cubic foot of heated area versus outside air kit

[Shari]

I went to our city inspection office and learned our plans will work the way we want - but I learned something about OAK's (outside air kits):

I was dickering back and forth with the inspector saying we won't know if we need an OAK until the stove is installed versus his statement we had to install the OAK before he would sign off on installation. It seems by our city code there is a certain cubic foot requirement to make sure the stove installed is not burning 'inside' air for combustion creating a possible carbon dioxide problem. When I provided approximate room sizes and he compared the btu's of the Oslo to our home's cubic feet in just the living room, dining room & kitchen and determined we are not required to install an OAK. We were warned however not to install any inside doors to our living room, dining room & kitchen floor plan as any doors would affect cubic foot requirements for room air required by the stove.

Can other knowledgeable persons chime in on this discussion so other wanna-be wood burners could possibly tell in advance if they will need an OAK? I don't know the fire codes our inspector was referring to. (Sure hope someone knows what I'm talking about otherwise I'm sitting in a room talking to myself waiting for my Oslo to come home! )

EDIT: I think the figures he came up with were if the stove put out 100,000 BTU's then we would need 5,100 cubic foot - but my recollection of the figures might be wrong. I know the Oslo does not put out 100,000 BTU's - that was just an example he used.

Shari

 

[John_M]

Shari, Some municipalities are now requiring that all new wood stove installations must have an outside an OAK. Let me speculate on your local requirements: When you close the doors to the living room where Ozzie is located the room is now reduced in size by all the rooms you closed off. Ozzie still need just as much oxygen to breath so now he must get that oxygen from about 1/2 the cubic footage as the original rooms with open doors. This could result in less oxygen and negative pressure in that room, requiring that any extra needed oxygen be sucked in through electric outlets, leaky windows, kitchen exhaust fan, etc.

Perhaps your local regulation is intended to reduce health hazards caused by less than perfect inside air. I am not certain that regulation does that, but it is probably the intention.

My stove has a properly functioning OAK and I have what the doctors describe as "dirty" lungs. I am easily reduced to coughing, wheezing, asthma, and other respiratory discomforts during times of less than perfect indoor air. To further improve the condition of my inside air, I open a window next to the wood stove. The top of the window is lowered about 3/8" during the time I am burning. I feel no cold drafts from this opening because the cold air is immediately heated by the stove. So far this winter I have had no respiratory distress. I attribute this success to the OAK and the slightly open window near the stove.

Just my $.02.

Best,

John_M

 

[Shari]

John, Thanks for contributing to my (vaguely) worded post. Yes, you are correct the municipality I live in is looking closer at indoor air. The inspectors comment was 'we are tightening up our requirements and looking at carbon dioxide issues relating to wood stove installations'.

I've read here when people install their stoves they find out at a later date they need an OAK. The point of my initial post was to hope that one of the stove seller's on the list, or one of the many fireman on the list, could post the 'formula' for needed interior cubic foot compared to stove BTU requirements. I didn't catch the formula our inspector used and thought that formula would be great info for future stove installations.

Does anyone know what this formula is? Craig? Todd? BeGreen? FirefighterJake?

Shari

 

[Battenkiller]

I don't see how anyone can determine intake air based on BTU output in a wood stove. Oil and gas burners typically need from 20-40% excess air delivered to achieve complete combustion. The service tech has all kinds of gadgets and meters to check your installation in those devices. Wood stoves OTOH are notorious for needing even more excess air to achieve combustion. This is necessary because air cannot be delivered to the fuel in a uniform and predictable manner like in a oil or gas burner, or even in a pellet stove. There are many reasons why this is so, but they all point up to the fact that wood/wood gases that are not meeting oxygen molecules inside the burn zone are going up the chimney as unburnt fuel. Excess air is always needed beyond what is theoretically used in combustion.

Each stove/operator/fuel combination will has different requirements, and those requirements will vary throughout the burn cycle. Loading the stove with small loosely packed splits will require lots of excess air, not only because the fire will be burning faster, but because there will be numerous points of ignition throughout the box as volatile gases are evolving from the wood all over the place. This is a very chaotic situation compared to the slow and even combustion of a bed of coals. It is necessary that oxygen and sufficient ignition temperatures are present at the same time and place in the firebox or these gases will not ignite.

I'm sure "Ozzie" will do this better than my old Vigilant because of it's superior secondary burn characteristics, but you will still need much more excess combustion air than a gas burner on a BTU basis. How much? Maybe Jotul has the answer. In any event, make up air in the home can be supplied in dozens of ways besides a direct connection to the stove air intake.

I find it interesting that in Canada, outside air supplies were once mandatory for stoves and fireplaces, but that requirement has been reversed based on research done on the subject. According to studies, OAKs are not only ineffective in preventing air spillage from wood stoves, they could potentially be hazardous in certain windy conditions.

 

[Todd]

I never heard of this requirement around here. I found this, I think it's from WA state, maybe it will help you figure some numbers.


http://www.houseofcraig.net/combustion_air_calc.html

 

[Corey]

I don't know of any specific 'formula' but we've done some 'ballpark' figures before. As a general rule burning gaseous hydrocarbons, you need about 15:1 air/fuel ratio (just like in your car) for good combustion. Most stove operate a little 'lean' - ie more air than needed, so probably closer to 20:1 air fuel ratio. If you break that down into an equation:

(BTU per hr stove output) / (~8600 btu per lb wood) = stove wood consumption in pounds per hr

(stove wood consumption lb/hr) x (20 air:fuel ratio) = lbs/hr air required for combustion

(lbs/hr air required for combustion) / (0.075 lbs per cubic foot) = air cubic feet per hr for combustion


So, given 100,000 btu/hr stove:
=======================

(100,000 btu/hr) / (8600 btu/lb) = 11.6 pounds wood per hour

(11.6 lbs wood/hr) x (20 lbs air per lb wood) = 233 lbs air/hr

(233 lb air/hr) / (0.075 lbs/cf) = 3100 cubic feet/hr

(or roughly all the air in a 20x20 foot room with 8 foot ceilings - every hour)


You mentioned 5100 cfh, this calculates 3100 cfh, but clearly, there is room for 'fudging' in most of these factors...btu/lb of wood, air:fuel ratio, and I don't know if there would be a factor for the stove efficiency as well. The above is assuming you burned 100k btu of wood which would probably be 70k btu to the house in a 70% efficient stove. If nothing else, just throw in a 40% 'safety factor' and there is your 5100 cfh!

As for the cubic feet of the space the stove can draw from, I don't know why that would explicitly make a difference other than the fact more cubic feet of space 'in general' should have more exterior windows, doors, etc and more chance for outside air leakage. But if you had one super tight, insulated 'great room' with 20' vaulted ceilings and a big floor, you could probably meet the CF requirements and not have much outside air infiltration. Likewise a spread out ranch house with 8' ceilings may fall short on the CF, but if every wall is an exterior surface with a somewhat leaky window/door you'd have plenty of outside air coming in.

 

[jcims]

(or roughly all the air in a 20x20 foot room with 8 foot ceilings - every hour)

That's some nice stubby pencil work there. Now i can tell my wife that I'm as light as the air in our living room.

 

[Corey]

Heh - weight may be the same, but it is apparently the density which counts, at least that is what my wife always says. At least she always talks about me being dense!