Air intake to wood stove

[rockinmywaypa]

I work nights and have a lot of time to think as I work and last night I got to thinking about putting in a line to supply fresh air to the woodstove. I am going to build a porch next year anyway and thought that while I have it dug out I'd put in some underground lines to warm up the air before it enters the house. So,air enters inlet, runs underground,warms up and enters house. My question is how many feet should I run minimum to have an increase from say 20 below to earth temp before it enters house. Will there be a problem after awhile with stagnant smells from the line. I know all new installations nowadays have a dedicated inlet but I haven't seen one during the installation process.

 

[s219]

Hmm, if you're talking about an outside air kit, I think the stove will be more efficient the colder the air is. But don't quote me on that, it's just something I remember hearing once. I think the greater the temperature difference between intake and chimney, the better the efficiency.

 

[Beltzington]

Don't have an answer to your question but I did use an outside air kit to a woodstove located in our basement. Used 4" galvanized pipe and the vertical and horizontal length was about 14'. Never had any issues with it for the 10-years we used it. From my understanding the purpose of an outside air kit was to prevent sending heated air up the chimney. Since this effectively creates a closed system I am not sure your gain will be worth the effort. The setup I had would run you out of the basement and you had to keep a close eye on the flue temperature or you ran the chance of a chimney fire. If I were to do anything differently I would have wanted some easy way to close the outside air inlet when the stove was not being used. Good Luck with your project

 

[rockinmywaypa]

What got me to thinking about this was that I am effectively replacing all the air that goes up the chimney with cold outside air coming in through any cracks around doors and windows and especially around the area at the top of the basement wall. Insulating is one thing but I still need to get air into the house to replace the draft in the chimney. I thought that air as warm as the ground around the house would take less energy to raise than the colder surface air. I hadn't planned to plumb it directly into the stove, just into close proximity so that hopefully the rest of the house may stay at a more static pressure and draw less air through the walls. I got to worrying that it may create an environment that could smell and wondered about others experience if it's been done before.

 

[94BULLITT]

Here is some info on a outside air kit.

Sweep's Library - Why burn outside air?

 

[crazyal]

What I did was use 4" PVC pipe and then built a box around it with the pipe centered inside. I then used spray foam to fill around the pipe, maybe 2" or more on each side. The stove is up against an outside wall so the pipe isn't very long. I used a clothes drier vent after replacing the door with a screen to keep stuff out of the pipe. It's worked fine so far and when the stove doesn't have a fire in it I can't feel any coldness from the pipe.

 

[Baby Grand]

Cold air is dense air. A cubic foot of dense air has more oxygen in it than a warmer, less dense cubic foot of air.
So, for a given CFM flow rate, the cold dense air will give you better combustion.
If CFM flow rate is limited by inlet size or flue velocity, then this would help make more heat.
Now you guys have me thinking about my own set up, as I do seem to have some smoke getting into the room.
I think I need an outside air feed to my downstairs stove so I'm not pulling a vacuum on the house or having to leave a window open.

 

[KennyG]

Warming up the air is technically an advantage, since less of the heat of the fire is required to warm up the air to combustion temperature. However the difference is minimal. My house stove (basement) has an inlet in the wall above grade and then the 3 inch pipe comes down the wall to the basement floor. In my pole barn, I've got a penetration through the metal liner panel ceiling and a 2 foot long standpipe into the attic area.

Per Grand's question, the outside air intake helps with draft. I have serious negative pressure in my basement (that I'm still working on) but the stove with outside air intake works perfectly. The only catch is that I have to be very careful and slowly open the door to refuel it to allow the pressure to equalize or I get a puff of smoke back into the room. To use an outside air intake, you have to have a "airtight" stove. Stoves that are approved for mobile homes have the capability to use outside aire intake.

 

[dfkrug]

Sweep's Library - Why burn outside air?

That's a good link, Roger.

My house is very tight, so I had to install an outside combustion air intake.
It was not too hard to install when I was building the house, but adding it
later can be a bear. Mine goes under the slab and up an outside wall to a
screened intake. All modern wood stoves seem to offer a "mobile home" kit to
bring in outside air.

It works well, but starting a fire requires leaving the door to the stove
open until the draft is strong enough.

 

[94BULLITT]

That's a good link, Roger.

My house is very tight, so I had to install an outside combustion air intake.
It was not too hard to install when I was building the house, but adding it
later can be a bear. Mine goes under the slab and up an outside wall to a
screened intake. All modern wood stoves seem to offer a "mobile home" kit to
bring in outside air.

It works well, but starting a fire requires leaving the door to the stove
open until the draft is strong enough.

Thanks, there is tons of info on that site. When it is real cold outside and there is a down draft I light a bunch of newspaper to get some heat going up the chimney to get a draft going. I have also heard of people using sterno cans.