How many CFM goes up the flue?

[kgrant]

I know it will be different between every stove and how they are burned. Anyone know of a study on how many CFM a wood stove uses?

 

[HotCoals]

Tons of it?

 

[oconnor]

I recall reading over at woodheat.org that an EPA stove vents about 30-40 CFM - less than your dryer or bathroom fan/range hood.

 

[LLigetfa]

Good enough reason to have an OAK.

 

[Alan Gage]

Great question and I look forward to the responses.

I'll be building a small house this spring and think it will be sealed tight enough that I'll need to worry about some sort of air exchanger. I've been reading up on passive vents in an exhaust only system and it sounds like it would be a good way for me to go. Basically you add a few outside vents in certain areas of the house (closets for one) and when the kitchen/bath fans are turned on they draw in fresh air through the passive vents as they exhaust. If they don't get used enough by the occupants during normal living they can be place on timers. Another option I've heard is instead of outside air for the wood stove letting it be the exhaust source by drawing inside air. This got my wondering just how many CFM they flowed.

Alan

 

[bogydave]

Depends on the stove, damper setting, draft of each flue is different.
Some times lots, sometimes not much.
If you don't have a air/heat exchanger, pulling some fresh air into the house is healthy for you. IMO

 

[WidowMaker]

A: A wood fireplace consumes a VAST amount of oxygen, and the chimney updraft simultaneously vacuums a HUGE amount of both burned and unburned air out of the house. This evacuation of air is known as the chimney flow rate, and it is measured in cubic feet per minute (cfm). A typical open fireplace with a brisk fire burning will create a flow rate of somewhere around 500 cfm out the chimney, which is enough to totally evacuate the air from a 1,000 sq.ft. house every fifteen minutes. Meanwhile, replacement air must squeeze in through tiny openings around doors, windows, etc.. If the house doesn't present enough openings to the outside atmosphere to allow static pressure stabilization, the atmosphere inside the house will remain at a lower pressure (and a lower oxygen level) than the atmosphere outside the house as long as the fire is burning. The tighter the house, the more pronounced the effect.

Frome here

http://chimneysweeponline.com/hooa.htm

 

[ajreid]

Alan Gage
Just get a couple kids that dont know how to shut the door.

 

[thechimneysweep]

WidowMaker, that quote is about open fireplaces. The range I've seen posted for controlled-combustion wood stove fires is more like 30-40 cfm.

 

[Battenkiller]

Depends on the burn rate. If you want to make a lot of heat, you need to burn more pounds of wood in a shorter time span. In order to do that, you need more combustion air. If you want to provide a slow, steady heat release, you need less combustion air. In each case, though, you need more air than the theoretical amount for complete combustion because there is never a perfect mixing of wood gases and air inside a stove. This is aptly called "excess air". Failure to supply enough excess air will result in not all of the fuel gases getting burned. The concept of excess air isn't the same as just opening up the draft more (although that will provide more air), but is more a matter of stove design. At any given burn rate (pounds wood/hour), a stove that is designed to require more excess air will draw more CFM into the intake than a stove designed to need less excess air.

I've seen in books that the typical air-tight wood stove will use between 10 and 50 CFM depending on how much heat is needed and the design of the stove. Some stove designs need less excess air and still get excellent combustion efficiency through superior gas/air mixing. Others (like my old VC) need more excess air to get a clean burn. All other factors equal, the stove that is stingier with excess air will send less heat up the chimney and transfer more heat into the home. This is all part of the overall heating efficiency of the burner, a number than is real hard to pin down just by looking at specs. An extreme case of this is a pellet stove, which has an extremely high combustion efficiency (>96%), but a much lower overall efficiency due in part to the high amount of excess air needed to get that high combustion efficiency.

 

[Renovation]

Great question and I look forward to the responses.

I'll be building a small house this spring and think it will be sealed tight enough that I'll need to worry about some sort of air exchanger. I've been reading up on passive vents in an exhaust only system and it sounds like it would be a good way for me to go. Basically you add a few outside vents in certain areas of the house (closets for one) and when the kitchen/bath fans are turned on they draw in fresh air through the passive vents as they exhaust. If they don't get used enough by the occupants during normal living they can be place on timers. Another option I've heard is instead of outside air for the wood stove letting it be the exhaust source by drawing inside air. This got my wondering just how many CFM they flowed.

Alan

Hi Alan,

In case you're not familiar, search the threads here for info on heat recovering ventilation systems--HRVs. They save energy by exchanging heat between the exiting and entering air, and are now code in some places. Seems like one of those and an OAK is the way to go to get both fresh air and energy efficiency.

EDIT: Thanks Nate--it's an HRV.

 

[nate379]

That is how my house is. Pretty much a poor mans HRV. My bath fans are controlled by a humidistat. I set it to 30% in the winter and 50% in the summer.

Great question and I look forward to the responses.

I'll be building a small house this spring and think it will be sealed tight enough that I'll need to worry about some sort of air exchanger. I've been reading up on passive vents in an exhaust only system and it sounds like it would be a good way for me to go. Basically you add a few outside vents in certain areas of the house (closets for one) and when the kitchen/bath fans are turned on they draw in fresh air through the passive vents as they exhaust. If they don't get used enough by the occupants during normal living they can be place on timers. Another option I've heard is instead of outside air for the wood stove letting it be the exhaust source by drawing inside air. This got my wondering just how many CFM they flowed.

Alan

 

[Alan Gage]

Hi Alan,

In case you're not familiar, search the threads here for info on heat recovering ventilation systems--HRVs. They save energy by exchanging heat between the exiting and entering air, and are now code in some places. Seems like one of those and an OAK is the way to go to get both fresh air and energy efficiency.

EDIT: Thanks Nate--it's an HRV. [/quote]

I'd originally started looking at HRVs but was put off by the price and the fact I had a hard time finding one small enough for a 750 sq. ft. house. And all the ones I could find ran constantly. Seems to make more sense, at least to me, to have a system that is a little more controllable so it can pump more air when when needed (people actually in the house, cooking, bathing, etc).

I'll have no air conditioning so in the summer the windows will be open all the time anyway. For the me the amount of recovered heat offered by a HRV wasn't worth it.

Alan