make up air

[loneeagle15]

next year my wife an myself are going to reside our house adding possibly rigid foam insulation and definitely house wrap
I am building my hearth pad as we speak it will consist of 5 layers of backer board
now for my question can i bring my makeup air in through the air space under my stove and also can I use the air in my ventilated crawl space or do I need to take my duct for the make up air all of the way outside?
we can not get an inspection where we live all the county will give permits for is electrical and gas work?
Thank you

 

[Shane]

Should be able to. Where is your stoves intake air? Also as for extending the vent all the way outside this is usually depends on whether you have a ventilated or un ventilated crawl space.

 

[loneeagle15]

in the very middle of the stove on the back at the bottom

 

[jtp10181]

Read the directions for the stove for where and how to hook it up. Often times you can have it come up under the bottom so it is totally hidden.

If I remember correctly every manual I have read specifically says not to draw from crawl spaces or attics, to always run the ducting to the outside.

 

[builderbob]

I have read a few manuals for zc wood fireplaces---not woodstoves---and all of them say not to end the air intake duct in garages or *unvented* crawlspaces. I think they want to avoid it taking in fumes from say car gas, or perhaps gas furnaces in garage etc. Or from dead sealed off space in unvented crawlspace. A ventilated crawlspce does not present those dangers.

The manualsdon't say not to intake air from *ventilated* crawlspaces. They just don't talk about ventilated crawlspaces.

Some here have said it's ok. Others have commented you might just draw into the house a "musty" smell from the crawlspace. Also, it seems the manuals specify bigger and bigger duct the longer you have to run it. Like they say 4" if 10 feet or less, then 5" if over 10, then 6" if over some longer length.

So, if yu would have to run the duct pretty long to get to the very outside, tht might be a consideration to just go to the crawlspace under the stove area. Or if it isn't terribly far all the way to outside wall, might be better to just do it all the way.

Another newbie researching fireplace questions, BTB

 

[Gooserider]

next year my wife an myself are going to reside our house adding possibly rigid foam insulation and definitely house wrap
I am building my hearth pad as we speak it will consist of 5 layers of backer board
now for my question can i bring my makeup air in through the air space under my stove and also can I use the air in my ventilated crawl space or do I need to take my duct for the make up air all of the way outside?
we can not get an inspection where we live all the county will give permits for is electrical and gas work?
Thank you

Your stove manual rules, and if it doesn't say, you probably should check w/ your dealer or stove maker's tech support to be sure.

However, you should have no problem going down through the hearth to connect an Outside Air Kit (OAK), as long as you follow any requirements they give you about protecting the pass-through opening (probably not a big deal) and making the ducting from appropriate material.

My understanding is that you can draw from a ventilated crawl space as long as the venting is adequate to supply the required air volume, though as others have mentioned there is some risk of getting a musty smell. Some have actually argued that a crawl space is BETTER than an outside wall vent as it avoids the (IMHO mostly theoretical) problem of a wind induced pressure imbalance causing draft issues.

Gooserider

 

[elkimmeg]

goose brings up a good point presurization on the winwardside of a home it positive pressure leeward side negative pressure
since winds shift it is hadr to draw from the leeward side ont theory is to have a tee or y and pull from two locations one opposite sides of the home to nuetralize pressure
the wind shifts the negative side will draw air out of you fire and have negative effects to burning

I will check code but I am pretty sure combustion air has to be taken fron air out side your insulated living space

I know it can't take air from an area of other combustioin appliance locations

Also it can't take air from hazzardeous location such as a garage.

I also know there is language concerning crawl spaces this is what I have to look up.

the deffinition of outside air is air taken from outside the envelope comunicating directly with the " outside"

or directly connected to the outside

 

[webbie]

John Gulland (the expert in these matters) and others have concluded - after careful studies in Canada - that outside air is generally not the solution to some of the problems that folks think it is. In short, it appears that most wood stoves are better off without outside air - of at least that is the way I read virtually all the studies. This same conclusion was drawn a couple decades ago by Jay Shelton, one of the first scientists who published a number of studies. Here is an article by Gulland that mentions it:
http://www.woodheat.org/planning/successfulfireplace.htm

 

[Shane]

FPX has started recommending mounting their blowers indoors in the colder climates as well as allows their air intakes to be installed in properly ventilated crawlspaces and attics. This of course is contingent on AHJ approval.

 

[builderbob]

John Gulland (the expert in these matters) and others have concluded - after careful studies in Canada - that outside air is generally not the solution to some of the problems that folks think it is. In short, it appears that most wood stoves are better off without outside air - of at least that is the way I read virtually all the studies. This same conclusion was drawn a couple decades ago by Jay Shelton, one of the first scientists who published a number of studies. Here is an article by Gulland that mentions it:
http://www.woodheat.org/planning/successfulfireplace.htm

For a thorough discussion in favor of outside air, and rebutting the above mentioned proponents of inside air, see Tom Oyen's (chimney sweep and stove shop owner) article at following link.

http://www.chimneysweeponline.com/hooa3.ht

BTB

 

[builderbob]

Sorry, make it THIS link:

http://www.chimneysweeponline.com/hooa3.htm

BTB

 

[tradergordo]

We had a big discussion about this last year, try the search. At any rate, I can't believe that after that discussion, the chimneysweeponline still contains quotes like:

"a strong enough wind were to blow at a downward angle onto the side of the house opposite the outside air intake termination"

"Even if we were to accept the theory that exhaust backdrafting through the outside air intake is remotely possible in certain extreme wind conditions, we must realize that, in the absence of an outside air connection to the firebox, this same extreme downdrafting would fill the house with poisonous wood exhaust."

Both of these statements show a lack of understanding of the problem with wind induced pressure related problems associated with outside air connections. The wind DOES NOT need to be blowing down your chimney, nor will any of various chimney caps resolve the issue. From the second quote - there wouldn't BE any downdrafting without an outside air connection so the whole statement is silly. But the rest of their comments seem pretty accurate to me - after much buzz, and some anedotal reports of sparks flying out of outside air terminations and people who have heard of people with problems - as far as I know, no one was able to find or produce documented cases of housefires for example, that resulted from an outside air connection. I think they may have some place in unique circumstances but they're probably not appropriate or neccesary for most houses and too often they are used as a bandaid to fix a bigger problem like a poorly installed or located chimney/flue.

Sorry, make it THIS link:

http://www.chimneysweeponline.com/hooa3.htm

BTB

 

[Gooserider]

Full disclosure - I don't have an OAK, and am not sure I could put one in this house if easily if I needed to. However after reading the arguements pro and con, I tend to fall on the favorable side of using an OAK, especially in new tight construction, or in old construction that has been tightened up a lot. I'm not fanatical about it though.

It seems to me like the arguements against the OAK are mostly hypothetical, and I haven't seen any actual definite evidence of HARM resulting from using one. The arguements in favor of an OAK are IMHO slightly stronger on balance, so it seems to me like a case of "doesn't hurt, might help"...

Secondly, if one is doing major remodelling to create a hearth or stove location, redoing a fireplace, etc. it is going to be easiest to install an OAK at this point rather than going back and trying to install one later - If you decide it isn't wanted / needed, it wouldn't be that hard to plug, but it would be harder to install later if you found it was needed. That said, I wouldn't go to great lenths to add an OAK if it made the project more complex, nor would I do major renovations just to add one, absent strong evidence that it was needed. IOW, I'd only put one in if it were pretty easy.

Bottom line, the stove is going to need to get air from someplace. It can get it from an OAK, or it can get it from inside the house. The OAK is easy, but if pulling from inside the house, you are competing with all the other air consuming appliances, both other fires and things that pull air out of the house like driers and exhaust fans. In a leaky house this is no big deal, but since we generally want to stop leaks, I see no harm in adding an OAK as long as it isn't overly difficult to do so.

Gooserider

 

[thechimneysweep]

As anyone who has read my posts on this forum (and my wife) already knows, lack of understanding and I are no strangers. However, since every wood-burning installation my company has done in the past decade has included a direct connection to outside combustion air, I think I've got a pretty good grasp on this issue.

Given the way the outside air connections are designed on today's woodstoves, it would be next to impossible for any wind velocity short of a tornado to create enough negative pressure on the leeward side of a house to cause backdrafting through an outside air intake located on that side of the house. As discussed in this forum previously, there is simply no documentation that this phenomenon has ever occured.

The folks at woodheat.org seem to hedge their bet a little concerning this issue: Take a look at the sketch at the bottom of their page at http://woodheat.org/outdoorair/outdoorairmyth.htm : See the little arrow of wind coming down off the peak of the roof and down the chimney? This sketch illustrates a COMBINATION of leeward-side depressurization and chimney downdrafting, which woodheat.org evidently believes could be the one-two punch that might create backflow through the outside air intake.

On our website, we try to make the point that this combination of forces would only affect installations where the wind direction and velocity, chimney location and outside air intake location were situated in a very specific juxtaposition, and that installation of a bernoulli or venturi cap would harness the same wind force to reverse the wind-driven backflow down the chimney.

As to the idea that backdrafting refers only to outside air systems, that's not the case. Backdrafting is just what it sounds like; the reversal of flue gases in the chimney, causing them to flow downwards, regardless of the cause. The point we're trying to make on our website is that backdrafting severe enough to overcome chimney updraft and blow exhaust out an outside air intake would blow the same exhaust into the room through the draft intakes in non-outside air installations.

 

[Highbeam]

I'm sold that it is a case of can't hurt, might help. I'm also an engineer that even knows what Bernouilli did. Anyhow, in my case the OA connection is required and mine is connected to the ventilated crawlspace per the stove manufacturer's instructions. This is an ideal way to go in my opinion since the crawlspace is ventilated on both windward and leeward sides and should be balanced. I have actually found that the crawlspace connection alone is a mini chimney and actually sucks air from the crawlspace into my home. Once connected to the stove the OA system can only help draft.

I also had a non-OA equipped stove prior to this one and could always feel the air rushing towards the stove. Maybe this was a convection current and maybe the stove sucking air. Won't know until fall.

I will consider the termination point of the OA system in the crawlspace and also consider the one in a million chance that smoke could spew out of it and be sure that there is clearance to combustibles. This is not very necessary though but my theme has been very conservative.

 

[tradergordo]

chimneysweep - woodheat.org specifically says:
"Evidence of wind-induced reversals in combustion air ducts is becoming more common now that so many systems have been installed. When servicing systems with direct outdoor combustion air ducts, look for soot or staining inside the duct. If there is any evidence of reversal, disconnect the duct and plug the hole in the house envelope."

I would like to know how many outside air kit ducts you have inspected after a stove has been in service? Do you personally inspect them when you do sweeps? If not, why? If so, have you seen soot or staining inside any of them?

Take a look at the sketch at the bottom of their page at http://woodheat.org/outdoorair/outdoorairmyth.htm : See the little arrow of wind coming down off the peak of the roof and down the chimney? This sketch illustrates a COMBINATION of leeward-side depressurization and chimney downdrafting, which woodheat.org evidently believes could be the one-two punch that might create backflow through the outside air intake.

You are right in that it sort of depicts two totally unrelated problems and since they are only trying to illustrate one problem perhaps they could have used a better picture - but the fact is that the chimney could be place in ANY position in the picture (on the left side of the roof for example) and the negative pressure would still cause (at least in theory!) the backdrafting.

The point we're trying to make on our website is that backdrafting severe enough to overcome chimney updraft and blow exhaust out an outside air intake would blow the same exhaust into the room through the draft intakes in non-outside air installations"

I understand what you were TRYING to say, however this is also "hedging your bets" to use your phrase. The point from woodheat is that by using and OAK you are making yourself more suseptible to backdrafting (due to wind/pressure issues) and you are making a totally different point that IF you were to have backdrafting (from a totally different source) then you would actually want an OAK. This is a valid point - but kind of sidesteps the issue. Someone with backdrafting of ANY kind has a problem that should be fixed (could be "wind down the chimney" that you speak of, or strong venting appliances, for example).

 

[tradergordo]

I'm sold that it is a case of can't hurt, might help.

I still need more convincing before I can come to that conclusion.

Hardly anyone talks about indoor air quality as it relates to this issue. Some people in the industry know that after houses started being built "extra tight" with tyvek wraps, etc. indoor air quality started to suffer. Now the experts are recommending elaborate solutions such as air exchangers. You don't HAVE to spend big bucks on an exchanger because your woodstove IS an exchanger, but ONLY if you are NOT using an outside air connection. I WANT my stove to pull fresh air into my house and expel stale air continuously.

From woodheat.org again: "It has been suggested that a direct combustion air supply to a woodburning appliance would eliminate its air consumption impact on other chimney vented combustion equipment in the building. However, when their doors are closed, wood heating appliances exhaust comparatively little air from the dwelling. Wood stoves with gasketed doors consume in the range of 10 - 25 cubic feet per minute of air, which is a tiny amount of air, much less than an oil furnace, for example. Since the air consumption of a wood stove is so low, the risk of reversal of a ducted combustion air supply outweighs any advantage gained by bringing air from outdoors"

 

[thechimneysweep]

Trader, I'm a bit long in the tooth these days to clean chimneys, but the Sweeps who work for our company still sweep thousands of flues each year. Direct connection to outside air is mandated in the code jurisdictions we service, and because any blockage of the intake port will adversely affect combustion, inspection of the outside air intake is performed on each sweep. In 27 years, we've never observed any sooting or other signs of exhaust backflow through an outside air system. In fact, I've never met any Chimney Sweep, or fireman, anyone else for that matter, who has ever observed signs of this phenomenon.

Still, anything is possible, so our installers are taught to get outside air from a ventilated crawlspace whenever possible, and, wherever there are prevailing winds, to avoid terminating on the leeward side of the house.

In the meantime, woodstove manufacturers who incorporate outside air intakes have long since learned to avoid an airtight path from the intake plenum to the outside air terminus. Severe pressurization on the chimney end or depressurization on the outside air intake end are significantly reduced by openings to the room which exist in the flowpath to or through the stove. These openings exhibit themselves in instances of severe room depressurization (where the house itself is creating the vacuum), as they allow exhaust to enter the room, a phenomenon known as spillage.

 

[builderbob]

chimneysweep - woodheat.org specifically says:
"Evidence of wind-induced reversals in combustion air ducts is becoming more common now that so many systems have been installed.

So woodheat.org says what they say.

What concerns me then are what others say. My brethern hearth owners in Washington state are required BY THE STATE to use outside air. Are state regulations endangering them? Are the Washington State regulsators a bunch of nitwits?

The zc woodburning fireplace manufacturers say---install outside air intake. In fact they even say you void your warranty if you do NOT follow those install instructions. Manufactures like Napoleon, Travis, RSF, KozyHeat(Hussong?), Lennox, Quadrafire, etc. Are all the engineering staff for all those outfits a bunch of nitwits?

Are they all wet?

Given Chimneysweep's pro and con discussions, given Goose's excellent logic, and given I don't want to go counter to my unit manufacturer's instructions and thereby void any warranty, I do believe I will go with outside air.

No offense to woodheat.org, but seems too many "so-called" authoritative sources stand on the other side of this question.

BTB

 

[Todd]

I'm sold that it is a case of can't hurt, might help.

I still need more convincing before I can come to that conclusion.

Hardly anyone talks about indoor air quality as it relates to this issue. Some people in the industry know that after houses started being built "extra tight" with tyvek wraps, etc. indoor air quality started to suffer. Now the experts are recommending elaborate solutions such as air exchangers. You don't HAVE to spend big bucks on an exchanger because your woodstove IS an exchanger, but ONLY if you are NOT using an outside air connection. I WANT my stove to pull fresh air into my house and expel stale air continuously.

From woodheat.org again: "It has been suggested that a direct combustion air supply to a woodburning appliance would eliminate its air consumption impact on other chimney vented combustion equipment in the building. However, when their doors are closed, wood heating appliances exhaust comparatively little air from the dwelling. Wood stoves with gasketed doors consume in the range of 10 - 25 cubic feet per minute of air, which is a tiny amount of air, much less than an oil furnace, for example. Since the air consumption of a wood stove is so low, the risk of reversal of a ducted combustion air supply outweighs any advantage gained by bringing air from outdoors"

I like that take on your wood stove air exchanger. Most people get sick in the winter time because of stale/germy air inside. People, open your windows once in awhile, get some fresh air!

Your right that wood stoves use so little air it probably doesn't matter. Maybe I'm wrong but I think these studies were made assuming the massive air use of fireplaces. For example my fireplace took air from inside the house and if I didn't crack a window I could feel the pressure difference. Kind of felt like a stuffed up head cold. It probably sucked more heated air up the flue than it was releasing into the room. And of coarse when not in use it leaked cold air into the house. Last year I installed airtight doors and oak to it. Now it doesn't have those effects and it's more efficient. I can really feel the difference.

 

[tradergordo]

ChimneySweep - I appreciate your feedback, and I believe you. I think at this point we have to agree that the risk of flow reversal CAUSED by an OAK is minimal if it exists at all - based on your 27 years of observation. My primary concern now are people needlessly drilling holes though their walls and indoor air quality. Specifically you said:

...woodstove manufacturers who incorporate outside air intakes have long since learned to avoid an airtight path from the intake plenum to the outside air terminus.

My first question is to the people here using OAK - are you also incorporating "room openings" in this pathway?

I don't understand how this is any different or better than cracking a window next to the stove? How do you prevent cold outside air from pouring into the room when the stove is not firing? At least you can close a window, how do you close your OAK "room holes"? Do you have any pictures of a typical installation? I thought I remember you (or someone else?) mentioning they just use standard round metal dryer vent + standard metal dryer outside termination (available inexpensively from the big box home stores). So do you just drill holes in this vent? IF so, where do you drill them? How many? What size?

Also - your own site implies that using an OAK would prevent flue gasses from spewing into the room in the event of a flow reversal - however this is totally at odds with the fact that you only advocate OAKs with "room openings" - which presumably would not prevent flue gasses from venting into the room during a flow reversal.

 

[Highbeam]

I think Tom was referring to intentional leaks in the path between the firebox and the hookup point of the stove to the OAK duct. The woodstove manufacturer has no way to incorporate leaks into the site built ducting.

From my examinations, the Hearthstone allows a nearly airtight seal between the OAK stub and the firebox. You can actually look into the intake manifold and see the throttle body of the primary air control and then a path to the unthrottled secondary manifold.

A leaky OA system could indeed spew either crawlspace air or chimney air into the living space when the fire is not drafting which would not be desirable from an air quality point of view. I used three inch semi rigid aluminum dryer duct for my OA. No holes were called for in my manual.

As far as using the stove as a fresh air exchanger you can't have it both ways. In one post the amount of air consumed by a stove is said to be extremely minor and then in another post it is said to be your air exchanger and serve a vital function. Myself, if I want cold fresh air I will open a window.

 

[tradergordo]

As far as using the stove as a fresh air exchanger you can't have it both ways. In one post the amount of air consumed by a stove is said to be extremely minor and then in another post it is said to be your air exchanger and serve a vital function. Myself, if I want cold fresh air I will open a window.

"minor" is relative, it moves about 10 - 25 cubic feet per minute, which is better than nothing (i.e. what you get with an OAK). I'm not sure how much ventilation is recommended by the air exchanger experts. But too much (as in an open fireplace) and you end up with cold drafts. Less, and you wouldn't be exchanging your air enough. The problem with the "do it yourself" method (i.e. opening a window every now and then) is that most of the time people don't realize when their indoor air quality is poor and its also better to have continuous circulation and periodic bursts.