it?s less efficient. With a 25-foot height, you might do fine with a smaller flue.
The BTU/hour is used to determine the minimum flue size for a gas burner. There?s also a maximum size. If the cross section of the flue is 7 times bigger than the minimum for the appliance, draft may be inadequate because the appliance won?t produce enough flue gas to keep the flue warm.
That?s the problem with wood stoves! Idling, they may lose draft because they don?t produce enough flue gas to keep the flue warm. I?ve found that a suitable cap can make a big difference.
I read years ago, that scientists proved that a humming bird (or was it bumble bee) could not fly. It's proven method, but technology says it's not possible. No convincing some people, after scientists say
I wasn't advocating a cold chimney. That was Bob F that brought that up. I didn't address that part of his question because "I don't know". To be honest, I don't know why it was recommended I have 4 more feet extended on my wood stove chimney. My questions were more out of curiosity. All I said was the chimney worked better on cold nights.
It may have been I just needed more chimney and *I thought* it was we needed more exposed chimney. As I recall we had about 16' of chimney. Maybe the recommended amount was 20'.
I agree. That is basically what I was trying to explain in my earlier post for gonjah. But the link that you provided explains it better.
Gonjah is wondering why someone suggested adding 4 more feet to the existing chimney, and my guess is that the extra 4 feet would bring the chimney into code and prevent down drafts by placing the top of the chimney at least 2 feet higher that anything within 10 feet of the chimney.
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