Outside Air Inlet location

[planner steve]

I'm building a home on a slab foundation. I will have a PE Super 27 wood stove and desire to have outside air for combustion. I believe the house will be sealed tight enough that it is a necessity. The stove will have a chimney pipe that will be about 21 feet high.
I had proposed that we run the outside air supply line down from under the stove pedestal, under the slab, through the foundation wall, and emerge outside the house.
My contractor has proposed that we run the line to the back of the stove, into the wall, then up to the attic and out the gable. The would make the pipe run 8 feet up from the entry to the stove, then sideways 25 feet to exit the house at an elevation of about 9 feet. The line would be covered with significant blown cellulose insulation as it runs through the attic.
Are there problems with this method of supply? Why not just have the supply pipe extend above the insulation in the attic and skip running it horizontally to the gable? The wood stove will not be the main source of heat, so should I add a damper to shut if off when the stove is not in use to prevent air infiltration through the make up air line?
Appreciate your thoughts.

 

[madison]

The only thoughts I have are:

if the supply line is "in the wall" and it is an exterior wall - you would have a potentially uninsulated area .... interior wall not an issue regarding the insulation.

In the slab - my thoughts maintenance would be an issue. Depending on the design of the home and attic insulation, I would draw it straight from the attic if building codes allows.

The only issue I can see potentially with your idea, the air supply when run vertically outside the evelope of the house will act like a chimney and would not draw air down to the stove....

 

[Squisher]

Your through the slab and staying below your stove height would keep it out of contact with combustibles and make it less of a potential chimney.

-if the house is de pressurized to a level greater than chimney draft, spillage will occur from any available opening in the appliance and venting system.

-oak's can reverse flow direction if the outdoor weather hood is exposed to wind induced negative pressure in excess of chimney draft, this can be a fire hazard.

 

[planner steve]

The only thoughts I have are:

if the supply line is "in the wall" and it is an exterior wall - you would have a potentially uninsulated area .... interior wall not an issue regarding the insulation.

In the slab - my thoughts maintenance would be an issue. Depending on the design of the home and attic insulation, I would draw it straight from the attic if building codes allows.

The only issue I can see potentially with your idea, the air supply when run vertically outside the evelope of the house will act like a chimney and would not draw air down to the stove....

The outside air supply line would run through an inside wall. If I draw from the attic, the end would be about 11 ft above the stove to be safely above the insulation. I had the same thought about this being a "stack" that would draw air up. I suspect during stove operation this won't be an issue other that perhaps when starting a fire. It would seem the draw of the wood stove chimney would be significantly higher and pull air in the outside air supply line. However when the stove is not in operation, I may be continuously venting some amount of air back out through the outside air line.

 

[planner steve]

Your through the slab and staying below your stove height would keep it out of contact with combustibles and make it less of a potential chimney.

-if the house is de pressurized to a level greater than chimney draft, spillage will occur from any available opening in the appliance and venting system.

-oak's can reverse flow direction if the outdoor weather hood is exposed to wind induced negative pressure in excess of chimney draft, this can be a fire hazard.

I suspect I won't achieve the air tightness of some members, but the house will have an ACH50 of <2.0. I'll have an electric clothes dryer, a low volume range vent hood, and bathroom vent fans which all could reduce the pressure in the house. I'm making provisions to add these units if indoor air quality requires fresh air make up. (broken link removed to http://www.ventilation-system.com/cat/twinfresh-standard/)

I agree if I go with the up through the wall option, having the opening on the outside wall may introduce wind effects. It would seem that drawing air from the attic might be desirable for that reason.

It would seem that going under the slab reduces these issues. The under slab option will likely cost me $500 to $1,000 more if I can convince my contractor to do it. That's just the reality of doing something out of the ordinary with contractors. So if the through the wall and into the attic option will work, I'm willing to do it.

Hopefully someone on the forum has already done it and will see this post. In the meantime, I'll try to see if there is a way to determine how much this chimney effect will actually be. I know it has to do with height of stack and temperature difference, but there may be other factors as well.

 

[Squisher]

I'm sure someone will correct me if I'm wrong, but I think it's blaze king that has specific wording of the OAK not being higher than the stove, like it would have to be in a basement install say.

In the WETT courses up here they show a video of a home in Ontario that was backdrafting through the oak while running. This was due to high winds outside. And the oak was at the same height as it entered the stove. It seems from their courses and literature wett is not really for oaks at all, yet often local authorities or provinces have mandated otherwise.

Hopefully a few of the experienced installers will pipe in with their thoughts.

 

[bcrtops]

I think it best to have the OAK inlet below the stove. In this case, I think you would be well advised to (try) & get a hold of a technician from the stove manufacturer & ask their advice on this installation. Note: that most all the schematics/drawings of OAK installations I have seen do show the inlet below the stove -- thinking there is a good reason for this.

 

[lsucet]

Till now i am not seeing any manufacture recommend the installation of an OAK higher that the bottom of the fire box. they recommend straight out thru the back wall at same high of the connector in the back or down thru crawl space that is ventilated. I know and hear about cases that people do it different like in basement installed but it is not recommended. if you have the choice just do it like it should and avoid have two chimney connect to the stove. you also can give it a try without first and see how react and go from there.

I don't think you should do the attic install at all. Just my opinion.

 

[BKVP]

If you have a day,Ishtar basement you could upsize to 4" and run it along the floor, then drop a nice decorative box over the full run. That way you can stay below the bottom of the firebox.

Going above the bottom of the firebox creates the real possibility of creating a second chimney. Under the perfect storm scenario, the two could reverse roles and that would be cataclysmic!

 

[blades]

Under the slab, done before the slab is poured should not be a grand or better for 25 bucks or less of pipe.

 

[electrathon]

I think if you go up you will effectively be fighting a chimney draft issue. Keep it low. I would run 4" PVC plastic under the slab. Direct connect to your stove.

 

[planner steve]

I think if you go up you will effectively be fighting a chimney draft issue. Keep it low. I would run 4" PVC plastic under the slab. Direct connect to your stove.

I'm going to push to do the outside air line under the slab. I see there is some debate about pipe material. PVC would seem to be best for longevity, but don't think it would be wise to connect that to the stove. I have the pedestal with the stove. Pacific Energy literature says to just have the make up air emerge under the pedestal with no need to connect to the stove. I have reservations about that being a source of air infiltration during majority of the time the stove is not operating. Not sure how to make transition from PVC to metal, but I suppose I'll figure something out.

 

[electrathon]

For the time the stove is running as long as you are very close to the stove I may agree that is good enough. It would be better though to feed direct to the stove. When the stove is off it is a real big deal. I would transition to a flex aluminum drier duct where it exits the floor. A shut off would be good, especially is it is not going to be in constant use.

 

[Attaboy]

I think science is the best advisor on the subject.

http://www.woodheat.org/the-outdoor-air-myth-exposed.html

 

[iPREFERPi]

I believe the concern for Carbon Monoxide poisoning is behind most AHJ's requirements for relief Air and combustion air.
If any combustion appliance is starved for Oxygen it will attempt to draw the oxygen (1/3 of the fire triangle) from the easiest source. In the case of Wood stoves that may be the smoke pipe/chimney. It may also be other appliances using pilot valves etc.

 

[planner steve]

I believe the concern for Carbon Monoxide poisoning is behind most AHJ's requirements for relief Air and combustion air.
If any combustion appliance is starved for Oxygen it will attempt to draw the oxygen (1/3 of the fire triangle) from the easiest source. In the case of Wood stoves that may be the smoke pipe/chimney. It may also be other appliances using pilot valves etc.

After getting everyone's thoughts and reading everything I can find, here is my take.

The problem in a tightly sealed house with a wood stove is that various devices like bathroom vent fans, range vent fans, clothes dryers can all create a situation where the pressure is reduced in the house. This can result in either reducing the draft of the wood stove, starving it of air, or in extreme cases pulling air back down the chimney. The outside air kit serves as a hole punched in the wall to prevent this from occurring. But the outside air kit is just one method to solve the problem. Crack open a window would be another. Or devices like the Condar Air Supply Ventilator.

The last sentence of the woodheat.org article sums it up for me. You have to minimize depressurization.

This for me answers the question about putting the outside air intake up in my attic. You wouldn't want to do that as the natural draft created will counteract the hole in the wall you are trying to create.

Seems the safest thing to do is put a 4 inch outside air supply under the slab and try to figure out a way to plug it when not in use.

I am going to explore the use of the Twin Fresh units I mentioned above. It appears the controls for these allow them to be put in continuous supply mode, where they each supply 30 cfm of fresh air into the house. They also operate in a cycle mode where they run in supply mode, then reverse and exhaust. Maybe I just switch them to supply mode when the stove is running and forget about outside air altogether? In theory it should work, but I like simple systems. Like a hole in the wall.