Thread: Attic as make up air

Just wondering how an attic can be used for (FAU) make up air and have fireblock/firestop regulations.

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Conceivably one could install a fire damper in the duct run and it could be Ok, but why would someone want to go to that expense. Is that a screw damper mechanism and the stat wiring going through the run?
What a lousy set-up in the pic. For stuff like that I do write up the lack of fire separation. However, and this is my own personal bias, I hammer the air quality issues.
So much poor HVAC work involves poor air quality and introducing contaminants into the occupancy space.
"Mr. Client don't you just love this picture. It shows all the crap you and your family are going to breathe in if X doesn't get fixed."

I know you're in CA so the requirements are probably different; but fire-blocking for a single family house is defined in the IRC section 602.

In the case of make-up air ducts, the opening between the attic and the room is not considered a 'concealed draft opening'.

Originally Posted by Darren Miller

I know you're in CA so the requirements are probably different; but fire-blocking for a single family house is defined in the IRC section 602.

In the case of make-up air ducts, the opening between the attic and the room is not considered a 'concealed draft opening'.

Because it's a closet?

Originally Posted by Markus Keller

Conceivably one could install a fire damper in the duct run and it could be Ok, ...

That would not be okay, that could shut off the make up air and create problems with the appliance still operating, ending up killing people.

Originally Posted by Darren Miller

In the case of make-up air ducts, the opening between the attic and the room is not considered a 'concealed draft opening'.

And opening through a ceiling in a room (any room, living room, bedroom, closet, etc.) is not considered "concealed space" and therefore no fireblocking is required (think of the older homes with whole house fans - same thing ... a big open hole in the ceiling to the attic).

*IF* the ceiling was part of a fire-resistance rated ceiling-attic system, then, no, you would not be allowed to install a make up air duct through that system and into the attic, it would need to go all the way through the attic and exit out the roof (or gable end wall, etc.), in which case the duct would need to meet the minimum required gage for going through that space, probably 26 gage or thicker, not likely the standard 30 gage which would normally be used for that.

Originally Posted by Jerry Peck

That would not be okay, that could shut off the make up air and create problems with the appliance still operating, ending up killing people.



And opening through a ceiling in a room (any room, living room, bedroom, closet, etc.) is not considered "concealed space" and therefore no fireblocking is required (think of the older homes with whole house fans - same thing ... a big open hole in the ceiling to the attic).

*IF* the ceiling was part of a fire-resistance rated ceiling-attic system, then, no, you would not be allowed to install a make up air duct through that system and into the attic, it would need to go all the way through the attic and exit out the roof (or gable end wall, etc.), in which case the duct would need to meet the minimum required gage for going through that space, probably 26 gage or thicker, not likely the standard 30 gage which would normally be used for that.

How come "if"? IRC 602.8/302.11 doesnt specify fire rated or not, nor did I see any exclusions.

Originally Posted by Marc Mazza

How come "if"? IRC 602.8/302.11 doesnt specify fire rated or not, nor did I see any exclusions.

Because you are not reading R602.8 in its entirety, you are only picking out selected portions of it and trying to ask a question which is not applicable to that section, and which is answered in the rest of that section: (bold and underlining is mine)
- R602.8 Fireblocking required. Fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space. Fireblocking shall be provided in wood-frame construction in the following locations.
- - 1. In concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs; as follows:

- - - 1.1. Vertically at the ceiling and floor levels.
- - - 1.2. Horizontally at intervals not exceeding 10 feet (3048 mm).
- - 2

. At all interconnections between concealed vertical and horizontal spaces such as occur at soffits, drop ceilings and cove ceilings.
- - 3. In concealed spaces between stair stringers at the top and bottom of the run. Enclosed spaces under stairs shall comply with Section R311.2.2.
- - 4. At openings around vents, pipes, ducts, cables and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion.
- - 5. For the fireblocking of chimneys and fireplaces, see Section R1003.19.
- - 6. Fireblocking of cornices of a two-family dwelling is required at the line of dwelling unit separation.

I will try to define "concealed" for you:
- You are in a storage building in the back yard, you are standing inside looking at the studs, the rafters, and the metal skin sides and roof attached to the exterior side of the studs and rafters.
- Nothing is "concealed" from your view, right? You can see everything.
- Now, however, you decide to install plywood on the inside of the studs to allow you to hang tools on ... where there used to be a stud, metal skin of the exterior, and another stud, you now see - plywood ... the space between the two studs, between the plywood and the metal skin is no longer visible - it is "concealed".

Now let's take that a step further, you are in a closet in the house with the studs exposed, drywall on the back side of the studs at the adjoining rooms, there are no "concealed" spaces in that closet, now install drywall on the closet side of the studs - you have now created concealed spaces at each stud bay, and those concealed spaces much now be fireblocked.

Now to a fire-resistance rated ceiling: your standard single family house or town house simply has a ceiling between the living space and the attic, it is there to serve as the membrane to separate the living space from the attic space, it can be drywall, plywood, or (theoretically) not there at all.

Now you have a condo on the first floor with a condo on the second floor, then the attic: the structure is common property to all of the owners, and the first floor condo is required to be fire-resistance rated from the structure above and around it, likewise the second story condo from the structure below, around, and above it.

Did that make sense?

Yes, got it thanks...conceled. And thanks for the explanation and your patience, it helped a lot.
So this would be applicable if the make up air vent was in the wall of the closet, like so many are-not the ceiling.

Originally Posted by Markus Keller

Conceivably one could install a fire damper in the duct run and it could be Ok, "

Sorry Markus, but actually it isn't ok. Anything that can be closed won't cut it. It's the reason that grills are accepted and registers are not for use in combustion air provisions.

Eric and Jerry I agree with your points. I stated conceivably because, yes if the fire damper closed and cut off combustion air, it would/should mean that the unit is on fire. At that point cutting of air should be a good thing.
I always prefer to see combustion air from the exterior.

There seems to be some confusion on this thread between make-up-air and combustion air.

That damaged cabling doesn't belong in the combustion air/fresh air vent either.

Marc: This "closet" containing forced air furnace and whatever else, does it "communicate" with the living space, the garage, or the outdoors? (IOW where does the "door" to the closet open from?).

As I recall, it is fairly common to have a non-communicating-with the living area furnace closet, either opening to the outdoors or to the garage, in most parts of California, especially less recent construction.

If that is the case, then the separations and "communication" or "non-communication" determine whether the compartment is considered part of the living space, the garage, or the attic (the source of fresh-air for combustion).

I see only one fresh air vent/opening and something beyond the cabling. If cat I below (draft hood) and only opening is above, issues with curtain effect, etc. possible.

Originally Posted by H.G. Watson, Sr.

There seems to be some confusion on this thread between make-up-air and combustion air.

That damaged cabling doesn't belong in the combustion air/fresh air vent either.

Single opening combustion/make-up air is allowed with respect to size and location.

I must admit I am not real clear on the code drawing that shows the opening in the ceiling being to outside air. Other drawings refer to openings in the ceiling to a vented attic.

I agree HG. I think the OP is talking about combustion air in this case. What is in the pic is realistically not make up air.

Correct, I had it backwards.
To ensure that all fuel-burning appliances operate safely, air must be provided while they are operating. Without enough air, your house can quickly become polluted with unhealthy gases, including deadly carbon monoxide (CO). Carbon monoxide is odorless, colorless, and highly poisonous. The air needed for the safe operation of all fuel-burning
or combustion appliances is called “combustion air.”

Replacement air for all exhaust appliances is also critical. When a ventilation fan operates, it draws air from inside the house to the outside and creates or contributes to a slight vacuum in the house. The resulting negative pressure can be serious. If the negative pressure is strong enough, it can cause the furnace and other fuel-burning appliances to backdraft dangerous gases such as CO into the home. All exhaust fans, including clothes dryers, must have an adequate supply of air. The air needed for this purpose is called “makeup air.”

Marc M.:

By the way, what is that in the upper left corner of your picture with insulation packed up against it, strapping supporting, and very close to wood? A b-vent for the exhaust? Offset immediately below the attic floor?

FAU B vent.

Originally Posted by H.G. Watson, Sr.

There seems to be some confusion on this thread between make-up-air and combustion air.

That damaged cabling doesn't belong in the combustion air/fresh air vent either.

Marc: This "closet" containing forced air furnace and whatever else, does it "communicate" with the living space, the garage, or the outdoors? (IOW where does the "door" to the closet open from?).

As I recall, it is fairly common to have a non-communicating-with the living area furnace closet, either opening to the outdoors or to the garage, in most parts of California, especially less recent construction.

If that is the case, then the separations and "communication" or "non-communication" determine whether the compartment is considered part of the living space, the garage, or the attic (the source of fresh-air for combustion).

I see only one fresh air vent/opening and something beyond the cabling. If cat I below (draft hood) and only opening is above, issues with curtain effect, etc. possible.

H.G. Are you trying to make me think I'm loosing it! Your post wasn't nearly this long at 10:00 .