Good idea?

An outside air kit would alleviate all of the issues with the idea you have, and it wont void any UL listings.

A number of manufacturers use this principal for their appliances, Rinnai on demand water heaters and one gas fired fireplace unit I installed utilize a concentric flue/intake. the combustion air is drawn in around the flue pipe. it is a pipe inside another pipe. I believe most applications use an induced draft for this, and they obviously burn gas, not a solid fuel.

EatenByLimestone said:

It's not a bad idea, but I can imagine that there are little hurdles that would have to be overcome when doing it. A cool chimney isn't the best at stopping creosote formation. The drag of pulling combustion air the length of a chimney may make it hard to start a fire.

Matt

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I agree with Matt on this one. When pulling down the cooler outside air, the insulation space would be cooler than it should be and therefore the creosote would be the issue. However, I believe the creosote issue would only be more prevalent on the higher portions of the liner where the cooler air enters as the fresh air intake. The second issue is startup air, just as there is a draft for exhaust gasses and smoke there will be a draft for the air intake, not sure how that would work if the pipe temps are so warm that the fresh air will want to rise up and out.

Since the fire would call for fresh oxygenated air to survive, would it create it's own so tp speak 'draft'? Not sure in which direction would be stronger, being pulled into the firebox or the heated air going up.

Hmmm... interesting question and clever idea.

I'm thinking the downsides outweigh the benefits.

Of course one downside is a cooler chimney, thermally making an interior chimney more exterior, which is bad for all the usual reasons.

A consequence you may not have considered is that now the intake becomes effectively it's own chimney, of the same height as the exhaust chimney, drawing against it and reducing net draft. OAKs can do this initially, if it's outside inlet is higher than the stove (like in a basement install), but the effect vanishes once cool air starts flowing into it and cools it down. Air surrounding a flue would stay warmer than exterior air, and continue to draw against the stove while in operation.

But thanks Adabiviak for the clever idea, and sincere Kudos on innovative thinking. The rule for innovation is "Fail, fail, fail, kill!"

Adabiviak said:

Thanks for the replies everyone.

With this in mind, I have another question on a different topic. Given a woodstove with no external air supply, the air has to get through the house's "shell" somewhere to get to the stove's intake. Where's the preferred breach? Kitchen/bathroom ceiling vents (since they're not likely to have a valve to close them anyway)? Leaks around doors? Open a window?

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You're just full of questions and ideas, aren't you? Perhaps you should change your name to Curious Adabiviak.

Regarding your Q, personal pref I suppose.

One common downside of using room air is that, since air leaks in through whatever gaps it can find and moves towards the stove, distant areas tend to get cold. Perhaps cracking a window close to the stove would minimize this? It might be better to do that than have colder bedrooms/bathrooms.

Adabiviak said:

Could you use the insulation space of a double-walled stove pipe for drawing external air in to feed the stove? Benefits include using external air to drive the fire (for obvious reasons), using the "wasted" heat of the chimney to pre-heat the incoming air for better combustion, keeping the chimney line even cooler, and not requiring a second hole in your house for an external air line to the stove. Cons include requiring a specially designed chimney cap (to make sure smoky air isn't pulled in), a specially designed stove (to be able to use external air coming through this passage), and specially designed stove pipe (since this channel would need to make complete runs through whatever bends exist in one's stack). Also the potential for more creosote buildup in the chimney with this cooling action? Anyway, does this sound like a good idea?

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This has always appealed to me as the most logical way to minimize stack plus OAK heat losses. The counter-current setup helps to maximize heat exchange between the two concentric flows. This set-up almost certainly requires a well functioning catalytic afterburner to minimize excessive creosote deposition and a small fan to prevent loss of draft.

I don't think it would be necessary to modify a stove already equipped with a dedicated OAK port, since the incoming air flow could be Teed-off just above the stove and fed into that port, although the esthetics of such an arrangement could be a bit challenging from an interior design perspective.

A similar Tee-off would be necessary at the chimney to try and keep the two flows separated. However, as long as the catalytic converter works well enough to keep CO concentrations low and stove plus pipes are nice and tight a few percent of recirculated flue gas should not pose a real technical problem (though probably running afoul of some building codes).

If you draw combustion air from the same height as your chimney outlet, you have in effect created two optional paths for the smoke to go, while at the same time killed your draft. On a forced air or forced exhaust appliance, this is not a problem, because the fan moves the necessary air in the proper direction. On a wood stove with no fan moving your inlet or exhaust air the chimney must be a certain amount taller than where the air comes in. If I understand correctly, this chimney height measurement is from the bottom of the stove, or where the air inlet is. If you move your air inlet higher than the stove, then the air inlet should be considered the bottom of the chimney for height requirements.

What's to keep the two air flows from reversing directions? That would make for a really hot outer chimney.

sixman said:

What's to keep the two air flows from reversing directions? That would make for a really hot outer chimney.

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As I suggested in my previous post, the "what" can be a small, dedicated fan. Controlling flow direction and speed should also help the efficiency of the heat exchange process between the two concentric flows, thereby potentially offsetting the cost of operating the fan.

"Kitchen/bathroom ceiling vents (since they’re not likely to have a valve to close them anyway)?"

My bath and kitchen fans all have flappers or valves of one type or another to minimize the amount of air drawn into the house when they are not in use. They don't produce a perfect seal, but certainly reduce the flow.