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My house is built on a slab. Within the slab are "channels" which act as ducts for the HVAC system.

I assume it is partially lined with metal but based on photos from an HVAC-cleaner, it's hard to tell how far that metal goes from the registers.

Long term, I plan on re-routing all of my HVAC outside of this slab ductwork as it is extremely dirty, probably contaminated with all kinds of mold and insects, and essentially a big mess. This house was built over 60 years ago.

In the mean-time, I am about to re-do a single room's floor. It is currently concrete (carpet had been ripped up) but is going to be replaced by porcelain tile. This specific room is really close to the furnace (with a direct wall-vent a previous owner "cut in") so I am not concerned about its temperature.

I want to fill in, or otherwise seal off, the slab-duct and register in this room so I can tile over it. Long term, I want to do quit using all of my in-slab ducts.

Can I simply seal off the duct at this register? Or do I need to fill the entire duct in the slab in this room? What material (concrete/cement? specific type?) should I use in either case?

Thanks!

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4 Answers 4

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I would remove the register and pour ready-mix concrete into the channel to seal the old "ducts" and fill up the hole in the floor. Shouldn't cost you more then $20, if that.

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I see this is an older Question. But yes, you want to fill that room's entire duct for structural stability with gravel or even expanding foam. While it's been perfectly fine for this long, the heat removal could allow freezing now or later & invite cracking or collapse of that minimal section.

If you'd prefer to just keep the in-slab ducts, if cleanliness was addressed long term. Then, these can be scrubbed out (not just vacuumed) & paint sealed with a few layers of sprayed-on paint via a hose. I don't know of anyone who could do it personally, but it really should be quite quick & simple with wonderful results.

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Not sure why Community popped this up as active since it has old answers, but I think you wasted a lot of time and money unless there was some issue with the HVAC system performance you weren't disclosing. If you're concerned about debris in the ducts, just have someone clean them out. There are services that specialize in that.

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  • I never filled them off or sealed them. I'm still using them. Definitely need to get them cleaned out, although the prices I've seen quoted are easily $1,000 or more. And this only really provides hvac to the first level of my house.
    – Kurtis
    Commented Sep 21, 2016 at 14:02
  • That's cheaper than re-routing all your ducts unless you DIY. You'd need to make sure you size the trunks and runs correctly. Not sure how you'd re-route anyway since you apparently have a second floor.
    – topshot
    Commented Sep 22, 2016 at 1:38
  • This is why Community has been bumping this question for more than 5 years now. I believe I've resolved the issue. ;)
    – FreeMan
    Commented Dec 10, 2020 at 15:46
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when you pour concrete into slab duct you will have expansion from new cement. I would think you need to insert some expansion material to prevent cracking the floor slab. Foam may do the same thing. My slab from 1952 had dust, neighbor has leaches. Old metal pipes rust out that they used for slab ducts. local companies refuse to work on them.

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  • When answering it help full to explain the reasons why you are giving the answer you are and provide as much detail as possible. Please explain or provide some documentation to support your assertion that new concrete expands. What does dust and leaches have to do with this? Also, explain what " Old metal pipes rust out that they used for slab ducts. " means why it is relevant to the OP.
    – Alaska Man
    Commented Mar 5, 2018 at 1:58
  • Actually, the term “expansion” is a misnomer. The concrete “contracts”. Most of the contraction occurs in the first 28 days. (Control joints are installed to control where the concrete contracts.) Then, after the concrete reaches its stabilized size, it could expand depending on temperature (if it heats up significantly), size of concrete unit, etc.
    – Lee Sam
    Commented Aug 7, 2021 at 3:37

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