Negative draft problems

[Fyrme]

Being it's winter again, I'm dealing with some negative draft issues in my shop, again...

The issue I'm having, is when I get a south wind, on a cold day that I've got the wood stove in my shop fired up, I have a bad down draft problem. Last year, I raised my flue height an additional 5', and put a different cap on it, which helped,,, a little bit, but didn't fix the problem.

The set up I have is a side wall exit near the eave, and 10' of exterior flue pipe. My roof is a 6/12 pitch, which puts the top of the chimney almost at the height of the roof peak. Using the 10'/2' rule, I exceed that by about 3'.

Is there anything else I can do to improve the draft on windward days, beside adding an expensive draft blower? I thought about adding another 3-5' of flue, but that will require a complete rebuild of the chimney with a heavy wall welded pipe, in order to support the height and weight.

I'm been considering building a forced air waste oil burner to avoid having to cut 6 ricks of wood to get me through the winters, but on days like today, a solution for this problem would be nice on my eyes and lungs, the rest of this winter...

 

[greenskeeper]

Can you slightly open a window, etc to aid in fresh air intake to the stove? Is your building "tight" ?

 

[Fyrme]

Building is not tight, and all doors and windows are on the leeward side, so it only adds to the problem

 

[danski0224]

The wind is putting your building under a negative pressure, which is overcoming the natural draft of your stove.

Assuming that I have the terminology correct: leeward side is the side sheltered from the wind.

At the very least, you need to put a hole in the building in the side facing the wind, with some sort of barometric relief to equalize the pressure.

A fan forced combustion air supply unit would also be advised. You would definitely need one of these with an induced draft assist.

 

[yeldogt]

I'm thinking the down draft over the ridge is still your problem -- did you get a regular wind cap with the adjustable wings? There are basically uninsulated cabins all over the place with wood stoves that don't do this in high winds.

 

[Fyrme]

The wind is putting your building under a negative pressure, which is overcoming the natural draft of your stove.

Assuming that I have the terminology correct: leeward side is the side sheltered from the wind.

At the very least, you need to put a hole in the building in the side facing the wind, with some sort of barometric relief to equalize the pressure.

A fan forced combustion air supply unit would also be advised. You would definitely need one of these with an induced draft assist.

Yes, leeward is the "downwind" side. Sorry, I'm throwing out fire service terms on you.

Flue is on the south side of the building, doors are on the north side. Wind out of the north works perfect, wind out of the south, far from perfect. I'm assuming the wind hits the steep pitch and pushes back to the flue.

Unfortunately my shop is not set up to create an opening on the south side.

I've looked at adding a chimney cowl, but u think adding 5' more to the chimney would be cheaper, even after rebuilding the entire flue.

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[Fyrme]

I'm thinking the down draft over the ridge is still your problem -- did you get a regular wind cap with the adjustable wings? There are basically uninsulated cabins all over the place with wood stoves that don't do this in high winds.

This is the style flue cap that I'm using now, minus the flashing. (Being dark outside, I can't get a pic of the cap) Before I was using the open style with the screen spark arrestor.

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[redidbull]

This article explains the 10 2 rule for chimney height. Make sure it conforms. Jim

https://www.acucraft.com/determine-proper-chimney-height-using-the-10-2-rule/

 

[danski0224]

Yes, leeward is the "downwind" side. Sorry, I'm throwing out fire service terms on you.

Flue is on the south side of the building, doors are on the north side. Wind out of the north works perfect, wind out of the south, far from perfect. I'm assuming the wind hits the steep pitch and pushes back to the flue.

Unfortunately my shop is not set up to create an opening on the south side.

I've looked at adding a chimney cowl, but u think adding 5' more to the chimney would be cheaper, even after rebuilding the entire flue.

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With all of your penetrations (windows) on the downwind side, there is a negative pressure zone on the downwind side of the building, and that negative pressure zone is communicating with the interior of the building through the penetrations. Of course, some pressure is getting in on the opposite side, and leaving through the window openings.

Pretty sure in the post 3 the building is described as "not tight".

There is no other explanation, especially if the wood stove works fine when the wind is pushing on the side of the building with the windows. Lots of places for wind to get in, not so many to get out on the other side.

The poorly sealed wall/sill plate/soffit/eaves would be a constant on either side if the building size and shape are comparable.

If your chimney/flue conforms to code right now, I would be surprised if making it taller will fix the problem. Maybe you will create other problems, like possible flue gas condensation or creosote buildup.

I'd try opening a door on the north side when you are experiencing these problems. Maybe set up some sort of smoke source to confirm chimney draft. Before doing anything else, unless your chimney is not code compliant.

 

[DenisG]

I haven't used one, but maybe something like this? - wind cap -
https://www.luxurymetals.com/wind_d...MI3tOJi7ea4AIVgbfACh3irwL-EAQYASABEgKdtPD_BwE

 

[Unhdsm]

Is that flue (stove) pipe or insulated chimney pipe?

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[yeldogt]

A burning fire creates heat that drives the draft -- remember ...wind is creating positive pressure on the side it is hitting .. negative on the other. I'm thinking the problem is the flue .. not the building.

 

[Fyrme]

I haven't used one, but maybe something like this? - wind cap -
https://www.luxurymetals.com/wind_d...MI3tOJi7ea4AIVgbfACh3irwL-EAQYASABEgKdtPD_BwE

Yeah, that's the 'cowl' i was referring to.

Is that flue (stove) pipe or insulated chimney pipe?

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The pipe pictured is single wall duct that I used for a temporary flue for my pressure washer. The stove chimney is single wall stove pipe. The only place I have double wall is where it goes through the wall.

This article explains the 10 2 rule for chimney height. Make sure it conforms. Jim

https://www.acucraft.com/determine-proper-chimney-height-using-the-10-2-rule/

Yes sir. I am within specs.

I'm thinking part of my problem may be the elbows slowing down my draft, and allowing the wind to overcome it. I know a straight flue is ideal, but I REALLY, REALLY don't want a hole in my roof.

 

[yeldogt]

This is the style flue cap that I'm using now, minus the flashing. (Being dark outside, I can't get a pic of the cap) Before I was using the open style with the screen spark arrestor.

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Do they rate that for wind ?

 

[Unhdsm]

Yeah, that's the 'cowl' i was referring to.



The pipe pictured is single wall duct that I used for a temporary flue for my pressure washer. The stove chimney is single wall stove pipe. The only place I have double wall is where it goes through the wall.



Yes sir. I am within specs.

I'm thinking part of my problem may be the elbows slowing down my draft, and allowing the wind to overcome it. I know a straight flue is ideal, but I REALLY, REALLY don't want a hole in my roof.

If you can, measure draft on a non windy day. I suspect its marginal at best, especially if you have an EPA stove. Chimney just may not be warm enough to draft.

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[Firebrick43]

Yeah, that's the 'cowl' i was referring to.



The pipe pictured is single wall duct that I used for a temporary flue for my pressure washer. The stove chimney is single wall stove pipe. The only place I have double wall is where it goes through the wall.



Yes sir. I am within specs.

I'm thinking part of my problem may be the elbows slowing down my draft, and allowing the wind to overcome it. I know a straight flue is ideal, but I REALLY, REALLY don't want a hole in my roof.

The chimney needs to be insulated. It not only the elbows, it's the quick cooling
causing low draft conditions. Danski theory on negative pressure due to leaks around Windows and such is probably correct. With minimal draft due a cool chimney now you have inversion.

I have never seen a chimney that's properly sized, insulated, and exits at or near the peak have draft problems. I have seen plenty of side mount or chimneys near the eaves have draft problems. Many people claim to have a good chimney but when they open the door puffs of smoke come out. A proper one allows no smoke to escape and the house or building will not even have a hint of smoke smell.

 

[danski0224]

It seems that many of the chimney insulation kits are designed for so-called lightweight corrugated stainless steel and aluminum retrofit chimney liners.

In commercial HVAC work, black iron kitchen hood exhaust ducting is insulated with ceramic-based insulation blankets, two layers are used in my area to get to the required thickness. This material is pretty heavy and certainly not inexpensive. It is held in place with stainless steel bailing wire or large stainless steel hose clamps.

The chimney should be checked with a draft gauge under varying operating conditions.

And the other question is whether or not the flue system is part of an approved installation by the stove manufacturer, or is this a cobbled together home brew problem?

 

[Fyrme]

It looks like a new double wall chimney is what it is going to take to make this thing work in ALL wind conditions. I priced a custom kit yesterday and it was nearly $1000. I just don't get it. It's pipe, inside another pipe, why is it so expensive? I've been looking for used double wall on CL and Marketplace since I put the stove in last year, but it's hard to find. I think I will just deal with the situation I have now for the rest of the winter, and figure out how to build it better before next winter.

 

[wrenchguy]

The wind is putting your building under a negative pressure, which is overcoming the natural draft of your stove.

Assuming that I have the terminology correct: leeward side is the side sheltered from the wind.

At the very least, you need to put a hole in the building in the side facing the wind, with some sort of barometric relief to equalize the pressure.

A fan forced combustion air supply unit would also be advised. You would definitely need one of these with an induced draft assist.

this. the wind going around the building with the right variables is sucking the air outta ur building. just because the windows and doors are closed doesnt mean they're not leaking. overhead garage doors leak big time.

 

[yeldogt]

this. the wind going around the building with the right variables is sucking the air outta ur building. just because the windows and doors are closed doesnt mean they're not leaking. overhead garage doors leak big time.

It can do both -- and often pressurizes. The roof design has much to do with it --

What people don't understand ---- a roof will act as a wing. It's increasing the lift as long as the air is not directed into the flue.


Lower pressure inside -- is actually pulling through the flue up and out ..... watch a building in a hurricane -- the early failures in a storm are when the roof flies off. It's a wing .....

 

[danski0224]

It looks like a new double wall chimney is what it is going to take to make this thing work in ALL wind conditions. I priced a custom kit yesterday and it was nearly $1000. I just don't get it. It's pipe, inside another pipe, why is it so expensive? I've been looking for used double wall on CL and Marketplace since I put the stove in last year, but it's hard to find. I think I will just deal with the situation I have now for the rest of the winter, and figure out how to build it better before next winter.

Why is it so expensive? UL certification and liability.

Woodburning factory prefab fireplace chimneys are triple wall (no insulation) and have a proprietary locking system. This type of pipe usually does not come apart without destroying it. Not the same as a wood stove, but mentioned because it doesn't come apart.

You probably need a Class A chimney system. The Hart and Cooley TLC Chimney System is one UL certified and engineered system product that is readily available.

That stuff is heavy, and expensive.

Of course, it needs to be installed to the manufacturer specifications. There is a complete installation and spec guide on the Hart and Cooley website. Comparable to prefab woodburning fireplaces, there are limits to offsets and the length of offset. Horizontal for much more than the minimum required for a properly installed stove to building walls, in order to get the chimney outside, is a no-go.

If your installation is a homebrew hodgepodge, hopefully you won't burn the place down and be without insurance coverage... Of course, that's if you can even get insurance coverage using a UL certified stove AND a UL certified chimney system. There's another reason why the stuff is so expensive...

Of course, you will still need to address building deficiencies and combustion air deficiencies.

 

[danski0224]

It can do both -- and often pressurizes. The roof design has much to do with it --

What people don't understand ---- a roof will act as a wing. It's increasing the lift as long as the air is not directed into the flue.


Lower pressure inside -- is actually pulling through the flue up and out ..... watch a building in a hurricane -- the early failures in a storm are when the roof flies off. It's a wing .....

Yes, and there is a lot of guessing without diagnostics.

I'm betting that the OP has a homebrew hodgepodge that is not installed properly... and wondering why it doesn't work...

Building dynamics is probably pushing it over the edge as a guess.

 

[greenskeeper]

It looks like a new double wall chimney is what it is going to take to make this thing work in ALL wind conditions. I priced a custom kit yesterday and it was nearly $1000. I just don't get it. It's pipe, inside another pipe, why is it so expensive? I've been looking for used double wall on CL and Marketplace since I put the stove in last year, but it's hard to find. I think I will just deal with the situation I have now for the rest of the winter, and figure out how to build it better before next winter.

It's insulated between pipes correct? That keeps the flue temperature elevated and increases the available draft.

 

[Fyrme]

It's insulated between pipes correct? That keeps the flue temperature elevated and increases the available draft.

No, the outer chimney is not insulated double wall. It is only single wall, which is what I am learning, is most likely my problem. I see single wall sch40 chiminey's all over the area where I live, and either they somehow work as they should, or those people are dealing with negative draft, like myself. That is why I will be not only changing my side wall flue to a straight through the roof flue, but once it hits the roof penetration, it will be double wall all the way up to the cap. Hopefully I will be able to piece something together buying the pieces off Marketplace and CL over the summer.

 

[Fyrme]

It's insulated between pipes correct? That keeps the flue temperature elevated and increases the available draft.

No, the outer chimney is not insulated double wall. It is only single wall, which is what I am learning, is most likely my problem. I see single wall sch40 chiminey's all over the area where I live, and either they somehow work as they should, or those people are dealing with negative draft, like myself. That is why I will be not only changing my side wall flue to a straight through the roof flue, but once it hits the roof penetration, it will be double wall all the way up to the cap. Hopefully I will be able to piece something together buying the pieces off Marketplace and CL over the summer.