Fresh Air Intake for a Woodstove or Wood Heater w/ Round Intake

-

Lots of people don't realize it, but when you use a heating appliance that is vented to the outside, every cubic foot of air that goes up and out the stack has to be replaced in the room. With the regular old fireplaces and pot belly stoves we know and love, that replacement air is going to be many, many cubic feet per minute and is going to be in the form of cold air seeping in through cracks, under doors, around windows... any way it can get in. Otherwise there would be a vacuum formed in the house, your ears would pop, the canary would die and eventually your house would implode.

Well, okay, I got a little carried away... actually, if your house is that tightly sealed, your wood burning device would burn slowly and it would be difficult to get a good draft going up the chimney. A lousy draft = a smoky wood burner or fireplace with a lot of the smoke ending up indoors.

My old house is nowhere close to being tightly sealed. When I had a good fire going in the wood burner, I could put the back of my hand up to the crack in the front door jamb and feel the cold air being pulled in. A lot of the work of the heater was going toward heating that cold air! I finally decided to do something about it. But what? The air intake on the front of my stove was a round design with a built-in adjustable damper and it swung open with the door when it was opened to add wood.

Step 1 Adapter

-

After tossing around many, many ideas that eventually got dismissed because of complexity or cost or being a pain to remove every time I wanted to add wood, I thunk this one up. It went together so easily that I was kicking myself for not thinking of it sooner.

While replacing old rusty flue pipe, I noticed that a 6" 90 degree elbow perfectly fit over the round air intake on this stove. All it needed to secure it was a small angle bracket, pop rivited to the elbow and held to the stove door with sheet metal screws.

I had just bought some dryer vent hose for another purpose, and my quick-as-a-snail brain put the two together.


Yes, I've got rocks on my stove. They serve as a small thermal mass that slowly releases heat into the room after the fire has gone out. The smooth rock on top of the granite block is my bedtime foot warmer on really cold nights. Behind the chunk of granite is a pile of aluminum ingots from my backyard foundry. Sometimes the granite gets replaced by a big pot of lima beans or corn on the cob!

On top is a small amethyst "cathedral" that I put there just because I think it looks purdy.

Comments

[cardinaler]

"every cubic foot of air that goes up and out the stack has to be replaced in the room."
Please correct me if I am wrong, but I do believe a lot of the exhaust is coming from the wood being converted to a gas. As an example, the average weight of air is 0.0807 lbs. per cubic foot. So a 10 pound log when burned will produce 123.92 cubic feet of exhaust gasses. More if you consider that hot air takes up more space than room temperature air. The wood doesn't just disappear. I think the amount of mass that is lost is almost immeasurable.

[Rainh2o]

I have seen people put a coil of copper pipe on the stove, then run it over to a radiator with a pump and a fan on the radiator....warms 2 places at once

[lemonie]

Do you know how much heat you're losing out of the flue? I have a vague idea that it would be good to use the flue gas to pre-heat your air intake, via some kind of heat exchanger, but not much more than that. ? L

[vincent7520]

I think my father's experience somewhat answers to your question (see my recent post).

Have a good week

[lemonie]

Thanks

L

[Davetech]

"Do you know how much heat you're losing out of the flue?" Yes, a lot. I usually point a fan at the flue to rob some of the heat before it makes its exit. I have thought about putting fins on the flue. But I have friend that owns a tree service and he dumped a huge dump truck of oak and pecan on the back of my lot so the wood is practically free. Just a bit of labor to cut it up and split it.

[lemonie]

Thanks for the reply - I can see some of the flue, I know you can get a lot of heat out of them. Or on the case of one I know a shed full of smoke... L

[vincent7520]

I love your stove and your big granite rock ! …
Here is another contraption my father made a long time ago. It worked perfectly (cannot show any pictures or drawings as he passed away over 10 years ago).
Here is the idea : a small low rev fan forces fresh from the outside of the house into a pipe that runs through the stove WITH NO CONNECTION WITH THE FIRE (ie. at this point fresh air does not feed oxygen to the fire) then the pipe runs along a wall and ends where the room is coldest. So the outtake lets in FRESH WARM AIR.
And the process of feeding oxygen to the fire goes on as usual. But the result is quite different :
1) Incoming fresh air is quite warm, and this heats the room as well ;
2) the room is slightly over pressured by this incoming hot air forced in by the fan and this prevents small cracks in doors, around windows, small holes letting fresh but COLD air seep in. In fact I remember having tested the room for leaks and see a piece of cigarette paper being slightly sucked out !… (a teen ager never believe his pa ! …).
This has worked for more than 40 years and was still in operation when we had to sell the house.
If this experience can help I'll be glad !…

[uberdum05]

You should put a T in the flue pipe and pipe back in some of the heated flue gas back into the fire to make it even warmer so then you can always add fresh cold air as well through another T in the cold air supply line

[glorybe]

             I can see why comfort might increase quite a bit but not efficiency. After all you are heating cold air from outside the home with your firewood. But as far as stopping cold drafts which might cause many people to use wood I don't see an efficiency gain from a mechanical perspective. I know this debate has gone on for many years.
             I do think that a carbon monoxide detector is vital for any stove burning fuel. If snow,ice or a fallen branch blocked an air intake or even a possum trying to keep warm it could create a deadly condition inside the home.

[bo88y]

---Cold air is denser than warm air, and contributes more O2 to the combustion than an equal volume of warm air.
---The chemical reaction of the burning releases far more heat than what's required to heat  the air an extra 30-60 degreesF which is relatively trivial. And likely, the denser cold air should compensate for some of the added heating.
---The main reason for having a dedicated air-inlet is to control WHERE the combustion air comes into the house. Without it, the combustion air comes from the heated interior, and this creates a negative pressure throughout the whole house, drawing in cold drafts wherever they can get in. This means that little trickles of cold air have to come in from all the leaky windows all over the house, cooling the air throughout the house. In a tight house, the lower infiltration rate can interfere with the chimney draft, making it tough to get a good fire going. So either  way, this kind of system helps. This last factor makes the biggest difference in making this kind of system more efficient.

[resago]

very smart. Just add a box fan blowing on the stove to increase further heat transfer.

[1flyby1]

On my exhaust pipe, about 4 foot high I installed a t-fitting. I then place a 90 degree elbow facing down and enough pipe w/ another internal damper down to about 6" off the floor. The damper is just a little ways down from the elbow. Leave the new damper closed. Once your fire is hot and burning well if you adjust your new damper right it will start pulling cold air from the floor instead of heat out of your stove. You will feel the heat difference almost immediately. It will take a little while to find the right setting on the damper not to open not to closed. Also close this damper before you open the stove or the draft stops and you'll get a face full of smoke.

[budmeisner]

You can also add a stack robber on the flue pipe- this helps capture lost heat....