How exactly does an interior vs. exterior chimney affect the neutral pressure plane?

[jasonrohrer]

I've read a lot of things about how exterior chimneys work more poorly than interior ones. If the chimney passes through the heated living space (like the upper floors of the house), then it will obviously be warmer than an exterior chimney.

What I'm having trouble pinning down is how this warmer chimney, combined with stack effects in the house and the location of the neutral pressure plane, will affect whether the (unfired standby) chimney will tend to draft air into or out of the house.

The general idea is that a warm interior chimney will tend to draft up and out during cold winter months, even when no fire is burning. But why is this true? If the stove is below the neutral pressure plane in the house, it should still suck air in, regardless of whether the chimney is warm or cold, right?

I've found three articles that kinda wave their hands about this issue:

A non-commercial service in support of responsible home heating with wood - The Evil Outside Chimney

The Wood Heat Organization was formed to support the public in the responsible use of wood energy in the home. We fulfill our mandate by providing reliable information, by conducting research into wood heating-related issues and by representing the public interest in discussions of policies that...

www.woodheat.org

Backdrafting | The Chimney Sweep

www.thechimneysweep.ca

The Hearth in the House as a System

And there's one article from Jotul that shows how a tall interior chimney has a higher NPP than the house:

All about wood stove chimneys | Jøtul

No Jotul woodburning stove or fireplace insert can function properly without a good chimney. Think of the chimney as the engine that drives the Jotul wood heating system. Think of its fuel as heat. Think of the power it puts out as draft. The more fuel (heat) you give this engine (chimney), the...

www.jotul.com

They even show a diagram of this:

But this isn't explained in any great detail. What would a similar diagram look like for an exterior chimney?

It seems like the "stack effect" in the chimney itself would only work if the chimney was closed at the top, so that the pressure could build up there. And yeah, it's taller than the house, but how does this help? Even in this diagram, we're still showing negative pressure all the way down the chimney... is the idea that the pressure at the bottom of the chimney is even MORE negative than the pressure in the house? So it will suck air into the chimney from the house?

And how would this change if the chimney was exterior and colder? Maybe the whole chimney is at outside atmospheric pressure in that case?

How does total chimney height affect this? Do taller chimneys naturally suck air out of the house more? If you make your exterior chimney tall enough, will that eventually overcome the problems associated with a cold chimney? What if your exterior chimney was 3x taller than your house?

And for interior chimneys, is as tall as possible necessary, regardless of construction cost? For example, on a tall 2-story house with the woodstove on the first floor, if you have an interior chimney that exits the roof near the eves of the roof, vs. a much taller interior chimney that exits near the peak of the roof, how much better will the taller one draft than the shorter one?

(And yes, there are other problems with a cold exterior chimney, like condensation of flue gasses and build-up of deposits, but I'm just talking about draft issues here).

 

[EbS-P]

I don’t like the concept of a neutral pressure plane. The chimney works because the pressure is higher at bottom than the top. The force up is greater than the gravity downwards.

Now consider hot air inside the house rises and escape through cracks this creates a siphon effect. The house is now at a lower pressure than outside because the air left so it’s going to suck air and the route that is the easiest to get into the house will be the one that most of the air flows into.

I could really care less where you draw some line that you think signifies neutral pressure, neutral to what outside under what conditions( colder and windier just means my house is more depressurized and sucks more outside air in. ). Basements are hard because they are the bottom of the stack effect all the hot air in the house wheat up leaving the cold basement at a lower pressure.

Really good air sealing will help reduce stack effect.

Back to the question interior vs exterior. Cold equals less buoyant force which means less of a pressure difference between top and bottom of the chimney. The result is that it flows less air and what that means for the NPp I don’t know. In the same house venting different amounts of air I would guess that at some really large flow up the chimney the area near the chimney will be at a negative pressure so it move the NPP further from the stove ???? Best guess.

 

[jasonrohrer]

Thanks!

We're faced with a situation, when adding a wood stove to our existing house, where the easiest path for the new pipe is through an exterior wall and up the outside of the tall, two-story house. Of course we would obey the 10-foot-2-foot rule at the top.

There are two bad things about this new chimney pipe:

1. It has two 90 degree bends in it, where it passes through the wall, along with a few feet of horizontal (along with passing through the wall, it has to clear the eve overhang on the way up).

2. The vast majority of the chimney pipe is on the outside of the house.

Of course, we would use insulated pipe on the outside, which I'm guessing would help to keep the chimney hot once the fire is actually burning, but won't help when trying to get a fire going with a cold chimney.


The other option is go go straight up through the house, passing through two heated rooms and an unheated attic on the way to the roof.

This would be a straight pipe with no bends, and the majority of the pipe would pass through heated living space.

Constructing this alternative is much more intensive. Cutting holes in two ceilings and one floor, along with a roof penetration and all that such a penetration entails. And the pipe itself will run through our master bedroom, consuming valuable floor space there, and also just looking out of place with the decor up there.

This stove will be on the first floor of our house, in the living room (not in the basement). Our house is quite tall, with high ceilings on the first floor. Chimney pipe will be something like 20-25 feet tall either way.

If our stove is going to have a night-and-day improvement in performance from a straight, interior chimney, it will be worth it.

But if the performance gain will be negligible.... it's a lot of extra trouble to put it through the house.


There's also the question of ease-of-cleaning, which has trade-offs as well.

The exterior pipe will likely gather more creosote, and the bends won't help, but we could also put access ports at the exterior bend, to make it possible to clean the whole thing without going on the roof.

The interior straight chimney would gather less creosote, but require standing on the (very high) roof to clean from top down.


This stove is primarily for emergency heating and occasional "it would be nice to have a fire tonight" use, so cleaning issues aren't our primary concern.

 

[moresnow]

The interior straight chimney would gather less creosote, but require standing on the (very high) roof to clean from top down.

There are stove models that can be accommodate a bottom up cleaning. List a make/model you are interested in and somebody can advise on its ease of bottom up cleaning.

 

[EbS-P]

I’d do exterior. You have plenty of height. I would get it as high in the interior wall as you are comfortable with double wall inside with 2 45s. Clean out on the bottom of the exterior class A.

This is a common install two story exterior class A. I would be more concerned about the stove you will chose than the venting system. And who will be doing the work if it’s not a DIY.

 

[begreen]

1. It has two 90 degree bends in it, where it passes through the wall, along with a few feet of horizontal (along with passing through the wall, it has to clear the eve overhang on the way up).

There are no 90º elbows in chimney pipe and elbows on exterior installations are generally discouraged. Some chimney pipe manufacturers forbid them.

The other option is go go straight up through the house, passing through two heated rooms and an unheated attic on the way to the roof.

This would be a straight pipe with no bends, and the majority of the pipe would pass through heated living space.

Constructing this alternative is much more intensive. Cutting holes in two ceilings and one floor, along with a roof penetration and all that such a penetration entails. And the pipe itself will run through our master bedroom, consuming valuable floor space there, and also just looking out of place with the decor up there.

Chimney pipe going through a room must be protected, typically by boxing it in a chase. This is a visual bump out, but can be covered with sheetrock and painted to match the room.

This stove is primarily for emergency heating and occasional "it would be nice to have a fire tonight" use, so cleaning issues aren't our primary concern.

Personally, I would come up with an alternative that enhances the home value in this case. Perhaps consider a good looking gas or wood fireplace in the living room as an alternative.

 

[gthomas785]

I've read a lot of things about how exterior chimneys work more poorly than interior ones. If the chimney passes through the heated living space (like the upper floors of the house), then it will obviously be warmer than an exterior chimney.

What I'm having trouble pinning down is how this warmer chimney, combined with stack effects in the house and the location of the neutral pressure plane, will affect whether the (unfired standby) chimney will tend to draft air into or out of the house.

The general idea is that a warm interior chimney will tend to draft up and out during cold winter months, even when no fire is burning. But why is this true? If the stove is below the neutral pressure plane in the house, it should still suck air in, regardless of whether the chimney is warm or cold, right?

I've found three articles that kinda wave their hands about this issue:

A non-commercial service in support of responsible home heating with wood - The Evil Outside Chimney

The Wood Heat Organization was formed to support the public in the responsible use of wood energy in the home. We fulfill our mandate by providing reliable information, by conducting research into wood heating-related issues and by representing the public interest in discussions of policies that...

www.woodheat.org

Backdrafting | The Chimney Sweep

www.thechimneysweep.ca

The Hearth in the House as a System

And there's one article from Jotul that shows how a tall interior chimney has a higher NPP than the house:

All about wood stove chimneys | Jøtul

No Jotul woodburning stove or fireplace insert can function properly without a good chimney. Think of the chimney as the engine that drives the Jotul wood heating system. Think of its fuel as heat. Think of the power it puts out as draft. The more fuel (heat) you give this engine (chimney), the...

www.jotul.com

They even show a diagram of this:

But this isn't explained in any great detail. What would a similar diagram look like for an exterior chimney?

It seems like the "stack effect" in the chimney itself would only work if the chimney was closed at the top, so that the pressure could build up there. And yeah, it's taller than the house, but how does this help? Even in this diagram, we're still showing negative pressure all the way down the chimney... is the idea that the pressure at the bottom of the chimney is even MORE negative than the pressure in the house? So it will suck air into the chimney from the house?

And how would this change if the chimney was exterior and colder? Maybe the whole chimney is at outside atmospheric pressure in that case?

How does total chimney height affect this? Do taller chimneys naturally suck air out of the house more? If you make your exterior chimney tall enough, will that eventually overcome the problems associated with a cold chimney? What if your exterior chimney was 3x taller than your house?

And for interior chimneys, is as tall as possible necessary, regardless of construction cost? For example, on a tall 2-story house with the woodstove on the first floor, if you have an interior chimney that exits the roof near the eves of the roof, vs. a much taller interior chimney that exits near the peak of the roof, how much better will the taller one draft than the shorter one?

(And yes, there are other problems with a cold exterior chimney, like condensation of flue gasses and build-up of deposits, but I'm just talking about draft issues here).

A chimney only has 2 reference points for pressure. Indoors at the bottom and outdoors at the top. A column of warm air will have a lower density than a column of cold air. Since the top reference pressure is (relatively) constant, this means the buoyancy effect (draft) created at the bottom of the chimney will be stronger when the chimney is warm than when it's cold.

The presence of the chimney hole will create local disturbances in the neutral pressure plane of the house. It's not like creating a horizontal hole through the wall which is the same elevation outside as inside. This difference should explain why it's possible for the chimney to draw even if it's in the basement.

 

[begreen]

Here is an example of a chimney chase bump-out in a room.