Interesting discussion elsewhere on "Makeup Air" for woodstoves

[Shari]

We don't have an OAK. Our local codes do not require it.

Interesting comment by our permitting department was they wanted to make sure the 'room' the woodstove would be located in did not have a door which could be closed. They also stated the 'room' required a certain cubic foot (I forget what # they stated). Since our woodstove is in our livingroom which is open to our kitchen which is open to a bedroom hallway and also open to a short hall and a laundry room we had no problem with sufficient cubic foot.

 

[WidowMaker]

Hey folks it's all outside air, just a matter of how it get to the fire box....

 

[oldspark]

Not sure I want to feed below zero air directly to the fire box.

 

[WidowMaker]

Now that would be intersting to know howmuch you redure stove top temp by feed outside air as opposed to room air..

But I still prefeer to feed 0* air directly to thr fire box as opposed to dragging it all the way thru the house to the stove..jmho

 

[HotCoals]

OAK's are rarely needed.
The more I search the nets and read..the more I'm certain I don't need one.
The amount of air going into a stove is less then you would think.

 

[semipro]

We don't have an OAK. Our local codes do not require it.

Interesting comment by our permitting department was they wanted to make sure the 'room' the woodstove would be located in did not have a door which could be closed. They also stated the 'room' required a certain cubic foot (I forget what # they stated). Since our woodstove is in our livingroom which is open to our kitchen which is open to a bedroom hallway and also open to a short hall and a laundry room we had no problem with sufficient cubic foot.

Interesting. The size (volume) of the space that the stove pulls from really makes no difference. Its the amount of combustion air that that space can provide that matters and that's dependent upon how "tight" it is.

 

[semipro]

Not sure I want to feed below zero air directly to the fire box.

You'd rather temper it a bit by mixing it with heated air from within your house?

 

[semipro]

OAK's are rarely needed.
The more I search the nets and read..the more I'm certain I don't need one.
The amount of air going into a stove is less then you would think.

I've measured it and it was more than I thought. I did this because I was curious how a 4" diameter OAK could flow enough air to fill my 6" diameter stove pipe. After all, a 6" diameter pipe has more than double the cross section of a 4" diameter pipe (28 versus 12.5 square inches).

If curious....the answer lies in the temp difference between the air coming in and that going out since gas expands when hot. Using the simplified Ideal Gas law, basically Pressure times Volume all divided by Absolute Temp (PV/T) of the incoming gas stream should equal that going up the chimney. It worked out.

 

[LLigetfa]

Not sure I want to feed below zero air directly to the fire box.

You'd rather temper it a bit by mixing it with heated air from within your house?

One way or another, the air will be warmed up to what the stove needs. It doesn't go directly to the firebox. It first passes through warming chambers.

I prefer not to have the -40° air flow across my floor en route to the stove and for it to steal what little humidity I have as it passes by.

 

[LLigetfa]

The amount of air going into a stove is less then you would think.

Even the "experts" can't agree on it but I've seen numbers between 10 and 25 CFM. I've also seen numbers for air infiltration on a tight home being around 75 CFM, so the stove could add 25%.

"Need" is subjective and should perhaps be replaced with "Want".

 

[LLigetfa]

I've measured it and it was more than I thought.

Since we don't know what you thought it was and what you measured, we cannot take away any of the knowledge you gained. I wish I had a way to measure it. Simply holding my hand in front of the OAK intake, I know I would not want that much cold air coming into my home.

As for the size (volume) differential between the OAK and the flue, there are other factors to also consider besides just expansion. A flue is also sized for resistance loss. With the air having 20% oxygen, the byproducts of combustion probably make up more than what is consumed. There is also pressure differential to consider.

 

[oldspark]

Not sure I want to feed below zero air directly to the fire box.

You'd rather temper it a bit by mixing it with heated air from within your house?

Yep, I would rather have passive OAK.

 

[semipro]

The amount of air going into a stove is less then you would think.

Even the "experts" can't agree on it but I've seen numbers between 10 and 25 CFM. I've also seen numbers for air infiltration on a tight home being around 75 CFM, so the stove could add 25%.

"Need" is subjective and should perhaps be replaced with "Want".

Add the stove's requirements of 10 to 25 CFM to that of:

- bathroom vent fan approx 75 CFM
- range hood 180 (downdraft 400) CFM
- clothes dryer 175 CFM

If all were running at the same time, even if your house leaked at 400 CFM there would still be a deficit of available combustion air for a wood stove without an OAK.

 

[oldspark]

Not sure I want to feed below zero air directly to the fire box.

You'd rather temper it a bit by mixing it with heated air from within your house?

One way or another, the air will be warmed up to what the stove needs. It doesn't go directly to the firebox. It first passes through warming chambers.

I prefer not to have the -40° air flow across my floor en route to the stove and for it to steal what little humidity I have as it passes by.

Not sure I know of these warming chambers you mention, not all OAK's have warming chambers do they?

 

[semipro]

I've measured it and it was more than I thought.

Since we don't know what you thought it was and what you measured, we cannot take away any of the knowledge you gained. I wish I had a way to measure it. Simply holding my hand in front of the OAK intake, I know I would not want that much cold air coming into my home.

As for the size (volume) differential between the OAK and the flue, there are other factors to also consider besides just expansion. A flue is also sized for resistance loss. With the air having 20% oxygen, the byproducts of combustion probably make up more than what is consumed. There is also pressure differential to consider.

I looked for my spreadsheet with the calculations but can't find it but remember what I observed. Pressure differences were considered but not used in calculations as they appear negligible. Velocity of the air flow was measured using an electronic flow meter built for measuring stack emissions. That and pipe diameter can be used to calculate flow rate. I merely mention the size difference between OAK and flue and my calculations in case others had wondered.

Even if a wood stove flowed only 5 CFM I'd still want an OAK on it though its requirements may be small when compared to to those of other equipment within a typical house. That's really irrelevant.

Its not the impact on stove performance that concerns me as much as the potential back-drafting that might occur under heavy demands on indoor air by other devices. The air required for powered and combustion venting requirements will come from the source of least resistance whether house shell, OAK, or flue. Its smart to avoid using the last.

 

[oldspark]

fact... according to the 2003 international mechanical codes, all newly constructed fireplaces shall be equipped with provisions for outside combustion air

Fireplaces suck way more air than a woodburner so I can see the code for them.

 

[MarkinNC]

As I recall there is a screen on the bottom of my stove where the OAK hooks in and I KNOW there is one on the outside.

I calculated that my 1600 SF house has about 12,800 CF of air assuming 8 foot ceiling (a little more if you have some vaulted ceilings like I do). So if a stove consumes 10 CFM of air per minute, it would take about 10 hours to consume the air in a home. Or put another way, the air would have to seep back into the house to replace itself every 10 hours or so. Then you have the issue of if you ever open any doors.

In my opinion, the lack of scientific evidence or evidence based data is a major flaw of the article. It's all conjecture. There was also an assertion (at least it was interpretation), that because EPA certified stoves are more "efficient" they consume less air. I would submit they require they require roughly the same amount of air to combust the fuel, the difference is how much heat goes into the space it's heating, instead of up the chimney.

 

[oldspark]

This issue goes way back and not just with the wood burners, I remember reading years ago just going out side and opening the doors will replace more than enough air, I am not for or against the OAK's but its seems like it's kinda hard to get a handle on good info.

 

[begreen]

Tom Oyen has a couple good articles on this topic. Like a flue damper, the need varies depending on the installation and the state. In WA, OAKs are required for all new home installations. As fuel prices go up and people demand better construction techniques for insulation and sealing, houses get tighter. If there is insufficient make-up air, an OAK becomes necessity for some installations.

(broken link removed to http://www.chimneysweeponline.com/hooa.htm)
(broken link removed to http://www.chimneysweeponline.com/hooa3.htm)

 

[Treacherous]

In WA, OAKs are required for all new home installations.

Look like Kittitas Co has the following stance on wood stove installation in new houses.


OBTAINING A RESIDENTIAL BUILDING PERMIT
(broken link removed to http://www.co.kittitas.wa.us/cds/ApplicationChecklist.pdf)

Since an OAK isn't required by Lopi does this mean OAK optional even in new construction?

 

[madison]

Are there stoves manufactured that have an air tight seal to the OAK? ie. Only air from the OAK supply side gets into the stove?

 

[LLigetfa]

Are there stoves manufactured that have an air tight seal to the OAK? ie. Only air from the OAK supply side gets into the stove?

I put a positive shutoff on my OAK so that it doesn't draw constantly when the stove is not used. I can snuff out the fire if I close it. The zipper (doghouse) air draws from the room but I put a shutoff on it too.

 

[begreen]

Are there stoves manufactured that have an air tight seal to the OAK? ie. Only air from the OAK supply side gets into the stove?

It depends on the stove. The Jotul Castine for example is a direct connect. Cut off the oak and you cut off the fire. PE is the opposite in that the air just spills into the bottom of the stove. If you cut off the OAK air, the fire is going to still burn, though perhaps not as robustly. But it's not air-tight.

 

[begreen]

In WA, OAKs are required for all new home installations.

Look like Kittitas Co has the following stance on wood stove installation in new houses.


OBTAINING A RESIDENTIAL BUILDING PERMIT
(broken link removed to http://www.co.kittitas.wa.us/cds/ApplicationChecklist.pdf)

Since an OAK isn't required by Lopi does this mean OAK optional even in new construction?

In this case, it's the building code that requires the OAK for a woodstove in new construction, not the stove maker. That's why when you go up to a site like Northern Tool you'll see the caveat: Not available in CA and WA. Not for mobile home installation.

 

[LLigetfa]

Judging by the writings of a few posters, I think some people may have the impression the OAK is to prevent all the oxygen in the home being consumed and people suffocating. Now an open air appliance could consume all available oxygen and leave behind all the other elements, just like a person locked inside an airtight box would suffocate. A stove will not extract just the oxygen from the home and unless it has a leaky smoke pipe, will not put what it didn't consume from the air back into the room. The stove acts more like a weak central vac simply to suck air out of the home raising the neutral pressure plane (NPP).

It's when the NPP gets raised that other passively vented appliances like a furnace or water heater that does not have dedicated outside air, has the petential to reverse their flue and draw CO into the home. Likewise, a bathroom fan or clothes dryer could do the same thing.

When there are multiple appliances passively vented (including the wood stove), there will be winners and losers WRT flue reversal.