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Thread: Woodstove Outside Air Intake -- Through Wall or Floor

  1. #1
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    Default Woodstove Outside Air Intake -- Through Wall or Floor

    All,

    I have the option of piping an outside air intake to my woodstove either directly through the floor below (run length to get below 2x10" joists w/ fiberglass batt insulation, cabin is ~4' off the ground) or through the wall it is sitting next to (run length of ~2' inside and through 2"x6" w/ fiberglass batt insulation). Both runs would essentially be straight.
    ?
    But is there a distinct advantage to one route vs. the other--going through a wall vs. directly below to the floor. Going through the floor is straight forward and would be a straight run... but would require more work (cutting tile/cement board/etc.). Going through the wall would be a slightly longer run (part of which would come from having to use a couple 90* bends), but would be easier. So is there a engineering advantage (lets put the work required to install aside)?

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by awwshucks View Post
    All,

    I have the option of piping an outside air intake to my woodstove either directly through the floor below (run length to get below 2x10" joists w/ fiberglass batt insulation, cabin is ~4' off the ground) or through the wall it is sitting next to (run length of ~2' inside and through 2"x6" w/ fiberglass batt insulation). Both runs would essentially be straight.
    ?
    But is there a distinct advantage to one route vs. the other--going through a wall vs. directly below to the floor. Going through the floor is straight forward and would be a straight run... but would require more work (cutting tile/cement board/etc.). Going through the wall would be a slightly longer run (part of which would come from having to use a couple 90* bends), but would be easier. So is there a engineering advantage (lets put the work required to install aside)?
    If it were me, I would definitely go straight down. Sucking cold air in winter creates condensation on your intake, and the associated dripping. Going straight down through the tile hearth will allow you to caulk a nice clean air/watertight seal at the floor around your intake duct... No water damage to your sheet rock wall from condensation; no issues with penetrating the vapor barrier in your wall; no horizontal duct run to trip over or damage by dropping something on it.... 90* out of the stove and straight down. Count yourself fortunate you have that option, rent the appropriate hole cutter, and do a nice clean job of penetrating the tile. In the end, you'll be glad you went that route.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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  3. #3
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    Cold air falls. Down through the floor! Do Like iofthetaiga says.
    When seconds count, the cops are just minutes away.
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    The outside air intake was designed for modular homes not stick frame homes. The reason for an outside air intake was to make up combustion air for the stove so it would not cause a neg pressure inside the home. It is impossible for your home to be so tight that this problem could happen with vent hoods and fart fans that are in homes today.
    The result of a neg pressure in the home caused from the use of a wood stove was it would cause a back draft or puff of the chimney flue during the burning process.

    The next problem you run into with installing outside air: Hot metal and cold air tends to make the steel around the intake crack over time. simple law of physic hot and cold expansion and contraction.

    Just a little of my back ground before I moved to AK was I had a chimney cleaning and woodstove installation business in CO for over 15 years.

    If you must install the outside air go into the crawl space and hopefully it is insulated so the air is not so cold. If the stove is on a pedestal then all you have to do is cut a rectangle hole through your hearth and floor straight down. If you want to be fancy then take a floor register for heating duck and mount that through the floor to the crawler.

    If the stove is on legs then you have to run the galvanized duck from the stove through the floor and then paint black.

    I hope this helps and good luck

    Sweepint
    Wasilla, (when not overseas)
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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by sweepint View Post
    The reason for an outside air intake was to make up combustion air for the stove so it would not cause a neg pressure inside the home. It is impossible for your home to be so tight that this problem could happen with vent hoods and fart fans that are in homes today.
    I beg to differ. This is simply not true. The fact that very few contractors take the time or care enough to built a tight house, does NOT make it impossible! The last two homes I've built are so tight that a stove WILL NOT burn without direct feed air, unless a door or window is open in the home. If the fresh air intake to the house is closed, the standard issue GE kitchen exhaust fan will create negative pressure in the house strong enough that care must be taken when opening a door, as it will leap inward and pop you in the nose if you're not careful. In our climate, houses should be built tight and airflow into and out of the envelope should be controlled, not willy-nilly through random cracks and electrical receptacles (or backwards down your chimney and through the woodstove). Not only is it possible to build a house this tight, it's not even terribly difficult; exceeding the five-star fan door test used to certify AK homes is child's play. Nonetheless, even with a less than perfect envelope, providing make up air to a stove will dramatically decrease the amount of air being sucked through walls in undesirable places, such as water line chases.... In our arctic/sub-arctic climate little details like that matter.
    ...he who knows nothing is nearer to truth than he whose mind is filled with falsehoods & errors. ~Thomas Jefferson
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    Well Sir
    Then you should put a combustion air kit in the last two home you built. The other 99% of the home won't need it. I am not disagreeing with you on the need for a tight closed house but a house needs to breath as well.

    The point of the discussion was putting in a combustion air kit in the house. The point I was making was in 99% of the home don't need it and it will inherently damage the stove in time just from hot and cold. The same process happened in the older gas furnaces on the heat exchanger that caused them to crack as well. If you can preheat the air like they have done with direct vent systems that would be much easier on the stove over all.

    Sweepint
    Wasilla, (when not overseas)
    '' Livn' The Dream ''
    26' Hewescraft Cuddy, twin 115 Yam

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    Thanks all. The cabin I am building is a weekend warrior cabin and the woodstove is a BK Princess. There are two reasons that I'd like to install the fresh air kit--and both have to do with getting the cabin warmer and more comfortable for the wife and kids who don't appreciate the cold as much as I do. First, offset neighbors with BKs have reported they have been able to warm up the cabin noticeably faster with a fresh air lot. And second is to reduce the cold air draft.

    With that said I will look at routing through the floor. Between condensation and trapping the cold air, sounds like the way to go.

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    There's another good reason to go down through the floor: if you go through a wall then air will come IN most of the time. But if the wind is pushing against the opposite wall then there will be a negative pressure on the wall with the vent, and the air will go OUT. This could cause a back-draft in the chimney or, at the very least, cause your stove to burn poorly.

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    Quote Originally Posted by NunavutPA-12 View Post
    There's another good reason to go down through the floor: if you go through a wall then air will come IN most of the time. But if the wind is pushing against the opposite wall then there will be a negative pressure on the wall with the vent, and the air will go OUT. This could cause a back-draft in the chimney or, at the very least, cause your stove to burn poorly.
    Good point. Exactly why I posted the question to a larger audience!

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    I did a floor vent. Insulated through to crawl space.
    A bonus is having the coffee pot on a hot stove keeping warm and the 1/2&1/2 on top of the vent staying cold by the outside air.

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