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Question DetailsAsked on 3/24/2014

In-slab air ducts have mold and rusting. Is it better to clean them or seal off and install ceiling ducts?

Purchasing a 27 yr-old home, 3200 sq ft. The home (built in 1987) has in-slab duct work which I have had video-scoped. The structure of the ducts is OK, but there is an above average level of mold throughout. The joints show early signs of rust and have separated slightly in some places allowing small amounts of sand in. Would it be better for me to have them cleaned? Or should I spend the additional money now to install new in-ceiling duct work and avoid future problems associated with in-slab ducting? There is ample attic space for installing ceiling ducts.

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6 Answers


Seal off the slab ducts... First because of current and future air quality issues- especially if you have young children. Second because of heat/AC distrubution losses through the ductwork.

If you must run ductwork through the attic area, try to add additional insulation to/around the ductwork to minimize the heat/AC loss running through a hot/cold unconditioned attic.

FYI- You did not mention if the ductwork was carrying both heat/AC. If you have a fuel heating system as well located on the main floor above slab area, you want to be certain that the fuel heating system is completly isolated from the living area. A "sealed combustion" high efficiency system is perferred so that no exhaust gasses or combustion make up air can get into the living area.

Answered 6 years ago by hosey


Of course a lot depends on how difficult the access is for new ducts - if you have open floor joists in the basement, open attic space, etc to run them in, and amount of wall/ceiling repair that will be needed after install, etc. Can be very easy in a ranch with crawlspaces or open ceiling basement, to extremely costly in a concrete or brick construction house.

Another factor is how old is your HVAC system - if due for replacement, then a different type of system with high-velocity ducts or a mini split system or such might fit in well.

As far as the ducts go - if sand is coming in, then the ducts were illegal in the first place. An "in-slab" system has always been required (since at leat the 60's when I started in the building business) a trench under the slab to put the duct in, it was assembled taped, then concreted in place when the slab was poured - with concrete ALL AROUND it.

Your possible solutions:

1) move ducts up into house, per above and Hosey's comments - either inside house if possible, or in extremely difficult cases inside an insulated utilidor or plenum mounted as a bumpout on the outside of the house.

2) if you don't plan on staying in the house many years, and are willing to risk problems selling it with the same ducts a few years down the line, then cleaning and airsealing should last at least 3-5 years in most cases.

3) I have worked on a few commercial jobs like this, though the problem was water leakage into ducts or pipe/cable chases, where the ducts were repaired with a cast in place expendable pipe liner like is used to repair old water and sewer lines. Whether it could be done in yours would require a contractor to evaluate bends and such. I have also seen ducts like this remotely pressure washed and heated air dried, then thickly epoxy sprayed to provide a new liner - again, same system as used to seal leaking water and sewer mains. Personally, this is the first place I would go, but then I have a lot of experience with this type work. Your normal HVAC or plumbing company cannot do this - takes a specialty pipe restoration company. Pictures of examples of the two systems here:

Answered 6 years ago by LCD


can you advise who did your video scoping? I have the same issue but haven't been able to find anyone able to do the scoping. My house was built in 1968, i've been here 14 years and the vents have never been cleaned...

Answered 6 years ago by Guest_9135589


Guest 9135589 - if you can't find a local residential duct cleaner who does that, try calling local HVAC larger contracters with a public shop/showroom, google for it, and as last resort google for commercial building duct cleaners - they are much morer likely to have it.

If all else fails, then a sewer and drain cleaner with video camera could run his sewer camera through them - hopefully after a bleach wipedown, but will NOT be fully decontaminated by any means, if that creeps you out.

Answered 6 years ago by LCD


Can you link any of the images here?

There are applications that allow for the dispersment of sealing solutions throughout the ducting system that will seal it off against air leakage but I do not know of any scenarios where they can be sealed against any bulk moisture. If they have been neglected for years, its likely that a good cleaning and AeroSealing will make them last a bunch longer but I am no expert when it comes to in slab ducting.

Answered 6 years ago by WoWHomeSolutions


Slight glitch in my first comment - the lining system I mentioned that is used in pipes is an "expandable" liner, not "expendable" - is a fiberglass or polyester or kevlar like material sock (depending on application) that is resin impregnated and then "blown" into place like turning a sock inside out or like those party things that uncoil when you blow on them, then hardens in place, making a permanent water and air tight liner.

Answered 6 years ago by LCD

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