Question about outside ambient temperture and draft.

[Mo Heat]

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

It's complicated.

Abridged, near 50 word, version follows...

Influence of heat on draft:

Heated air (inside chimney) is lighter per given volume than ambient air (outside chimney).
The hotter the air, the lighter it becomes.
The lighter the air, the better it rises.

Inside chimneys remove less heat from air (in them) than outside chimneys.
This is why internal chimneys draught better.

Influence of chimney height on draft:

(Some other time.)

Influence of barometric inversion on draft:

(Some other time.)

Influence of humidity on draft:

(Some other time.)

Influence of lots of other things on draft:

(Some other time.)

I knew I'd regret posting that...

 

[MountainStoveGuy]

I understand the above post completly, but still not clear on the original question. So im assuming that the lower barometric pressure out side when its cold vs the high barometric pressure in your house makes your stove draft better. NOT the cold air surrounding the pipe on the outside making it draft better, i.e. the ballon rising faster in cold air vs warm air. Is it when the pressure is higher outside when its warmer the cause of a stove not drafting properly?

 

[precaud]

I think it may be confusing to look at it in terms of barometric pressure: one could also say that sound is a modulation of barometric pressure, but that's unnecessarily obtuse. The temp differential (indoor vs outdoor) is the biggest factor in the equation. If it's colder inside that outside, it will downdraft until you get the chimney air warmed and get that airmass moving upward...

 

[MountainStoveGuy]

I guess what i am trying to get at, is why would a stove not draft in 50 degree weather with a chimney that is 90 pecent inside the building envelope? That doesnt make sense to me since the inside of your house is in theroy 50 degrees plus. So is the reason in wont draft is lower pressure associated with colder temps? because it cant be relative to the out side of the actual stainless steel chimney being less the 40 degrees vs the inside temp of the flue. If the chimney is almost all inside the house it cant tell what temp it is outside.

 

[precaud]

I guess what i am trying to get at, is why would a stove not draft in 50 degree weather with a chimney that is 90 pecent inside the building envelope?

Methinks the question cannot be answered the way you asked it. Assuming you mean a cold system (i.e. 'static' draft), look at the event that is before you. There is a cold mass of air in the chimney that you want to get moving. It's trapped in a pipe between two large, loosely coupled air masses, the room air and outside air. Which of these does the chimney air most resemble temperature-wise? It will act more like that one. You can force the issue and change the whole dynamic by raising the temp of the chimney air, making it more bouyant than either of the other two.

So is the reason in wont draft is lower pressure associated with colder temps?

No. It's confusing to look at this event in terms of pressure, when that is exactly what you are trying to create... Plus, in the world of chimneys, pressure is a relative thing...

because it cant be relative to the out side of the actual stainless steel chimney being less the 40 degrees vs the inside temp of the flue. If the chimney is almost all inside the house it cant tell what temp it is outside.

There is no magical transfer of pressure through the stovepipe... again, assuming you mean a cold system, I think the only relevance of an indoor or outdoor chimney is whether the chimney air temp is closer to the inside or outside air... The air in an outdoor chimney will be closer to the outdoor temperature and will either be static or draw down. Once the stove is burning and the chimney air is hot and drawing upwards (i.e. 'active' draft), the outdoor chimney will absorb more of the heat and so run cooler, slower and providing less of what you are calling draft.

 

[webbie]

I guess what i am trying to get at, is why would a stove not draft in 50 degree weather with a chimney that is 90 pecent inside the building envelope? That doesnt make sense to me since the inside of your house is in theroy 50 degrees plus. So is the reason in wont draft is lower pressure associated with colder temps? because it cant be relative to the out side of the actual stainless steel chimney being less the 40 degrees vs the inside temp of the flue. If the chimney is almost all inside the house it cant tell what temp it is outside.

I don't think any of these posts mentioned the "house effect" or "stack effect" that has to be countered before a chimney can work. What this means is that many chimneys want to pull air IN as opposed to out when the fire is not going. This is due to a number of factors related to house pressure. Picture a fireplace on the bottom floor of a home - air is rising within the home, which pulls on the fireplace. Also, kitchen fan, clothes dryers, furnaces and other devices are pulling air from the home, creating even greater negative pressure.

This stuff occurs whether or not the chimney is inside the building. So, to answer the question, an interior chimney will function better and is more likely to work in warmer weather than an exterior one since it can more easily counter the stack effect with it's own draft. But there is no certain answer since houses differ.

 

[precaud]

That's a good explanation of the house effect, Craig.

Picture a fireplace on the bottom floor of a home - air is rising within the home, which pulls on the fireplace.

I think it's more accurate to say that it is circulating. It is rising by the heat sources and falling along the walls, which can pull air down a cold fireplace chimney.

So, to answer the question, an interior chimney will function better and is more likely to work in warmer weather than an exterior one since it can more easily counter the stack effect with it's own draft.

Agreed.

 

[Martin Strand III]

Exsquize me, but

I believe I did mention "stack effect" earlier (Pg 1) on this thread.

And I believe it has nothing to do with (but can effect) a chimney. It is the tendency for a house, best seen in taller ones, to act "like " a chimney by having warmer air exit out of top floors (where there is more + pressure) to the outside and create less pressure in the lowest floors (aka depressurized basement), where outside air tends to be drawn in.

I agree it has an effect on "draft" in a chimney, as do many other factors. However, as has been brought out in this rant, temperature difference between inside the house (and chimney) and outside ambient temperature is the "major" determinant of how well a particular chimney will draught.

Aye,
Marty

 

[Robbie]

Influence of barometric inversion on draft: This was mentioned above by Mo Heat.

Mo. it's time to bring this back up I think, I just had this roll through and would like to narrow it down as to exactly what it was for future reference.

My stove draft had been almost perfect until this last weather system went through our area, (Last 3 days-Tenn.) and ever since, my stove draft has been a little sluggish.

It started out a couple days ago when some system moved down from up in Canada I think, a cold front, but first we had a cold rainy day, clouds low and the smoke from my chimney stayed almost on the ground. This is the first day I noticed a sluggish draft.

This system moved out today, leaving blue bird skys, now comes the cooler weather, due to get in the 20s tonight. My stove is still not drawing tonight (yet) like it did last week, even with much warmer outside and inside temps last week ??

It was 60s last week and we had a fire that drafted really well off and on for a couple weeks.

Has to be pressures, just like this says from Marty S thread above....(“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.” )

I'm not exactly sure what the weather people said just went through our area here in Tenn., but what ever it was, it really messed up my draft for a few days.

I'm sure it will come back though, since it 's done so well so far.

Any more ideas or thoughts ?




Robbie

 

[tradergordo]

I think what a LOT of people don't understand is that a good chimney drafts ALL THE TIME, even when there is no fire. An outside chimney generally doesn't do this, and cold temps only make it worse (when not burning). But draft when the fire is going is better in all types of chimneys when it is colder outside.

 

[berlin]

"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"

MSG, what the heck are you talking about??

the reason coal burning is generally not recomended over 40º F is simply because coal has lower exhaust gas temperitures than wood due partially to the lower volitile content of coal vs. wood, there is more heat transfered from the coal bed than from the visable flames. for example anthricite < 10% volitiles ___ bituminous <35% volitiles and wood is all over the place at 50 -85% volitile content. the lower exhaust temps of coal especially while burning slow on a warm day prohibit establishment of decent draft in all but the best chimneys, this is why it is not advised; has NOTHING to do with a chimney being inside a building envelope, it doesn't matter where the chimney is or what fuel you are burning if you insufficient or complete lack of draft you will have smell, smoke, and CO issues.

For example, my same stove burning wood, putting out the same amount of heat will have a stack temp 100º -300º F hotter than coal putting out the same heat. I know this because from time to time, depending on what i feel like burning that day, if i have enough wood, i will periodically switch.

 

[Mo Heat]

Hi Robbie, This draft thing can be complex in certain cases. Even the generally accepted rules can get twisted if your house has a ridge, or trees, or even other houses around it. Just a little wind coming from a different direction can really change things at my house since I have all three. Sometimes my smoke blows straight down onto the back deck due to wind coming over the roof line and down onto my chimney. It's pretty weird, but luckily I have a 22 foot chimney that drafts ok even then.

Some stoves are better drafting than others. Then there's Berlin's volatiles factor variation with the different wood species (or coal). Similarly, wood size (small burns fast, large burns slower producing less volatile output), wood hardness, and moisture content. Chimney insulation, material, location (inside or out, relative location to roof line, wind direction, etc.). And on and on. So many factors that each chimney draft is probably more different than finger prints... no two are alike and draft will look different even on a day to day basis, at least to some degree.

If you have a marginally drafting chimney, the hard part is usually getting the draft started.

Berlin, Do you know of a wood species volatiles chart link? I couldn't find one doing a routine google search.

 

[webbie]

This is not quite a volatiles chart, but it does give starting and coaling qualities...in general, easier starting is more vols and better coaling is less:
https://www.hearth.com/what/woods.html

 

[Mo Heat]

Thanks Craig. That's an interesting chart and does help a bit.

There are two surprises under heating class. Apple and elm fall into the '2' range. I've never burned apple myself, but I've heard others on hearthnet rave about it. The elm is probably a borderline call from my experience. I actually like burning it myself although I agree with Warren that it can be a bear to split as it increases in diameter.