How much ventilation in my attic?
TRIGGER
| Posted in Energy, Heating & Insulation on
Good Morning All,
My name is Jack. I am new here so any tips on working this site are welcome. I would like to know how to figure how much ventilation I should have in my attic.
Also, where should the vents be? Eves, Dormer vents, gable ends, or what? Also, my furnace is drawing high and low combustion air from my attic. Dose this have relevence?
Thank’s, Jack
Edited 3/3/2003 8:58:22 AM ET by Triggerski
Replies
The PDF file I have attached does a nice job of explaining the reasons for and requirements of attic ventilation. Don't recall exactly where I got it from.
The furnace combustion air thing surprises me - Never heard of that before.
I'm also a bit concerrned about what you said about "drawing high and low combustion air" from the attic. Hopefully you mean there are 2 inlets, and not an inlet and exhaust
Profanity sucks
I have an F.A.U. closet that has a weatherstripped door isolating the furnace from the living area. There is a vent in the lid that is open to the attic, and anouther vent in the wall witch goes up the stud bay to the attic also. Maby this keeps gasses ''combustable air'' out? Why high & low vents on your typical water heater door? Thanks BOSS for the attic venting info.
First - What's an " F.A.U. closet" ?
Second - Don't have any idea what you mean by "Why high & low vents on your typical water heater door?" I've never seen 2 vents on a water heater.
Or am I just being dense, and everybody else understands this?The shortest distance between two points is how far apart they are
Boss, the high/low vents he's talking about mean combustion air sources located high up and down low. Sizing dictated by btu requirements of the appliance.__________________________________________
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
If I understand you right, you're saying he's talking about high and low vents like in a louvered closet door?
I can see that in a closet - So the heated air from the appliance doesn't build up. But I can't see why a furnace would be set up that way.
My only real concern was that some hack had vented the exhaust side of the furnace into the attic.Don't be redundant by repeating yourself
The source of air necessary for combustion, whether louvers in a door, wall, or fresh air piped into the closet/room, need to be at a high and low location. Exhaust needs to be sealed and exit via power vent or chimney.__________________________________________
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Why would combustion air need high and low sources? As long as the air can get in what difference does it make?
I agree that the combustion air needs to be exhausted to the outside - That was why I questioned his original post about it.My identity lies in not knowing who I am
BH,
As Calvin said, the codes require both a high and low source of combustion air when a combustion appliance is in a confined space.
A confined space is one where the cubic feet of the space are less than 50 times the btu capacity of the appliance.
Some combustion experts have questioned this requirement, but that's what it is.
I personally believe it makes sense: the high vent acts as a "safety valve" if the appliance is spilling flue gases, which are initially hot and thus will rise; the low vent will act as a "safety valve" if the flue gases cool between cycles and the CO2 (which is heavier than most components in room air) settles to the bottom of the space.
In either case, it is important to get the flue gases out of the space because if they build up in the space, they will reduce available oxygen and the appliance might start producing huge amounts of CO.
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Guess you and I were posting at about the same time.
I had never heard of such a code requirement. But - I don't really know much about HVAC. And furnaces in closets are unheard of around here, except in mobile homes.I am becoming increasingly worried that there isn't enough anxiety in my life
we use the international one and two family code here in oregon, Cht 20 defines combution air the code that I believe is the target for the national code.
To answer the question, attic ventilation is should meet the 1 square foot of net free air per 150 square of attic or building foot print. if using rigid or roof vents with eave or bird block vents divid the area equally (1/300 each area) I prefer this method because you can take advantage of convective heat flow to create passive air flow.
If you have a standard size furnace (150k btu or less)this amount of ventilation should be adequate for combustion, given one attic area for the house.
Are you saying you think it's O.K to vent combustion air into the attic ???
Can't believe anyone would think that's a good idea.Flashlight: A case for holding dead batteries.
I hope he's referring to combustion air for furnaces located in the attic!
FWIW,a 150 btu furnace needs 150 sg in of free vent space. If you follow the attic venting ratios, the venting for a 156 sq ft house would provide adeguate venting for a 150 btu furnace!?!
I don't think you'd need 150,000 btu for a house that small in most habitable portions of the earth!!!
_______________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Edited 3/4/2003 9:31:32 PM ET by Bob Walker
I should have bolded "should be adquate" furnace size and and house size are normally related, sorry about the confusion
Im starting to see the light. Thanks.
Elaboration------Ive replaced a roof truss system on a 4 unit 2 story apt. building. Fire was the peril. Each unit has a Forced Air Unit in its own closet witch is vented
thru the roof . Each FAU closit has the high and low vents in question, venting to 4 seperate attic spaces. I have fire walls in the attic above ea common fire wall on #1 & #2 floors. Each attic space is 22' x 32' with two 20sq.in. dormer vents but no eve vents. How do i figure B. T. U.'s reQuired?Then vents?
if I read you right you have 4 attics each with a furnace closet vented to.
If so you have 704 sq feet each
vents needed upper(roof) and lower (eave) of 338 sq inch net free air (nfa) each for a total of 675 sq in. vents should be stamped with a number than nfa just add up to exceed that for each attic
As to btu, each appliance will have or should have a label with the imput btu's
To confirm enough vent air I have been using 1 sq inch of venting for every 1000 btu. in both upper and lower vents. so as long as you do not have over 338,000 total btu of all appliance connected to that attic and the attic space is not restricted like low slope under 3/12 you shold be ok,
But like all great answer confirm with your local government inspector
Yes, you read correctly. I'll check what I have with your calks., and then call the authorities for there blessing.
Input appriciated, Triggerski
I was not saying to vent the exhaust gases to the attic, the question was is it ok to use the attic for combution air which I beleive is the situation.
using attic space for combution is a very common installation and is set out in both the international and uniform building code. It makes sence to me because proper ventilation makes the attic space function as the exterior. I perfer to see the vent s in the floor and ceiling of the closet and the walls and access door treated as exterior walls and door weatherstipped. This method to me make much more sence than using indoor air which if there is a combution problem ie the induction fan does not turn on yet the furnace still fires and exhaust gas accumilate around furnace, which i find happens more often than I like to see.
note:combustion air is the air used in combustion, dilution air, and ventilation of the appliance. air for combustion and dilution air becomes exhaust gas or flue gas which needs to be vented to the exterior