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Outside air for woodstove?

 
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Recently found out that the main reason for the constant flow of cold air in this ancient house is not really the fault of partially crappy windows/doors but in reality the wood stove which takes the air out of the room, thus lowering the pressure and this missing air has to be replaced. Now there are concepts to draw outside air directly to the stove. Which sounds reasonable. Any thoughts/experience to share?
 
Posts: 600
Location: Michigan
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Yes, if you can do it, i would. Hands down, unless you are purposely using its negative pressure for air exchange during the heating season.

I installed our first one in a panel that was secured in the window, a vertical 1x6 with a 4" hole. From there 4" galvanized duct to an adaptor i fit to the ash drawer opening. Removing the cleanout plug and installing a sheet metal baffle and louver to direct air from the center of the firebox floor toward the back yields a blast forge and fire starting couldnt be easier now.

2 large logs side by side a small pile of kindling, progressively larger sticks to wrist size branches, a log quarter or half on top. Blowtorch 10 seconds, close the door and fuggedabouddit!

Our air intake looks like a hot rod exhaust and travels from lower front center of the pedastal to the base along the block riser it sits on and makes a 90 up through the roofing from behind the stove, since it is now located in the center of the main living area.

I was cutting wood in february with a bow saw and man pulled sleds..... we track the temps inside and out and i will testify to a significant improvement in capacity (blast forge?) and fuel consumption was quite noticibly reduced.

Some factors, our house has block walls, and i like to burn clean and clear. This puts any damper control in the intake, keeping negative pressure in the heater, instead of backing up the vent. But we usually run it wide open and deliver heat over a long time to load the mass, in other words, the structure will soak it up and i let a fire burn downt to just the right coal pile before adding wood in order to control consumption or add one or two at a time depending, you know how it goes.

We never smell like smoke, nor does my house when im gone away a few days then come home to the lady diligently feeding the stove, and would have a fresh nose to notice. The amount of air passing through the duct is impressive and it will have such a low temp its scary, like a reverse burn sometimes.... -10F and lower! It is preheated largely on entering the plenum and has channels for air diversion to an air curtain behind its ceramic glass window. That was air coming into our house, one for one going out!!

There is much improved comfort and efficiency. The number seems to be around 20 or 30 percent depending on your envelope.

Backwoods home has an old article 'better wood heating' or better heating with wood'. The stuff in there is a gift and have confirmed in my applications his advice to be sound and an improvement for most or a starter into wood heat for many. It has helped us and others inpassed it to for a good many years.
 
pollinator
Posts: 1099
Location: Victoria BC
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How big is your stove and chimney pipe, Frank?

I've been puzzling over sizing for a cold air intake; would it be correct to say you feel that a smaller intake would be too small for your setup?
 
frank li
Posts: 600
Location: Michigan
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The 4 inch intake has worked great. All the respiration it needs. The intake could easily be a 3" the length to outside and amount of elbow figure into knowing and designing if it is to be a technical excersize, but i just went with available locally available materials and instinct.

The backwoods home articles author uses a chimney size intake. This is usually a balance rule for ventilation without the negative impulse a drafting stove has.

My understanding is also that the logs burning release expanding gasses, much larger than the logs along with expanding the intake air by drastic temperature change. That and the adaptor i made to introduce the outdoor air to the air curtain intake and ash drawer compartment has some leakage, about 1/16th" gap in some places.

It need not be dead airtight, but its technically better.

Not sure about cubic feet. Its 18" wide, 22" deep and probably 22" tall rough. There is fire brick, so i would have to measure inside. A small pedestal type. New England Stove Works 18- TR/50-SHW10

Has a 6 inch outlet and insulated chimney from the ceiling up. I couldnt imagine a 6" or 8".

The new high efficiency stove a friend bought and had installed has a 2-1/2" intake, maybe and 6 inch vent/chimney.
 
Mike Homest
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Hi Frank!

thanks for the thoughts. Dunno for sure what block walls means, perhaps those concrete blocks you fill with concrete?

The wall behind the stove will be a problem to get to the outside, unfortunately it is at this point about 1m thick! All natural stones, guess those were just flying around when someone build the house a couple of hundred years ago.  At least it has quite some mass and stays cool in the summer, without A/C.
 
frank li
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Location: Michigan
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Probably easier to go straight up. If you enter the firebox low, it wont become a chimney!

Our house only has stone where we made repairs, old woodstove vent was in the wall, some perches for beams and wher a couple partitions were broken, we repaired it with field stone.

It is hollow cinder block, no concrete fill.

Our parents have a house that is one big 8" thick pour, like an above ground basement with the walls sitting on a 5' deep footing. Awesome. Let it storm like crazy, you cant feel it or hear it. 54" wide doors allow large items in and out with ease and all the doors are commercial steel and their frames are cast right into the walls.

The only thing i would do different is to have 4"concrete, 4" foam, 4" concrete, with steel commercial shopfront type rool down shutters concealed in the awnings or overhangs!

I could never afford it, even a 28' x 36' but i live in it in my daydreams.

A meter thick, now thats a proper wall system for the baron's Keep!
 
Mike Homest
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Walls are not all that thick though a few parts about 1.2 m! Just natural stones, no cement or alike, just some mud between. I'd really like to know how long it took to built?

It seems one can assume roughly 12.5 m³ Air are needed to burn 1 kg of wood! There was something in the stove manual it would burn about 2.6 kg per hour. Meaning 32.5 m³ pretty cold air are sucked in from the outside each hour! We had this winter a minimum of about -12C, which is unusual cold. That we are alive proves that even with this plague the stove works quite well.
 
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Location: kelowna bc. zone 6 in the hills
wood heat
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I added an outside air source to my Blaze King Princess Cat stove this year. The intake at the back of the stove was 4" x 7"...the intake kit (which I didn't buy....made my own) is for a 4" round duct  which reduces the intake cross section by over half . I notice I can damp down the stove lower giving me longer burn times. I am curious....anybody know the burn physics here?
 
Mike Homest
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kelly purdue wrote:I added an outside air source to my Blaze King Princess Cat stove this year. The intake at the back of the stove was 4" x 7"...the intake kit (which I didn't buy....made my own) is for a 4" round duct  which reduces the intake cross section by over half . I notice I can damp down the stove lower giving me longer burn times. I am curious....anybody know the burn physics here?



Unsure, it seems the stove can much better suck in the air from outside through a tube directly then from the house. Also cooler air contains more oxygen, which is what you want to burn.
 
Posts: 499
Location: Rural Unincorporated Los Angeles County Zone 10b
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With 8 inch thick walls and draft free windows our house is pretty air tight so I made an outside air kit from plumbing scraps to draw air up from our basement which is passively vented to the outside. It supplies secondary combustion air which is always open. The other two air controls are on the front door and draw air from inside.



Even though it's just plastic it doesn't get hot because it's constantly being cooled by incoming air.
 
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