Question about outside ambient temperture and draft.

[MountainStoveGuy]

I have a stupid question, i have seen on many occasion that draft is worse with warm outside tempertures. That goes against the grain with me. Cold air is heavier the warm air, and more difficult to displace. Cold air condensates flue gas and creats creosote. The warmer the flue the faster the flu gas moves up the chimney. So how does 50 degree outside temps hurt any of the points? I have always been in the school of thought that the more pipe you have inside the building the better the draft. Cold pipe in my opinion is not happy pipe, i thought i new alot about chimneys but apparently not. I have seen the reference to warm outside temps affecting draft many times on fourms and websites. I even called simpson tech and they coulndt give me a answer. Please explain. I just called my sweep and he told me it was a old wives tale. there has to be more to it then that.
Thanks!
Ryan

 

[elkimmeg]

Without re-visiting why I got banned but a hot air baloon will rise threw cooler air faster due to tempature and and pressure characteristics. The warm air air baloon goes into cooler air and if no additional heat were applied, it would slow down as the warm air in it cooled. Now an inside chimney is partically warmer and produces less resistance to heavier downward cold draft air so it is easier to get going once things are in motion they tend to stay in motion. Once it reaches the outside think of the hot air balloon it now rises even quicker threw that cold air actually drawing more air out of the chimney as it leaves. If it hit warmer air it would have a lazy assent. Did I screw this up or did I make sense I confused now?

 

[MountainStoveGuy]

Elk, i knew you would have something for me, thanks for the responce.

So, on that line of thought, are you telling me that it is better to have a exposed chimney, for instance, the very edge of the building with lots of exposed pipe, then in the center of the building with the pipe inside the envelope of the house? Or better yet, a chimney that is ran on the side of the house ? I alwyas recomend that my customers keep as much of the chimney inside as possible, and to only go on the outside of the building (90 degree through the wall and up the side of the house) as a last resort. In pratice, not theroy, exposed chimneys on the side of the building do not perform well in my altitude and climate.
Before i posted this thread the only logical thing i could make of this topic was that the difference in temerture made the gas move faster, in otherwords if the flue gas and the out side temp were the same nothing would happen. There has to be a point of diminishing return relative to the cold air at some point will condensate. In other words this theroy might not be true if its -20 degrees out side. At some point the cold air will overcome the flue temp and condensate the flue gas OR the flue gas will cool and not rise. Am i makeing any sense myself?

One last point, maybe this never comes up in my area because during the burn season there are very few days that are that warm.

 

[MountainStoveGuy]

And one more thing, i think this theroy would only apply to a established draft in a chimney, and is definatly aginst the grain for starting your stove, i would think it would be easier to start your woodstove on a 50 degree day, but hopefully a coldfront rolls through a hour after, and your chimney pulls like a champ.

 

[Martin Strand III]

The "drive" for draft is more than just 'hot air rises'. Draft is a dynamic thing and does not come with the stove as many think.

The greater the difference between outside cold air and inside warm air, assuming all else is equal and copesthetic in chimney length and offsets, the greater the draft will be. This explains why on warmer between season days, the draft can be less than desirable and some coal stoves are not recommended to burn unless it's colder than mid 40's* F.

Aye,
Marty

 

[MountainStoveGuy]

Will a chimney draft better on a -20 degree day then a 30 degree day? I think it would draft better on a 30 degree day, at some point it will have to get so cold out side that the chimney will strugle to keep warm enough to maintain a decent draft. Man, i learn something new ever day.

 

[Martin Strand III]

MSG:

Like I said, the bigger the temperature difference inside/outside, everything else being equal with the chimney, the better the draft. So, let's see, is 30* colder or warmer than 20*....

Another factor to plug into understanding draft, besides temperature difference, is to understand that the house itself (even without a stove) acts somewhat like a chimney wanting to spew air out the top. Yea. Some more than others, of course. This is known as "stack effect" and accounts for basements usually being a poorer place for a stove than upstairs since a basement is more depressurized (area of lower pressure).

Aye,
Marty

Grandma used to say:"If you dodge the draft, you'll get into trouble with the Law."

 

[MountainStoveGuy]

I said negative 20 degrees. I understand the principal. My question is, at some point the out side temp will cool the pipe to the point of condensation, wont it? I am very fimilar with the stack effect and most other aspects of chimeny draft. Whats strange, in my neck of the woods its not talked about much, and the only thing i can figure is that we dont burn coal, so it we are not in a danger area if we burn above 40 degrees. I have been selling chimneys for 7 years, and this has never come up. I complety agree, and it makes total sense about the differantial temperture gradient effecting draft, its just not discussed at ALL here in colorado. Thanks.

 

[Martin Strand III]

MSG:

Sorry, missed the "-". Oh well...

Chimneys centrally located in the home (vs an outside wall) tend to hold heat (from the house) better, stay warmer, draw better, have less backdraft and are affected less by outside (bitter cold) temperatures. I'm afraid I cannot extrapolate your question to an outside temperature of absolute zero - if that were the case, chimney draft would be the least of your problems.

Aye,
Marty

Albert Einstein said, "Nothing happens unless something moves."

 

[MountainStoveGuy]

MSG:

Sorry, missed the "-". Oh well...

Chimneys centrally located in the home (vs an outside wall) tend to hold heat (from the house) better, stay warmer, draw better, have less backdraft and are affected less by outside (bitter cold) temperatures. I'm afraid I cannot extrapolate your question to an outside temperature of absolute zero - if that were the case, chimney draft would be the least of your problems.

Aye,
Marty

Albert Einstein said, "Nothing happens unless something moves."

That i complety agree with, and thats how i sell chimneys, trying to keep them in the building envelope of the house as much as possible. So what happens to draft with a COAL stove and a chimney that is mostly inside the building envelope? sounds like that can be dangerous! Or does atmospheric pressure play a part in this when out side temps are cold vs the inside house being warm? To summerize the question, if i understand properly, coal stoves need to be burned when the outside temp is below 40 degrees, if 85 percent of the chimney is in a 70 degree house, then what? Or if the atmospheric pressure outside, when its below 40 degrees, have somehting to do with this?

 

[Sundeep Arole]

Wind speed will cause a huge effect on draft, as with fast wind blowing past the top of the chimney you have a huge venturi effect which will basically make the chimney behave as though it were hooked up to a big
roughing pump. For this reason, comparing draft on a cold day vs. a warm day may not necessarily be a apples to apples comparison. A cold calm day vs. a warm calm day would be a more valid comparison.

Having a good draft does depend on your ability to heat up the chimney, and, as Elk said, on a cold day, the chimney may just lose heat to the environment faster then it gains from the outgoing smoke, and that could cause poor draft.

 

[MountainStoveGuy]

Im not putting wind in this equasion, i understand that wind effects draft. Actually im not talking about the quality of draft except how it relates to outside temperture, and how much is inside vs outside. Its been stated that you should not burn a coal stove below 40 degrees. If the flue is inside your building, like going through a catherdral celing at the peak, and only have 3 feet coming out of the roof. How does that rule apply?

 

[Sundeep Arole]

Only my guess, but I would then think the rule won't really apply. It might be difficult to get the draft started, as it would be hard to get the chimney heated, but once heated, it shouldn't cool down rapidly. So you should have good draft once the fire is going.

 

[MountainStoveGuy]

Only my guess, but I would then think the rule won't really apply. It might be difficult to get the draft started, as it would be hard to get the chimney heated, but once heated, it shouldn't cool down rapidly. So you should have good draft once the fire is going.

I think the chimney would be easier to start, because the flue is already room temperature.

I agree that the 40 degree rule woundt apply. But then if thats the case, all the coal burners out there have lots of exposed chimney?

And, on the last point you made, stated in another comment in this thread is that you would get stronger draft with greater differantial temps between the outside temp vs the inside flue temp. So in theroy a flue thats mostly in side the building wouldnt draft as well as one on the outside. At least thats what i have gatherd from this thread. Like i said before, i might have been giving the wrong advice all these years. I usually tell people to keep as much pipe in the house as possible. Maybe thats not the case.

 

[webbie]

I have seen the reference to warm outside temps affecting draft many times on fourms and websites. I even called simpson tech and they coulndt give me a answer. Please explain. I just called my sweep and he told me it was a old wives tale. there has to be more to it then that.
Thanks!
Ryan

My guesses:

1. What has been said about the differential between temperatures seems true. I would add that the difference between the outside temp and the chimney temp (as opposed to the house temp) would be the more important.

2. The humidity in the air, which is much higher at higher temps, make the air denser and therefore smoke has a harder time entering into the air once it hits the outside - which restricts the air flow.

3. Barometric pressure? Moon Phases? Perhaps the first has been studied in relation to smoke, the second probably not.

4. Wind, of course, can work either to the benefit or detriment of draft.....in fact, I think it most always has one effect or the other when over 12 MPH or so.

My experience disagrees with your sweeps view. So does the experience of lots of other folks. Did Dylan weigh in on this? If he believes it....well, it would show something!

 

[webbie]

I just called my sweep and he told me it was a old wives tale. there has to be more to it then that.
Thanks!
Ryan

Of course, you mentioned that you are in CO - land of generally low humidity. But here's something to consider, and something to look for:

On certain days I see the wood smoke coming out of the chimney relatively quickly and heading straight up.

On other days, usually foggy when snow is melting, etc, - the smoke comes out and heads complete sideways and down - lingering around.

My perception of this is that the first instance creates more draft since the chimney is actually being extended upward by the conditions, in other words the atmosphere is helping the situation. In the second instance it is like holding a piece of sheet metal a few inches above the flue tile...which brings up this point....

That pre-fab double wall chimneys (with restrictive rain caps) might not be as affected by some of these conditions as open masonry ones - which are usually the ones I observe.

Still, once all is said and done, I have had many experiences with customers unable to even light or operate coal stoves until the weather become cold 24 hours per day. So there is either a heavy-duty power of suggestion or some reality behind this "old wives tale".

 

[MountainStoveGuy]

Craig, we posted at the same time. Please see the comment above yours. I agree with your points.

I agreee that differential temps is true
We have no humidity here, well less then 5% most of the time. Maybe a factor?
I have no idea about the barometric pressure or moon phases.
Wind definatly affects draft
My sweep has 20 years of experence, and the reason he hasnt heard of this is that its simply not talked about in my region. (speculating) Maybe thats due to coal not being a fuel source here. Thats my guess.

So what about a coal stove with a chimney thats mostly in the house? dangerous?

 

[Martin Strand III]

Here's what Big Brother says about it:

"6. Fires burn best when the weather is clear and cold, because of reduced atmospheric pressure on the air in the flue-hence greater draft velocity. During periods of heavy atmosphere or rainy weather the temperature of flue gases must exceed normal temperatures to overcome the heavier atmospheric weight."

http://www.cdc.gov/nceh/publications/books/housing/cha7.htm

Aye,
Marty

 

[CK-1]

And one more thing, i think this theroy would only apply to a established draft in a chimney, and is definatly aginst the grain for starting your stove, i would think it would be easier to start your woodstove on a 50 degree day, but hopefully a coldfront rolls through a hour after, and your chimney pulls like a champ.

I've read somewhere at opening the damper on a masonary fireplace for about 30min will create draft using warm inside air.... Hell.. thats where most of the heat goes anyway....

 

[Martin Strand III]

Didn't like that reference?

Here's another about understanding "draft" from a more familiar source: https://www.hearth.com/what/fallstartup.html

"How to Operate Your woodstove in the Spring & Fall Seasons - by Ken Rajesky, Hearthlink International

Over the many years in Technical Service during the early Fall, we would receive calls from customers complaining of awakening in the middle of the night to a smoke alarm, and finding a smoke filled room where their stove was located. Obviously this was disconcerting and prompted a call to see what was wrong with their stove.
We would first ask how cold was it outside during the night and how they loaded and set the stove for nighttime operation. Almost 99% of the time, the answer came back that it was in the high forties, early fifties (Fahrenheit). These temperatures would make the house uncomfortable if some type of heat was not utilized so it made sense that the woodstove was used. The problem that caused the smoke to fill the room and/or house was not a mechanical failure or design defect but instead, a lack of draft. The stove was improperly operated for the season.

What do I mean by that? First, you need to understand draft, and how it works, so heres a quick Draft 101 course. First of all, draft is not shipped with stoves. If you dont have draft, you don't have a chance, even with the best of products. Draft will vary from house to house on the very same block because of the many factors involved even though the homes may have been built identically. Draft evacuates by-products of combustion (smoke / gases) by a pulling or sucking action. This action pulls air into the stove for combustion purposes, and at the same time, pulls gases out of the stove through the chimney connector (stovepipe) and flue (chimney).

Draft is determined by contrasts-----the temperature of the air outside of the flue, versus the temperature inside the flue. The greater the temperature contrast is, the stronger the draft. Chimney height, flue size, chimney connector configuration, fuel, and altitude are also important factors, but temperature contrast is a greater factor. That's why stoves typically operate wonderfully in the winter, and seemingly not so great during the other seasons. In the winter, the outside temperatures are colder, thus creating a greater contrast with flue temperatures. In the spring and fall seasons, the temperatures are warmer so the contrast isn't as large. As one might say, when the contrast ain't great, problems precipitate!
So, if the outside temperatures on a cool October night are not near freezing, what can you do to ensure a good draft? First, remember the basics of draft---contrasts. Next, remember that if its not real cold outside, you will somehow need to create that contrast by sending more heat up the flue than normal. You may even have to keep the bypass damper (if your stove has one) open for an extended period of time. This means sacrificing some efficiency but its a trade off that you must make. If its unacceptable, then turn on your furnace.

Accept the fact that you will probably need to build each fire from scratch so prepare yourself by collecting a lot of kindling & paper to make this easier. Then, begin to develop a good bed of coals (1-2" deep) before loading your larger pieces. Load just two pieces at a time. The idea here is to heat everything---the stove, the flue, the house, and then let the fire die out overnight.
Heat your home so its comfortable, but do not fully load the stove and set the air supply lever in the same manner as you would in January. If you do so, you will certainly lengthen your burn time. But, you will rob the flue of the crucial heat it needs to keep the temperature contrast high in order to sustain a good draft. If you insist on operating the stove in this way, the result will be the flue will cool, temperature contrast shrinks, draft slows down, and exhausts are not pulled out of the stove and house. The new result is a smoke filled room, smoke alarms blaring, kids crying, lost sleep, and anxiety. Who needs this when all you want to do is to stay warm and save money by heating with wood?
But, if you can understand the basic concept of draft, and how to run the stove differently for the different seasons, thats 90% of the battle. Because of the many variables, you need to be flexible with your stove operation.

The Simple Solution Summary - Burn the stove with less wood and more air - do not smolder! In recognizing this, you will be successful, and sleep better."

Aye,
Marty

 

[MountainStoveGuy]

That is a very good responce. And i do have a very good grasp on the draft, and what causes it and what doenst cause it. I did like that referance. What i keep asking, and maybe it has been answered in a different way is.. What if the chimney is mostly in the house. i.e. 14' of a 17' chimney is inside the building envelope. In that case it would never be below 40 degrees. So is the answer that there is lower pressure outside the house causing that chimney to function properly? Or, if your chimney is located mostly outside, it drafts because the air around the pipe is colder then the internal flue temp. I can see where the first instance would always be true, and the second instance on be true on some occasions. So which is it?

 

[Mo Heat]

I suspect I'm going to regret posting this, I know I will if Frank reviews and comments on it , but it was kind of fun writing.

Gravity. Mass. Warm air light. Cold air heavy. At least they are at chimney heights or when generally compared at reasonably similar heights. The more water added to air (humidity) the heavier it gets. Same with anything else added to air (like smoke). Heat added to air containing smoke gets the constituent air molecules bouncing off one another and they are therefore farther apart from one another and thus have less mass per volume and are usually lighter than ambient, unheated, air). This works best with gases when they can easily expand their volume. Light enough to overcome the added mass of smoke particles, humidity, perhaps the competing movement from basement air up the stairs, etc., and they will rise up the chimney.

Relative, all. Like panning for gold in dirt and water. The gold sinks, the water and dirt are more buoyant. If you drop a split, it will fall through the air because it is heavier (greater mass) than air. If it hits the floor, it will stop, due to densities greater or equal to the split (sort of--I think atomic resonance also comes into play, but I know little of that). If you drop it in water, it will float (lighter than water). We're talking about heated smoke and air inside a chimney relative to ambient air at the top of the chimney. If you have enough difference in masses of air within the chimney, compared to ambient air, draft (buoyancy) occurs.

Exposed chimneys allow heat to escape (conduct) from the air/smoke mixture, into the chimney itself (or through it). Thus, the molecules of the smoke/air mix slow their entropic movement (bounce less--less chaotic bouncing among molecules that normally is forcing them farther apart and reducing their mass per volume). So smoke/air is now cooler, has greater mass, and is less buoyant and less likely to rise. Internal chimneys help keep the air/smoke molecules bouncing around, keep them lighter (less mass), and keep them more buoyant (they rise better).

Barometric pressure is likely a difference in mass of air at different heights (sort of) due to lots of things (heat expanding (bouncing) molecules apart for less mass per volume being primary), although air gets lighter way up high for non-thermal reasons (atmosphere diffusing into space, I think) even though it's colder up high. But it's likely consistent in a relative way, regardless, if you are talking about 20 or 30 feet of altitude difference like with a chimney. Anyway, lighter is more buoyant, heavier tends to be less buoyant, at chimney heights. Things heated have less density or mass than things that are colder, at least relative to the same volume of the same stuff, especially gases with room to expand.

Barometric inversions are one of likely many complexities in this little scheme. Even though warm air (less dense) will rise through cooler air (more dense), if you put a cap on the chimney (that has a much heavier mass than the heated air/smoke) the air can't penetrate (metal cap has way too much mass/density/different atomic vibrational resonance) and so instead of rising through the cap, it is stopped. This is similar to a barometric inversion inhibiting lighter mass air (or smoke from the chimney) from penetrating the denser air (for a while and to a degree--eventually things mix, at least to a degree based upon the degree of agitation). There may also be some circular movement within the inverted gases driving things downward. Like wind directed off tree tops, downward, towards your chimney.

If you release an object with greater mass than air, it falls. If you release an object with less mass than air, it rises. Smoke is released from burning wood. It mixes with heated air (less mass) and it all rises (hopefully).

As you can tell, I'm no scientist. But I think this is all true and logical. Maybe Corey or katooom or some of the engineers will correct my misconceptions.

 

[Martin Strand III]

Mo:

I'm no engineer or scientist, but I can still say, "That (your post) was about as clear as smoke."

In 50 words or less, what are you trying to say?

Aye?
Marty

 

[JAred]

which way should the smoke be spinning again? What if I'm at the equator? Anyway I've smoked ciggarettes in front of my firepllace in the middle of july when it's 100 out and all the smoke goes up the chimeny. Are Some chimeny flues ect that poorly designed that they don't always have a natural draft? and if wind is blowing down your pipe should'nt you buy a cap? I just always assumed that people knew how to install chimneys and flue pipe.

 

[Jay Shank]

That is a very good responce. And i do have a very good grasp on the draft, and what causes it and what doenst cause it. I did like that referance. What i keep asking, and maybe it has been answered in a different way is.. What if the chimney is mostly in the house. i.e. 14' of a 17' chimney is inside the building envelope. In that case it would never be below 40 degrees. So is the answer that there is lower pressure outside the house causing that chimney to function properly? Or, if your chimney is located mostly outside, it drafts because the air around the pipe is colder then the internal flue temp. I can see where the first instance would always be true, and the second instance on be true on some occasions. So which is it?

I know I can set the draft on my stove(old air tight wodburner) when it 40* out side during the day and the will burn 450*. As it gets colder i will have to readjust or it
will just take off had it a 675*.Added no wood adjustd nothing but the temp drop 29*in about 6 hours went from 33 when i load the stove (around 8pm)to 4* at 2:30.So I know the colder it is the beter the draft.But the thing with coal is you don't have a whole lot of smoke. So if your draft sucks you may never know.Co2 as we should al know is a heavy gas and stays to the floor so if the chimney is not drafting properly..ie.warmer weather, blockage.we would not know(coal burns alot cleaner
then wood).But think your look at this wrong(My opinion cuold be wrong).An inside chimney keeps the flue gasses warmer vs outside chimney which as ato cool gas. And as Elk warmer gas seem to raise faster. aslo the gas going up the outer chimney tend to cool and may condese leaving creasote.Anyhow I was told with either coal or wood you need good draft but with coal you don't know your always getting it(no smoke jusy co2)