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Flex ducts & Fireblocking


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In the commercial world we used the rule of thumb of nothing over 5' for flex. Flex causes an inordinate amount of pressure drop. Flex was used to attach the diffuser to the metal duct run. Even most of the low-end homes I've seen still have metal or worst case duct-board.

How many feet of flex are you seeing Donald? Do the come right off the plenum with this stuff? New home, older?

Sorry for all the questions, just curious.

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I live in a flex world! I was specifically speaking about new homes, but even in 20 year old homes here, we have flex the majority of the time (it's that high quality grey flex usually)

Flex is ran from the plenum to the supply registers here. Sometimes in a high end townhome with open ceilings we'll see sheet metal or foam board. That's about it.

I really hate calling out the flex running through a fire blocked floor/ceiling, but I have no clue as to what should be there.

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Don - Earlier today I did a nice older house (85 years old) in Kansas City. It had 3 central HVAC systems to replace the old Boiler and window A/C units. One of the heating units was an upright furnace mounted in the attic.

When you pop the SMALL attic hatch (about 18" x 18") you can see the furnace at the far end of the attic (about 35' - 40' away).

There is no light of any kind; they have blown in extra insulation (about 10" deep); there is no SECURE walking surface other than kicking insulation out of your way and looking for a ceiling joist; the GREY flex branch ducts criss-cross the attic in front of the unit; the sheetmetal main trunk lines for return and supply air RUN immediately in front of the furnace blower door with a 4" clearance between ducts and furnace; no room for a work platform in front of the burners if you wanted one; 2' clearance between the upright furnace and the wood rafters and wood shingle roof above it.

Question #1 - can you find any defects with the above scenario?

Question #2? - If the inspector has to take his belt off, slip it through the gap between rafters and shingles and use that as a loop to hang onto while leaning over the ducts (many of which are already smashed for some reason)and trying to get the door and flashguard off the 20 year old furnace - do you think this unit has received / will receive typical servicing by HVAC contractors or even regular filter changes from the homeowner??

Dan in Kansas City

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  • 1 year later...

Where specifically in IRC (2000 I presume) does the code exclude Flex Duct for floor-ceiling penetrations? I've read "Chapter 16 Duct Systems" several times as well as the referenced section "R602.8 Fireblocking" and cannot find an exclusion. In order to use ducts in the first place, they must be Class 0 or 1-- which the flexducts are. Regarding thru the floor/ceiling penetrationos, fireblocking around ducts requires approved materials (R602.8, 4.).

Is there another code that applies to Residential (single family dwellings) that specifically excludes use of Class 0/1 rated flexduct for floor/ceiling penetrations?

Thanks in advance.

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  • 4 weeks later...

I know this is not a residential code quote, but the 2000 International Building Code section 715.7 states: Flexible ducts and air connectors shall not pass through any fire resistance rated assembly. Flexible air connectors shall not pass through any wall, floor, or ceiling.

Section 715 only applies to fire rated assemblies only.

Typically a residential house is type 5 construction which would not require fire dampers or have rated assemblies.

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Scott, thanks for reply, but I'm still searching for definitive IRC code citation/interpretation that prohibits flexduct thru floor/ceiling assembly penetrations in residential construction. I'm beginning think that my city's code enforcement dept. is confusing firestopping with fireblocking where the former applies to rated assemblies and the latter to non-rated typical residential construction.

Clearly, flexduct is not permitted thru a fire-rated assembly as in commercial construction or in residential multi-family. I have not, however, found a clear prohibition in IRC that prohibits flexduct in passing thru floor-to-ceiling or wall-to-soffit penetrations.

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