Cold air returns under slab(Long but worth the read)


Old 06-06-03, 01:20 AM
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Cold air returns under slab(Long but worth the read)

A little background:

My house is 53 years old and the returns are under the slab. I have to get a new HVAC system and since the returns take on water under the slab all the contractors have refused to put the new furnace over the hole in the utility room floor where the current return comes up under the furnace. They want to fill it in and run new returns through the attic. This adds $5000 to the price and will also cost me closet space, a doorway next to the furnace, attic access space and leave many returns on the ceiling. My supplies are already high on the wall. Not to mention releasing tons of plaster and insulation dust from when they have to move insulation and cut holes in ceilings for registers and ductwork. I don't like this idea but they say the water creates a health hazard. I've been here 18 years and no one has gotten sick yet and it's been doing this for years and there is no sign of any mold on the 34 year old AC coil, in the blower compartment or the supply ductwork for that matter. I live in NE Ohio by Lake Erie if it matters.

I recently discovered the four points where water is getting into my ductwork. The returns are on the outside walls and if you take off the vent you can see the foundation from the inside of the house. You can watch the water come through and fall down to the return like a waterfall during a downpour. My foundation is leaking in many places and needs waterproofing just like leaky basement walls. The house is a ranch on slab and the foundation is 3-4 feet deep in the ground. I've already started the digging and am going to have it waterproofed professionally with three different layers of sealer, mortar mix and tar along with new drain tiles and gravel.

My cold air returns are old red clay tile butted together with the main artery being an 18" round red tile with all of the others connecting to it. The house is 90' X40' so it's quite a network under there.

1.) If I get the waterproofing done and it stops the water, thus drying out the returns, are they usable again? Is it safe?
2.)I see no presence of mold. After I'm done with the waterproofing I'm going to have the returns cleaned by a duct cleaning service. This may be a crazy question but for peace of mind is there any product or company out there that can pump something(a gas maybe?) through that ductwork to kill any possible mold once the ductwork is dry? I would have that done only after they are proven dry and cleaned.
3.)Would drying them out completely take care of any potential mold without taking further steps? Thanks for any replies to this very long post.
Old 06-06-03, 07:23 AM
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It kind of sounds like you are in a loose/loose situation.
You can abandon the in-floor returns and loose living space or try to make do with what you got with the potential of future problems.
As long as your family is healthy then trying to repair what you have will be acceptable. When one develops a sensitivity to mold though it often takes only microscopic quantities to get a reaction.
The problem with your situation is that because slabs are inherently cold and yours is quite deep, condensation could conceivably form which is where mold starts. Also being 4 feet down there is the possibiliy of high ground water at certain times of the year which would cause water infiltration in spite of your foundation work.

Having said this you still need to provide better perimeter drainage anyway.
Regular duct cleaning will reduce accumulated dust which gives mold a food source and having your fan run continuously will keep the slab warm which will reduce the likelyhood of condensation due to a cold slab. Also you could look into the
possibility of trying to modify your system so that it will take a SaceGuard or similar high efficiency filter and a UV light.

Aprilaire site:

Allergy article ( kind of alarmist and written for families with problems but fairly accurate) :
Old 06-06-03, 05:02 PM
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you used the existing system, and problems occurred, would it be feasible to convert to new return ducting? 5 grand is one HECK of a lot of duct!!! i guarantee i could come up from atlanta and do it for 1/2 the cost. get more opinions from local contractors who can put their eyes on it, and have a better idea what is involved.

gregH....excellent post, i learned quite a bit from your link..thanx!
Old 06-07-03, 12:11 AM
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Thanks for the replies. That info helps a lot GregH.

If you used the existing system, and problems occurred, would it be feasible to connect to new ducting?
If something would go wrong it could be converted. It would cost a little money though. What I want to do is the waterproofing work and give it a little while before the new system. With the AC I don't know if that can be done. The furnace is 18 years old but works well. The AC is 34 years old and it is the whole reason for the system replacement. Last summer it started tripping the breaker on 85+ days. You could reset it after a few minutes though. The tech came out, checked the freon, replaced the hard start capacitor, cleaned the condenser coil and it didn't help. I even had the wiring and breaker replaced and brought up to spec. It's a 5 ton unit and even called for 60 AMP overcurrent protection but it had been wired with 8 guage and a 40 AMP DP breaker all of these years. That didn't help a thing but at least it's new for when I get a new unit.

I'm told I need a new compressor but they said it was too old to get one for it. It's an Armstrong condensing unit from the late 60's/early 70's and it's so big it has two fans on it. I'd like to spend the $1000 to replace the compressor and do the waterproofing and hold off on a new system for 1-2 years while I watch the ductwork so I don't go off in the wrong direction on this duct thing only to get burned by it later. I'm more than willing to speculate on the $1000 to do this right the first time. I'd like to avoid an all new duct system if possible. If you saw this house you'd understand. They were quick to tell me the compressor is irreplaceable but they sure brought up selling me a new system, almost in the same breath.
Old 06-07-03, 12:32 AM
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Re: IF

Originally posted by hvac4u
5 grand is one HECK of a lot of duct!!! i guarantee i could come up from atlanta and do it for 1/2 the cost. get more opinions from local contractors who can put their eyes on it, and have a better idea what is involved.
It's a huge undertaking. They have to go up through the ceiling into the attic, make a turn and then run a sheetmetal trunk at least 65 feet through the attic. They also have to fill in the hole in the floor and cut out a section of ceiling joist(nothing above but attic) and make a box for the large return boot to fit through. The whole thing will be sheetmetal and there will be 8-10 branches. 2-4 will have no more than ten feet of flex at the end of a hard pipe run due to space concerns. The attic doesn't have more than 4'10" of headroom at the center. The roof has a 5/12 pitch so there will be a lot of belly crawling going on. There is a finished floor through the center of the attic and much of it will have to be torn up along with a lot of insulation removed to run the new ducts to the ceilings and cut in the registers. Other returns will come down through closets and require cutting holes in walls and installing registers. I'm trying to get as many return openings at floor level as possible. My supplies are high on the wall, only 8-10 inches down from the ceiling. I don't think all returns high on ceiling or walls would be ideal, especially considering how cold the floor is in this house now. All of this large network of duct would then have to be sealed and insulated. When you add up how long it is along with the length of the branches, that's a lot of insulating and sealing.

I've had three local contractors out. All reputable with great reputations throughout the city. They've all been in business anywhere from 30-50 years. I've dealt with 2 of them before and have no complaints. They all said the ductwork was $4600-$5300 of their quote. You have to see this place to believe it. Maybe I'll try to post a pic of my setup tomorrow. Thanks for the reply.
Old 06-07-03, 12:52 PM
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makes ya wonder how they constructed homes 40 years ago. anyways, i doubt any contractor wants the liability of using the old returns. you may not see any mold at the openings but legionairs (s) or another form of bacteria could be developing in there and they might feel liable if something were to go wrong.
be on the safe side and go with duct work like they suggested.
Old 06-08-03, 10:25 AM
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a little salt

on my crow please! yes that is a very difficult task. why sheet metal? ductboard would be cheaper, and would not heat up like metal in an attic would, properly run, ductboard and flex would be a good system. many folks do not like duct board, but have seen many 20 yr old ductboard systems in excellent shape. i would not replace the compressor. if you do, you can get one that is compatible, i would think, matching tonnage, voltage, and probably some repiping. still..... $1000....i would replace the unit.

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