# cold air return on slab construction



## LeroyJnks (Jun 25, 2008)

I don't normally turn to the Internet for help, but I am facing a situation where 5 people have given me 5 totally different answers. I am involved in the design phase of a pair of duplex rental housing units, 2 levels, to be built on slabs. Client absolutely does not want basement or crawl space due to potential flooding, mold, animal, and maintenance issues. Client also wants forced air furnace and AC. The ducts are doable on the first floor with bulkheads and creative framing, but the cold air returns have me concerned. I'm from the school of a cold air return trunk in the basement ceiling with cold air return grilles in most of the above rooms. Without a basement, this is not possible in this duplex project. The other general involved and his HVAC guy are happy with one cold air return near the front door (plus a few returns upstairs). My Michigan winter experience tells me we should have more first floor cold air returns, even if the first floor is a very open floor plan. My HVAC guy is telling me to ditch forced heat and use radiant plus high velocity AC. 

How have others dealt with cold air returns in a northern climate in a new home with no basement or crawlspace?


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## kirbymurphy (Jun 5, 2008)

LeroyJnks said:


> I don't normally turn to the Internet for help, but I am facing a situation where 5 people have given me 5 totally different answers. I am involved in the design phase of a pair of duplex rental housing units, 2 levels, to be built on slabs. Client absolutely does not want basement or crawl space due to potential flooding, mold, animal, and maintenance issues.
> 
> Client also wants forced air furnace and AC. The ducts are doable on the first floor with bulkheads and creative framing, but the cold air returns have me concerned. I'm from the school of a cold air return trunk in the basement ceiling with cold air return grilles in most of the above rooms. Without a basement, this is not possible in this duplex project. The other general involved and his HVAC guy are happy with one cold air return near the front door (plus a few returns upstairs). My Michigan winter experience tells me we should have more first floor cold air returns, even if the first floor is a very open floor plan. My HVAC guy is telling me to ditch forced heat and use radiant plus high velocity AC.
> 
> How have others dealt with cold air returns in a northern climate in a new home with no basement or crawlspace?


First: I'm far from being an HVAC expert. 

The single story house we just bought is on a slab and the only CA return is in the base of the air handler. Not ideal at all. Winter heating isn't much of a problem, but summer AC comfort is bad. There is little circulation and we rely on ceiling fans and vertical tower fans to keep air moving.

My last house was also on a slab and had the vents back-to-back over the doors. As far as I could tell it did nothing for circulation. I presume you NEED at least a small vacuum draw on the CA lines.

I'm due for a new AH so I'm considering a CA plenum up to the attic from the AH and running some insulated flex tube to the far rooms. I've seen that done on other slab homes and it seems to work. Since warm air rises, I guess it will work.

If you can't run or they won't pay for a CA system, at least try and locate the AH in a central part of the home.


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## nap (Jan 27, 2008)

underground duct


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## meterman98 (Mar 15, 2008)

*to kirby*

I`m from n.w. florida and we don`t worry too much about heating down here.This is my 2 cents worth about you`re install,by the way i`m 16 year hvac guy 10 yrs install and 6 yrs. service and estimates on installing new systems.First off trying to cool 2 floors with 1 system hardly ever works,the bottom is cool and the top is hot in summer.But if you have no other choice I would install the air handler upstairs in a heater closet with up and downstairs on a zone system.Cut a return vent in the bottom of the air handler platform so it is in the ceiling of the 1st floor and cut another return vent under the door of the heater closet.And definately add 1/2 to 1 ton on top of what the manual j says.So on these 97 deg days this 1 system might keep up.


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## coolmen (Mar 25, 2006)

You must have low returns on any slab to remove the cold air that is there or you will be haveing cold feet. if cold air is no way possible then you best have low supply registers.


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