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Duct cleaning and sealing

FrankFulton | Posted in Mechanicals on

Much of our Zone 1 (basement and first floor) ductwork has been in a vented, unconditioned crawlspace for 70 years. The ducts are galvanized steel, and condensation marks are evident on their exterior. The placement of supplies and registers can also be improved in several rooms. Two well-regarded HVAC pros eyeballed the ducts and recommended keeping most of it, although a third stated that the more we can replace, the better. Tearing the galvanized ducts from ceilings and walls and replacing them would be a major project.

We are doing major air sealing and insulation in the house, including conditioning the basement crawlspaces. Thus, all the zone 1 ducts will be in conditioned spaces. We will then move onto updating HVAC systems once our current oil furnace dies.

How should we proceed with these ducts: duct cleaning, duct blasting, and/or duct sealing/aeroseal? Should we do this work before or after the basement crawlspaces are conditioned? If we seal/aeroseal now, will this work survive HVAC updates in the next few years?

Thank you.

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  1. walta100 | | #1

    In my opinion an unconditioned crawl space with any duct work is a horrible idea.

    1 All duct work leaks air so you need to move them inside the conditioned space or eliminate them.
    2 Floors over unconditioned spaces are often cold and uncomfortable. Most crawl space walls have a
    much smaller surface area than the floors above them so it makes more sense work in the walls.

    A few mini splits may allow you to remove the ducts.

    To my mind air duct cleaning is a scam to remove more money from your wallet than dirt from the air, avoid this unless it is a medical necessity. Even then the money is better spent moving to someplace more modern than trying to bring a 70 year old building up to today’s standards.

    To me sealing ducts work inside a conditioned space is pointless waste of time and money. As any leakage escapes into the conditioned space just not exactly where planed all is well.

    No one but you will have any idea if your future plans will disturb the existing duct work.


  2. GBA Editor
    Martin Holladay | | #2

    This is a tough question to answer. Walter is correct: Many duct-cleaning companies provide questionable benefits.

    It's hard to evaluate condition of your ducts without a site visit.

    Ducts are problematic, so it's always worth considering ductless equipment, especially if you are going to replace your furnace.

  3. Expert Member
    Dana Dorsett | | #3

    What DOE climate zone is this house located in?

    [edited to add]

    Is this the zone 4A 1920s bungalow?

    Uninsulated ducts in an unconditioned vented crawl space is pretty lousy for as-used efficiency, and any ductwork leaks an a vented crawl space can drive air infiltration rates through the stratosphere (and potentially pull in some not-so-healthy crawlspace air into the conditioned space.)

  4. FrankFulton | | #4

    Thanks. Yes, same 1952 Cape.

    1. Downstairs ducts
    Zone 1 galvanized ducts will soon be in unvented, conditioned crawl space. But, they are undoubtedly leaky, and perhaps very dusty etc at 70 years old and having been in vented unconditioned space for so long. What do you recommend in terms of cleaning and/or sealing and/or testing flow rates for future HVAC upgrade?

    2. Upstairs ducts (lots of flex in attic)
    Zone 2 flex will be sealed w/foam and buried as part of our current sealing/insulating project. When that 1994 AC needs to be replaced, I'm leaning toward ductless in zone 2, and geothermal in zone 1 (using galvanized ductwork from question #2). A best of both worlds approach, I hope.

    3. (BTW, please check our our air sealing/insulating plan here - opted for best practice with moving the thermal boundary.)

  5. GBA Editor
    Martin Holladay | | #5

    I'll repeat my advice: if you choose ductless equipment, you'll lower your electricity bill and remove worries about duct cleanliness.

    In general, dusty ducts are self-correcting. If the dust is loose and therefore worrisome, clearly it gets blown out (and eventually either gets filtered out at the furnace filter or vacuumed up when you clean your house). If the dust is stuck to the ducts, it isn't worrisome -- because it won't come out your registers.

  6. FrankFulton | | #6


    We've been advised that our second floor could be ideal for ductless (4 heads, 1 in each BR and 1 in the open stairway hall - perhaps we'd keep the bathroom fully ducted w/downstairs zone). So, we plan to install ductless on the second floor and reap the benefits you espouse.

    But, we were told that our first floor is too spread out and lacks good mounting spaces for ductless heads. (We will seek other opinions of course.) So, we will likely stick w/ducts downstairs (geothermal or hybrid).

    Thus, my questions pertained to the existing galvanized ducts in that zone. Your reply that "dusty ducts are self-correcting" is very helpful - the first I'd heard this. Other comments on our plan are welcome and appreciated.

    Thank you.

  7. furnacecompare | | #7

    Hi Emerson,

    >How should we proceed with these ducts: duct cleaning
    The EPA only recommends duct cleaning in one of these three scenarios:

    Your ducts have a musty odor due to mold growth
    Your ducts are "infested" with insects or rodents
    Dust and debris are visibly released into your home from the registers


    Balanced discussion here:


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