wood stove, fresh air intake size?

[Raspy]

No, it is absolutely true. If you open a window on the 2nd floor of my house, the air will go out and if you open one in the basement air will come in. The pressure outside is the same both places so the pressure must be higher upstairs. Buoyancy is based on differential pressure, not a force in itself. Less dense material rises because the difference in density causes a pressure differential.

Kenny,

Think about it a bit further. The air rushes out because it is warmer and therefore less dense. The pressure is the same. Refer back to my oil floating on water example. Pressure is the same, density is different. Ping pong balls float on water. Density is less. Iron sinks in water. Density is greater. Hot air rises in cold air. Density is lower. Warmer water rises in the ocean. Density is lower. Ice is less dense and floats. Helium ballons are less dense than the air around them and they rise. Hot air ballons rise becasue of density difference. All examples of density difference. Heat is simply a measure of the average molecular acticity. When the molecules have more energy, warmer, they cannot pack as closely as cooler ones. So the same number of molecules occupy a larger volume, at any given pressure, than the same number of colder ones. The warmer group will migrate up, or away from the gravitational source, to be replaced by denser packed groups.

Inside a static volume of a reasonable size, the air pressure is the same everywhere with only a minute difference near the bottom, compared to the top. On a bigger scale, the atmosphere has higher pressure near the Earth than at high altitude, just as the ocean has higher pressure at lower depths, but that is a static situation based on mass in response to gravity. Same thing as a stack of bricks. The lower ones experience more weight from the upper ones. This has nothing to do with rising hot air.

Hot air rises, not because of pressure, but because of density, mass per volume, in response to gravity. There is less mass in warmer air, per volume. It's oil floating to the top of water. Two fluids with different weights. Different mass per volume.

Your house works just like a fireplace chimney. Air is heated in the stove and rises through the vent pipe to escape out the top because it weighs less per volume that the room air. Warm air accumulates in the upper floor area and escapes out an open window because the warm room air is lighter than the outside air. It tries to rise to the top and will continue to rise outside until it equalizes in temperature with the surrounding air. All density difference driven activity. Imagine your house sitting completely under water and completely full of oil. Open upper and lower windows. What happens? The oil floats out and up to the surface. It is displaced by water entering the lower window until the house is full of water. Not a pressure difference, a density difference.

 

[Egon]

If'n your stove is not designed for forced air feed be advised it may get real hot!

 

[JRobyn]

We have a Charmaster "Chalet" wood furnace in the basement that I believe is rated as 140,000 BTU. It's their smaller furnace, intended for up to 2,000 sqft. We have more than that and a ginormous great room, so it mostly provides just supplemental heat to our heat pumps and a few baseboards under large glass areas. When it's got a good load in, it draws an almost unbelievable amount of combustion air and similar flue volumes, at least until we regulate it down.

When we were building, I sorta figured that the double garage doors would provide PLENTY of leakage for combustion. The guys at Charmaster convinced me to add a 4" duct (flexible dryer vent) through the sill plate and into the space underneath our front porch. So far it's been bug/vermin free. The Charmaster guys also gave me a great tip - make a bump up in the hose from the floor and then weight the open end down in a 5gal bucket. Put the bucket as close as possible to your combustion air intake. Natural convection through the loop/bucket will tend to "seal" any excess outside air.

 

[KennyG]

Well Raspy, we're getting kind of technical here, but there is a pressure difference. Think about this: Why is there atmospheric pressure? Because air has weight and gravity acts on a column of air. When the air is warmer, it is less dense, the column weighs less it will try to rise. The upward movement definitely increases the pressure at the upper part of the house relative to the basement. This is the stack effect that drives a chimney and it works for the house also, just less magnitude because of the lesser temperature differentials. If you investigate stack effect on houses, you will find that the pressure increases at an average rate of about 0.0006 psi per story. Not a lot but enough to generate some pretty noticeable flows into the basement. You don't notice the outflows upstairs so much because you can't feel the outdraft of warmer aire like you can the indraft of cold air. We talk about the need for venting (like JRobyn mentioned) to make up for combustion air, bathroom exhaust etc. but even a well insulated all electric house will have negative pressure in the basement.

 

[Egon]

Stack Air Flow:

[video]https://en.m.wikipedia.org/wiki/Stack_effect[/video]