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Community and Q&A

Running HVAC ducts under slab on grade?

airfix | Posted in Mechanicals on

I’m reviewing the HVAC schematic provided by my design consultant and notice he has two ducts running under slab on grade.  This allows the register in one of the rooms to be positioned under the window.

I feel pretty nervous about running ducts under slab.  It seems to be just not a good idea for several reasons such as heat loss, moisture ingress, damage from settling, animals not to mention access issues to repair etc.

Both the rooms have part of their floor plan (not very much) over a framed basement.  Does it make more sense to position the ducts in a less efficient location but run them in the floor joists or a chase than to run them under slab.

See attached picture – everything above the orange line is slab on grade.  Red is the proposed duct run and registers, green would be my suggestion.

What are my best options?


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  1. Expert Member
    Dana Dorsett | | #1

    A duct under a slab could easily become a difficult to service or repair water infiltration path or mold farm well before a building's lifecycle is up. Your instincts agree with mine.

    The notion that you have to supply the heating air under a window is really old school, a vestigial feature of the days of leaky (or open) single pane windows. Floor registers in your preferred locations with vanes to throw the air toward the far wall should be adequate if the windows are decent.

    Alternatively the ducts can come up from the basement toward ceiling level in the walk-in closet and run in a soffit just below the ceiling , feeding the supply to the office above window level at the closet. A soffited duct under the ceiling could supply the walk-in closet with a register directly off the run or a much shorter tee, directed either downward or across the under side of the ceiling.

  2. Yupster | | #2

    Assuming a relatively good envelope and windows, then you are correct. The problems created by putting ducts in the slab outweigh the benefit of blowing warm air over your window. Positioning registers at the perimeter is a leftover design parameter from back when windows were single glazed units. The leakiness of the window and surface temperature of the window itself caused people near the windows to be uncomfortable. By blowing hot air directly over the cold surface you could help keep the occupants more comfortable while reducing condensation on the windows. If you are concerned about distribution of the heat to that side, you can mount the register high on the wall and select a register with the appropriate throw to move the air to the exterior wall.

    Edit: I see Dana beat me to it :)

  3. airfix | | #3

    You guys rock thanks. I like the idea of the soffit in the closet and will likely go with that.

    With regards to mounting the register high on the wall, I'd suggested that to my HVAC consultant but he said the extra two 90 degree bends to get the air up the wall will reduce your flow rate into the room and it's better to run straight lines under the slab.

    Windows are Windsor R-0.31 or R-0.27 with their dual low-e coating. Hopefully leakiness won't be an issue so there should be no drafts but is there still an argument to putting the register under the window because that's where your majority of heatloss in the room would be and therefore a register placed under the window will reduce temperature gradient across the room?


  4. Expert Member
    Dana Dorsett | | #4

    Placing the register under the window increases the heat loss of the room by blasting 110F+ air next to the coldest surface of the room instead of leaving an undisturbed air film slowly convecting 70F air by the window.

    From a comfort point of view with a U1 single pane that was a loss worth taking due to the chillier surface driving a higher convective draft, but with U0.27-0.31 windows usually not, since the surface temp of the window is a few 10s of degrees warmer and convection draft isn't nearly as fast or cold as what you would get with a single pane.

  5. Jon_R | | #5

    You can use this calculator to see that yes, you can still get uncomfortable (say > 20 PPD) downdrafts near windows - especially with interior side low-e coatings. But an occasional few degrees warmer inside is an easy fix.

  6. Trevor_Lambert | | #6

    This tool has some serious limitations and / or flaws. For example, even with an outdoor temperature of 72F and a window that is effectively an R-40 wall, it says 1 in 11 people will still complain of drafts. If you set the outdoor temperature any higher than that, it says thermal comfort cannot be achieved at all. Raising the indoor temperature only increases the percentage of complainers. What conclusions can we draw from this, other than someone will complain no matter what you do? It certainly doesn't suggest that putting a register under the window is going to make any positive difference.

    1. Expert Member
      Dana Dorsett | | #7

      I believe Lstiburek is credited with a statement along the lines of, "Five percent of people are never comfortable." :-)

      If the register is at ceiling level through the small office closet an blowing blowing toward the corner labeled "DESK" any cold drafts or searing solar gain from that window would be pretty much moot while sitting in that corner, at least when the air handler is active.

  7. jberks | | #8

    I also suggest against it. I assume there is insulation under the slab, but also consider
    if the duct is above or below the vapour barrier.

    Taking from commercial HVAC concepts. a round ceiling diffuser provides better air mixing and essentially "washes" the walls including the window. I like those designs over undermount window diffusers.

    I'm my current build, I used dettsons smart duct system where rooms like this I have an outlet high on the wall, throwing conditioned air accross the ceiling towards the window.

  8. Expert Member
    ARMANDO COBO | | #9

    The biggest downside to installing ducts underground is condensation and air leakage, which in many cases lead to mold. Even in the dry SW, where underground ducts were a fad in the 80's and 90's, you can see some ugly results on remodeling jobs. The practice is very much avoided nowadays.

  9. Expert Member
    BILL WICHERS | | #10

    Move the vent locations to avoid the underground duct run. I have such an underground duct run in my house and there are corrosion issues and no easy way to fix it. You don’t want to have to saw cut a concrete floor in a residential setting to fix anything, it’s messy enough in commercial buildings.

    A few extra 90s in one run can probably be balanced with dampers in other runs to even out the airflow between vents. You might ask your consultant about using turning vanes in the 90 degree fittings to help limit flow reduction although I don’t know if that will do much in a small duct run.


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