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  1. #1
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    May 2007
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    Default Fireblocking and Combustion Air requirements

    Hello all,

    I'm trying to wrap my mind around something. Hopefully I can make this clear.

    There are requirements for fireblocking:

    [R302.11 Fireblocking. In combustible construction, fireblocking
    shall be provided to cut off both vertical and horizontal
    concealed draft openings and to form an effective fire
    barrier between stories, and between a top story and the roof
    space] .

    AND

    There are requirements for using "outside" combustion air. I'm specifically referring to the air vented into the attic (see figure G2407.6.1(1) from the IRC2015).

    So, we have the fireblocking requirements with the allowing of the "outlet" air from the HVAC or water heater closet?

    As I see it, a hole is a hole is a hole and the goal is to keep the fire out of the attic for as long as possible. Fires do happen in water heater closets or in HVAC closets. So the possibility of permitting ANY products of combustion (fire, fire gases) CAN make their way into the attic space.

    What am I missing? Explanations? Mr. Peck, hoping you'll chime in too.

    Thanks,
    Bruce

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  2. #2
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    Default Re: Fireblocking and Combustion Air requirements

    Bruce,

    First we need to address where fireblocking is required and addressed: fireblocking is for concealed spaces (such as stud spaces concealed within a wall, or the space which is concealed between a floor and a ceiling).

    A ceiling is not "concealed" ... you can see it.

    However, you now have a large hole in the ceiling, and that hole in the ceiling has no insulation, and is not sealed for air infiltration or exfiltration - both are energy code issues.

    If that hole through the insulation and the air infiltration/exfiltration is taken into account on the energy form, then pull combustion air through that hole, however, if the energy calculations does not allow that combustion air to work ... install a direct vent appliance.

    However, being as there is a non-direct vented appliance installed, you need to provide it with combustion air - and this can be from outside, from inside, or from outside and from inside (there are those options, but those are different issues than fireblocking).

    Now, if that ceiling was a rated ceiling, such as one would find in an apartment or condo building (they are one and the same) ... that would be a violation of the fire-resistance rating of the roof/ceiling fire-resistance rating. In a single family home there is no fire-resistance rating to be affected.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  3. #3
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    Mar 2007
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    Near Philly, Pa.
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    1,643

    Cool Re: Fireblocking and Combustion Air requirements

    Yes, it seems counter-intuitive to seal everything around a required hole. The code is good at contradicting itself.

    The hole in the wall, passive makeup air theory has been debunked by ASHRAE in a study they did of an actual subject house with measurements taken. The only reliable MUA is mechanical such as a fan interlocked to the appliance.

    As for those dopey missle silos up into the attic think about the physics involved:

    Let's say you have a two story home with this furnace in an upstairs closet needing MUA. You install the code prescribed pipes high/ low and walk away. Will attic air know to ooze down the tubes into the Combustion Appliance Zone (CAZ)? How does it know this? Was it trained? Of course, air is even dumber than the people who write some codes if you can imagine that. It doesn't know where to go. So what moves it? Pressure differential or delta P. If the pressure in the CAZ is lower than the ambient pressure in the attic, attic air will infiltrate into the CAZ. The ASSumption is that when firing, the CAZ will depressurize With Respect To(WRT) the attic causing infiltration of MUA. However, if this is happening in an upper level of a heated home, that level is typically positive WRT the attic. Therefore, CAZ air will tend to exfiltrate up the non-directional MUA silo into the attic unless something really powerful overcomes it. In other words, when filled with warm air at a positive pressure WRT the attic, it will draft OUT-not in. They should install directional arrows inside the ducts with lighting so the air can see which way to go...

    Ok, so what about a ranch home. Even a ranch home can experience Stack Effect and this same phenomenon, just at a slightly lower delta P perhaps. The thing is, what is really going to drive this is not some dopey passive hole in the wall but the danged fans and where the leaks are. If the furnace cabinet leaks to the return plenum it can suck CO out of the combustion chamber and distribute it all over the home. If the supply ducts leak enough they may pressurize the CAZ in spite of other return leaks. The actual CFM required for combustion is quite small. The CFM required for atmospherically vented gas appliances increases if you want to combat condensation and rotting pipes through draft hoods and baro. dampers. If you provide reasonably sized openings between rooms then fix the doggone duct leaks and balance the stupid ducts so each room is at equilibrium, we could move on to more important things like sealing the top of the thermal envelope so they wouldn't have to oversize the equipment in the first place.

    What started my rant? Oh, yes, those dopey dumb air tubes and fire behavior. In a fire they would make very nice shunts to bypass fireblocking and ensure rapid spread of fire into the attic. No, nobody is going to put fire dampers in a small, dopey passive MUA pipe in a residential application so dream on there. I do have a solution: build the house properly including an intact ceiling then ventilate the building properly and everything will work fine. In the event of fire in a properly ventilated home, it shouldn't matter unless some idiot ran ducts into the attic and back or installed a whole house fan or similar ceiling shunt. I would recommend a Firomatic 110 vac shutoff to the furnace but since that isn't code most places don't hold your breath.

    Lastly, once you have sufficiently recovered from my sarcastic rant, get yourself some unlisted low level CO monitors and smile at some of the goofy codes we write. Yes, 'we'. I sit on NFPA 211 and two UL STPs who write standards that get adopted into codes. I'm trying to fix some of the insanity but the codes typically run about 20 years behind technology and knowledge. Sometimes we get quick action on emotional issues before the pragmatist can speak out on them such as these stupid MUA issues and UL 2034. Meanwhile, make sure your MUA meets ASTM E-84 so it doesn't spread the flames too quickly.

    Keep the fire in the fireplace.

  4. #4
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    1,703

    Default Re: Fireblocking and Combustion Air requirements

    Quote Originally Posted by Bob Harper View Post
    Yes, it seems counter-intuitive to seal everything around a required hole. The code is good at contradicting itself.

    The hole in the wall, passive makeup air theory has been debunked by ASHRAE in a study they did of an actual subject house with measurements taken. The only reliable MUA is mechanical such as a fan interlocked to the appliance.

    As for those dopey missle silos up into the attic think about the physics involved:

    Let's say you have a two story home with this furnace in an upstairs closet needing MUA. You install the code prescribed pipes high/ low and walk away. Will attic air know to ooze down the tubes into the Combustion Appliance Zone (CAZ)? How does it know this? Was it trained? Of course, air is even dumber than the people who write some codes if you can imagine that. It doesn't know where to go. So what moves it? Pressure differential or delta P. If the pressure in the CAZ is lower than the ambient pressure in the attic, attic air will infiltrate into the CAZ. The ASSumption is that when firing, the CAZ will depressurize With Respect To(WRT) the attic causing infiltration of MUA. However, if this is happening in an upper level of a heated home, that level is typically positive WRT the attic. Therefore, CAZ air will tend to exfiltrate up the non-directional MUA silo into the attic unless something really powerful overcomes it. In other words, when filled with warm air at a positive pressure WRT the attic, it will draft OUT-not in. They should install directional arrows inside the ducts with lighting so the air can see which way to go...

    Ok, so what about a ranch home. Even a ranch home can experience Stack Effect and this same phenomenon, just at a slightly lower delta P perhaps. The thing is, what is really going to drive this is not some dopey passive hole in the wall but the danged fans and where the leaks are. If the furnace cabinet leaks to the return plenum it can suck CO out of the combustion chamber and distribute it all over the home. If the supply ducts leak enough they may pressurize the CAZ in spite of other return leaks. The actual CFM required for combustion is quite small. The CFM required for atmospherically vented gas appliances increases if you want to combat condensation and rotting pipes through draft hoods and baro. dampers. If you provide reasonably sized openings between rooms then fix the doggone duct leaks and balance the stupid ducts so each room is at equilibrium, we could move on to more important things like sealing the top of the thermal envelope so they wouldn't have to oversize the equipment in the first place.

    What started my rant? Oh, yes, those dopey dumb air tubes and fire behavior. In a fire they would make very nice shunts to bypass fireblocking and ensure rapid spread of fire into the attic. No, nobody is going to put fire dampers in a small, dopey passive MUA pipe in a residential application so dream on there. I do have a solution: build the house properly including an intact ceiling then ventilate the building properly and everything will work fine. In the event of fire in a properly ventilated home, it shouldn't matter unless some idiot ran ducts into the attic and back or installed a whole house fan or similar ceiling shunt. I would recommend a Firomatic 110 vac shutoff to the furnace but since that isn't code most places don't hold your breath.

    Lastly, once you have sufficiently recovered from my sarcastic rant, get yourself some unlisted low level CO monitors and smile at some of the goofy codes we write. Yes, 'we'. I sit on NFPA 211 and two UL STPs who write standards that get adopted into codes. I'm trying to fix some of the insanity but the codes typically run about 20 years behind technology and knowledge. Sometimes we get quick action on emotional issues before the pragmatist can speak out on them such as these stupid MUA issues and UL 2034. Meanwhile, make sure your MUA meets ASTM E-84 so it doesn't spread the flames too quickly.
    Not a rant.
    One can extract much information from this post.
    ie: stack effect, delta P., fire blockng, CFM required for atmospherically vented gas appliances, and the list goes on.
    Jerry, Bob.
    Much thanks.

    Robert Young's Montreal Home Inspection Services Inc.
    Call (514) 489-1887 or (514) 441-3732
    Our Motto; Putting information where you need it most, "In your hands.

  5. #5
    Join Date
    May 2007
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    719

    Default Re: Fireblocking and Combustion Air requirements

    Quote Originally Posted by Jerry Peck View Post
    Bruce,

    First we need to address where fireblocking is required and addressed: fireblocking is for concealed spaces (such as stud spaces concealed within a wall, or the space which is concealed between a floor and a ceiling).

    A ceiling is not "concealed" ... you can see it.
    Agreed that code refers to concealed spaces; however, (and using my best J. Peck code interpretation), the code says "AND"

    [R302.11 Fireblocking. In combustible construction, fireblocking
    shall be provided to cut off both vertical and horizontal
    concealed draft openings and to form an effective fire
    barrier between stories, and between a top story and the roof
    space] .

    So, an opening in a closet regardless the reason defeats the effective fire barrier between the top story and roof space.

    Now, as to the rest of your post, I believe I understand you are saying for the installer to calculate what is required for let's say a gas heating appliance. But if the unit requires combustion air that can be drawn from the attic and now we have a hole that ALSO allows a developing fire in that same closet to gain access into the attic....then how is that allowable?

    Bruce Thompson, Lic. #9199
    www.TylerHomeInspector.com
    Home Inspections in the Tyler and East Texas area

  6. #6
    Join Date
    May 2007
    Location
    Tyler, TX
    Posts
    719

    Default Re: Fireblocking and Combustion Air requirements

    Quote Originally Posted by Bob Harper View Post
    Yes, it seems counter-intuitive to seal everything around a required hole. The code is good at contradicting itself.

    The hole in the wall, passive makeup air theory has been debunked by ASHRAE in a study they did of an actual subject house with measurements taken. The only reliable MUA is mechanical such as a fan interlocked to the appliance.

    As for those dopey missle silos up into the attic think about the physics involved:

    Let's say you have a two story home with this furnace in an upstairs closet needing MUA. You install the code prescribed pipes high/ low and walk away. Will attic air know to ooze down the tubes into the Combustion Appliance Zone (CAZ)? How does it know this? Was it trained? Of course, air is even dumber than the people who write some codes if you can imagine that. It doesn't know where to go. So what moves it? Pressure differential or delta P. If the pressure in the CAZ is lower than the ambient pressure in the attic, attic air will infiltrate into the CAZ. The ASSumption is that when firing, the CAZ will depressurize With Respect To(WRT) the attic causing infiltration of MUA. However, if this is happening in an upper level of a heated home, that level is typically positive WRT the attic. Therefore, CAZ air will tend to exfiltrate up the non-directional MUA silo into the attic unless something really powerful overcomes it. In other words, when filled with warm air at a positive pressure WRT the attic, it will draft OUT-not in. They should install directional arrows inside the ducts with lighting so the air can see which way to go...

    Ok, so what about a ranch home. Even a ranch home can experience Stack Effect and this same phenomenon, just at a slightly lower delta P perhaps. The thing is, what is really going to drive this is not some dopey passive hole in the wall but the danged fans and where the leaks are. If the furnace cabinet leaks to the return plenum it can suck CO out of the combustion chamber and distribute it all over the home. If the supply ducts leak enough they may pressurize the CAZ in spite of other return leaks. The actual CFM required for combustion is quite small. The CFM required for atmospherically vented gas appliances increases if you want to combat condensation and rotting pipes through draft hoods and baro. dampers. If you provide reasonably sized openings between rooms then fix the doggone duct leaks and balance the stupid ducts so each room is at equilibrium, we could move on to more important things like sealing the top of the thermal envelope so they wouldn't have to oversize the equipment in the first place.

    What started my rant? Oh, yes, those dopey dumb air tubes and fire behavior. In a fire they would make very nice shunts to bypass fireblocking and ensure rapid spread of fire into the attic. No, nobody is going to put fire dampers in a small, dopey passive MUA pipe in a residential application so dream on there. I do have a solution: build the house properly including an intact ceiling then ventilate the building properly and everything will work fine. In the event of fire in a properly ventilated home, it shouldn't matter unless some idiot ran ducts into the attic and back or installed a whole house fan or similar ceiling shunt. I would recommend a Firomatic 110 vac shutoff to the furnace but since that isn't code most places don't hold your breath.

    Lastly, once you have sufficiently recovered from my sarcastic rant, get yourself some unlisted low level CO monitors and smile at some of the goofy codes we write. Yes, 'we'. I sit on NFPA 211 and two UL STPs who write standards that get adopted into codes. I'm trying to fix some of the insanity but the codes typically run about 20 years behind technology and knowledge. Sometimes we get quick action on emotional issues before the pragmatist can speak out on them such as these stupid MUA issues and UL 2034. Meanwhile, make sure your MUA meets ASTM E-84 so it doesn't spread the flames too quickly.
    So what you're saying is.....it's OK and perfectly acceptable. (mama always said I wasn't a bright man)

    - - - Updated - - -

    And lest I forget - thank you Jerry and Bob for taking the time to respond.

    Bruce Thompson, Lic. #9199
    www.TylerHomeInspector.com
    Home Inspections in the Tyler and East Texas area

  7. #7
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    Mar 2007
    Location
    Plano, Texas
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    4,144

    Default Re: Fireblocking and Combustion Air requirements

    Quote Originally Posted by JB Thompson View Post
    So what you're saying is.....it's OK and perfectly acceptable. (mama always said I wasn't a bright man)

    - - - Updated - - -

    And lest I forget - thank you Jerry and Bob for taking the time to respond.
    The operative words in the code here are "concealed draft openings".
    An opening in the ceiling is not "concealed" where openings inside a a shaft or chase would be concealed.



    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

  8. #8
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    Default Re: Fireblocking and Combustion Air requirements

    Quote Originally Posted by JB Thompson View Post
    Agreed that code refers to concealed spaces; however, (and using my best J. Peck code interpretation), the code says "AND"

    [R302.11 Fireblocking. In combustible construction, fireblocking
    shall be provided to cut off both vertical and horizontal
    concealed draft openings and to form an effective fire
    barrier between stories, and between a top story and the roof
    space] .

    So, an opening in a closet regardless the reason defeats the effective fire barrier between the top story and roof space.
    Doing my best self-impersonation ...
    - R302.11 Fireblocking. In combustible construction, fireblocking shall be provided to cut off both vertical and horizontal concealed draft openings and to form an effective fire barrier between stories, and between a top story and the roof space

    Using the "and" in the correct context of 'cutting off' AND 'forming'; along with 'between' AND 'between'.

    It's all about "both vertical and horizontal concealed" spaces and opening in those concealed spaces.

    Now, as to the rest of your post, I believe I understand you are saying for the installer to calculate what is required for let's say a gas heating appliance. But if the unit requires combustion air that can be drawn from the attic and now we have a hole that ALSO allows a developing fire in that same closet to gain access into the attic....then how is that allowable?
    By code, other than the energy code, no ceiling is required to protect an attic from a fire from below - not in one- and two-family dwellings. And the energy code could be met by putting the insulation above the roof sheathing, with exposed beams, and ... (drum roll ) ... no ceiling - right?

    So the code is now down to requiring combustion air ... and not allowing the opening because of the energy code - and the code does give other options for meeting the required combustion air ... which means that obtaining combustion air from the attic just might not be allowed - because of the energy code.

    Let them show the hole into the attic on the energy calculations, which they probably can do with some other trade-offs, in which case even the energy code may allow it, but at least it would be in the calculations.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

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