What powers draft in a chimney?

DaveGunter said:

Did you read the link that he posted?

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I read it, and fully agree with what it said, I've tried to make that case here before and failed because I couldn't say it so well. Summarizing it as "heat rises" is what I had a problem with.

TE

I agree with what Fossil said. From a cold start you are creating a "bubble" less dense gas. Imagine that bubble is near the bottom of the tall part a "J" shaped tube. It can either come out the short leg into your room or up the tall leg out of the house. If you already have a substantial negative pressure in the room due to a "chimney effect" going on in the house then that bubble of less dense gas goes into your room and you have a backdraft. Ideally though the bubble of less dense gas travels straight up the tall leg of the "J". Once it starts up that leg that bubble is creating a vacuum or an area of low pressure just behind it. That vacuum/low pressure area is what continues to pull air in the proper direction through the stove. The vacuum part of it is like getting a siphon started. Really a chimney is just an upside down siphon. The chimney can either siphon air into the house or out of the house?

Not sure if mine could hold a paper plate but I had a paper bag taped on it a few weeks ago and it was funny watching it go in and out.

In my old house with a pre-fab open fireplace, after learning the hard way, I always started a fire by lighting a slightly crumpled sheet of newspaper placed on top of the kindling, a big whoosh of flame up the chimney. That kick-started the draft back into the desirable direction. I think that is also an advantage of the top-down method for stoves.

TE

He is not talking about starting a fire, he is talking about the natural draft a chimney has with no fire.

I don't think there's such a thing as "static draft". But if you mean can a cold chimney draft, the answer is yes. See this page for reference... http://www.gulland.ca/fhs/temperature.htm. If your stove/fireplace is above the NPP (neutral pressure plane) in your home, air will escape up the cold chimney. If it sits below the NPP, which is usually the case but not a fixed rule based on the variables at play, then without the aid of heated air it will act as a port through which replacement air flows into your house. That's why opening a window often helps overcome downdrafting - you're giving the system an alternative to your chimney through which to pull in replacement air. The key is understanding that there is a NPP in your house and it moves around based on lots of variables.

It is about density hot air is less dense and goes up away from the ground because of that. A chimney will reverse flow if the outside air is denser than the indoor air and this could either be due to temperature or humidity the reason is not important only the relative density. A masonry chimney is both cold and damp so heavy air and greater water content so denser air. It might be counter intuitive but if a reverse draft just open the door or windows to have the room to have the same air density as outside. Even a small fire can get a draft going if inside and outside air density are the same. I am no stove expert but after 4 decades of working on race cars and bikes at frightening speeds I do know a bunch about air density and Bernoulli effects. Fastest to date is 394 mph and many in the high 200 to 311 range. If I can generate .5 psi difference between the top and bottom of a car that is 15 ft long and 5 ft wide that is over 5000 lbs down force so very small changes in density are a very big deal.

Highbeam said:

Some chimneys are built outside like in covered picnic shelters. The temperature is the same on all sides and locations of the chimney. The air at the top of the chimney is at a slightly lower pressure due to elevation. However, at any elevation the pressure inside the pipe is equal to the pressure outside the pipe. It should not flow.

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What makes you believe there's any vertical movement of air mass there?

Highbeam said:

I don't follow... let's simplify...

Some chimneys are built outside like in covered picnic shelters. The temperature is the same on all sides and locations of the chimney. The air at the top of the chimney is at a slightly lower pressure due to elevation. However, at any elevation the pressure inside the pipe is equal to the pressure outside the pipe. It should not flow. It is in equilibrium. This would not be unlike sticking a straw into your glass of water and expecting water to flow out of the top of the straw continuously

I think a lot of people think they know what is going on but really don't. I'm an engineer, most of my schooling was in hydraulics and flow. I am familiar with bernoulli's equation. I ought to be able to tell you why a cold chimney draws but I don't know either.

Once you add a heat source and start talking about smoke that is a whole nother thing. Could it be that a chimney does not flow UNTIL you add a fire?

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You can not generate draft out of hope or good intentions. But air density can just about suck the wood out of the stove. It is all about density differential and btu input no doubt not a hydraulic problen other than the relative efficiency of a pump and btus added from that problem.

Hey guys,

Needed a break at work today so I grabbed my Mark's Engineering Handbook and read up on "Stack Effect". It agreed with most of what was said before but said it was a function of two variables.
1) Delta P, or the pressure differential between indoors and outdoors
2) Delta T, the difference between the AVERAGE FLUE Temperature and the Temperature of the air directly outside of the chimney. They had some fancy formula's to calcluate induced pressure differential, but another fire came running by my cube so I was disctracted.

It also jived with what Wikipedia had to say about it. Good read if you have 5 minutes.

I'm with BeGreen on this one... I've seen a lot of basement fireplace inserts backdraft until a good fire was established. Sometimes in these situations (sometimes!) it helps to just crack the door and fill the insert/fsws with a ton of newspaper and let it rip.... I've seen top of the line woodstoves never really work properly in a basement due to pressure issues also. When it comes to basements I generally suggest pellet or gas as that is what I would do. On the flipside I've also wood burn like a dream in the basement. Tighter houses with lots of air sucking devices do make life a bit more difficult.

Badger said:

Hey guys,

Needed a break at work today so I grabbed my Mark's Engineering Handbook and read up on "Stack Effect". It agreed with most of what was said before but said it was a function of two variables.
1) Delta P, or the pressure differential between indoors and outdoors
2) Delta T, the difference between the AVERAGE FLUE Temperature and the Temperature of the air directly outside of the chimney. They had some fancy formula's to calcluate induced pressure differential, but another fire came running by my cube so I was disctracted.

It also jived with what Wikipedia had to say about it. Good read if you have 5 minutes.

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Yep delta P and delta T is air density...

Highbeam said:

I don't follow... let's simplify...

Some chimneys are built outside like in covered picnic shelters. The temperature is the same on all sides and locations of the chimney. The air at the top of the chimney is at a slightly lower pressure due to elevation. However, at any elevation the pressure inside the pipe is equal to the pressure outside the pipe. It should not flow. It is in equilibrium. This would not be unlike sticking a straw into your glass of water and expecting water to flow out of the top of the straw continuously

I think a lot of people think they know what is going on but really don't. I'm an engineer, most of my schooling was in hydraulics and flow. I am familiar with bernoulli's equation. I ought to be able to tell you why a cold chimney draws but I don't know either.

Once you add a heat source and start talking about smoke that is a whole nother thing. Could it be that a chimney does not flow UNTIL you add a fire?

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I think you're simplifying a little too much. In the ideal conditions you're stating, there would indeed be no draft, but... In your picnic shelter example, perhaps because the chimney sticks up in the air the sun shines on it and heats it a bit giving you a heated flue and a pressure differential. For your straw in the water... if the all of the water in the vessel is exactly the same temperature then the equilibrium you describe would exist. In reality these equilibriums at any point in time don't exist, so you're always (statistically) going to have flow one way or another through your chimney. If you're below the NPP that flow will be down, if you're above it the flow will be up. Apply heat to the stove and you tilt the balance.

Highbeam said:

There's more to it. It's not the delta P or delta T between one end and the other. Think about the straw in your water glass much higher P at the bottom than at the top but no flow. Even if your glass is 100 feet deep.

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With the straw in the bottom of the cup there is a column of water that directly offsets the pressure differential. If your straw is vertically submerged in 6 inches of water the pressure at the bottom of the straw will be 6 inches of water or .22psi. The weight of the water (density times volume in the straw) will exactly equal the area of the straw multiplied by .22 psi and the system will be in equilibrium. Now let's replace the water with a less dense fluid (blow air into the straw) and place our finger over the edge of the straw. When you remove your finger the .22 psi pressure differential purges the air from the straw and pushes water up to the surface of the water.

As long as the inside fluid and outside fluid have the same densities the system will reach equilibrium and no further flow will occur.

However, if the densities are different, a constant flow will occur. Going back to the straw, if you had a little box at the bottom of the straw that magically changed water into air there would be a continuous flow of air coming out of the straw that would be at the same rate that the magic box can transform water into air.

This is sort of how a chimney works, but instead of working with positive pressure it is working with a vacuum. I will admit it is more abstract, and I struggle with the details of the physics behind it, the slug of warm air in the chimney rises and is continuously replaced by more warm air over and over and over again... At least until reload.

Sorry to say a vacuum is a contrivance that is made by people as there is no such thing. Pressure is more properly measured in absolute terms. Typical units are inches of water and inches of mercury. If draft could be described in temperature it would be very simple. You are also converting solids to mostly gases and some solids such as soot that is mostly carbon. This process raises the internal pressures that reduces draft as many of the gases are heavier than air causing the need for a higher a temperature differential than a simple temperature based model might suggest. To complicate things more you have to heat the stove and chimney to a useable temperature first, That is one of the things kindling does by having a large area to mass ratio. I need to go to work and not up for doing the math today or any other.

ddahlgren said:

Sorry to say a vacuum is a contrivance that is made by people as there is no such thing. Pressure is more properly measured in absolute terms. Typical units are inches of water and inches of mercury.

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Yes, we've all taken Physics 101... thank you for the obvious. You can't argue that the concept of vacuum, pressure differential from atmosphere, isn't a useful concept when discussing draft.

ddahlgren said:

I need to go to work and not up for doing the math today or any other.

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? Not sure I understand the point in posting, "I don't feel like explaining it, so I won't."

webbie said:

As one example - take a non-cat EPA stove and hook it to a very short (10-12 ft) straight up insulated chimney. In my experience, it will work to beat the band.

Take the same stove and hook it up to a 28 ft tall masonry chimney which is outdoors - the customer will likely call and complain about slow starting and chimney reversal.

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So, going back to the OP, in which Highbeam was inquiring about the case if indoor temperatures getting as cold as (or colder than) outdoor temperatures, one could argue you're replicating the conditions of your cited outdoor masonry chimney no matter what your actual chimney construction, at least prior to lighting the fire and pumping some heat into the flue.

Highbeam said:

Thanks Badger, I know that this is mostly an academic type question at this point.

We seem to be back to needing a fire for a chimney to flow. Without adding energy to the system, there will be no flow.

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But there is.

Take a look at the pictures on this page of the wrapped building.

http://www.insulation-guide.com/house-insulation-basics.html

That's the Stack Effect. Any pipe to the outside below the NPP is going to suck inwards, and any pipe above the NPP is going to let air out. No house is "cold", there is always some heating effect even if just from the sun, so there will almost always be some stack effect. The mystery is not how or why down drafting occurs in a cold chimney, it is why do so many cold chimneys still manage updraft when cold? I expect many of those are uninsulated flues facing the sun which heats the column of air enough to overcome the stack effect.

TE

Did you look at the link I provided? You do not *have* to add energy to the system to get flow. There are always dynamics available to drive flow through a chimney. Adding energy biases the system so you get the desired flow. Maybe I misunderstood your original question?