OK guys,
Need some imput here.
In design stage of two story home.
We will use open web floor joists for upper floor system.
We are looking at using the 'plenum' between first floor drywall ceiling, and upstairs subfloor, as the supply system of HVAC, to supply both floors.  Simply install registers in floor or ceiling as needed.  Comments and recommendations appreciated. 
Also, we intend to use smaller air returns in each room/area, rather than one or two larger returns.  Several of these returns will be located inside the supply plenum.  How much insulation, if any, would be required on these return ducts?
Comments and ideas please.

I am not hvac so I will not comment on that. However be aware of issues with wiring in plenum spaces. Refer to NEC section 300.22 (B). Exposed NM-B (romex) will not be allowed.

Posted By wes on 27 May 2010 08:01 AM
OK guys,
Need some imput here.
In design stage of two story home.
We will use open web floor joists for upper floor system.
We are looking at using the 'plenum' between first floor drywall ceiling, and upstairs subfloor, as the supply system of HVAC, to supply both floors.  Simply install registers in floor or ceiling as needed.  Comments and recommendations appreciated. 
Also, we intend to use smaller air returns in each room/area, rather than one or two larger returns.  Several of these returns will be located inside the supply plenum.  How much insulation, if any, would be required on these return ducts?
Comments and ideas please.

my uneducated thought...  hot air rises, so vents in the ceiling seem like they would hold the heat at the ceiling & it wouldn't get down to where the people are....

Wes,

Are you are thinking of a displacement type underfloor air distribution system? We have used these systems in office buildings with good success, although always with a plenum created by the floor slab and a raised access floor.

My guess is that you are going to need some custom equipment to do it, however. I don't think normal residential equipment will deliver the temperatures and pressures you need.

Some caveats about displacement systems:

Office building displacement systems are typically cooling only. Office buildings usually spend most of their time in cooling mode (at least the interior zones) and underfloor systems work great for that because the loads are typically low and constant. The equipment runs continuously during normal working hours and comfort conditions are satisfied by adjusting the supply air temperature (as well as the dampers on the supply grills).

Because of the low velocities and relatively low operating delta T, displacement systems have trouble when the skin load or other local loads are much higher than the majority of the space. At some point, adding more supply grills won't get the job done and supplemental cooling is required. Perimeter heating is typically handled with a separate system.

Because Euro office floors are typically about 1/2 the size of USA, the perimeter zone is much more influential (as in a house) and I'll bet that in Germany there may be displacement systems that heat as well as cool, but I have never seen one.

Having said all that, I don't think there is any technical reason you could not use a tight distributrion plenum instead of ducts for a non-displacement (normal) system (I think dmaceld either did this or at least considered it) , but I would worry about future IAQ issues because you cannot access it for cleaning. I would also be concerned about creating condensation at the perimeter on hot humid days. As others have pointed out, wiring is a consideration.

Underfloor distribution systems give good flexibility (important in office buildings) and displacement systems provide good comfort because of the low velocity and low delta T. Since future flexibility is not usually a consideration in residential, I would not do underfloor distribution unless I could do a true displacement system, but I don't know if this has been done in a residence and I would be hesitant to be a ginea pig if it hasn't.

Bruce

Cleaning the plenum down the road will definitely be a problem. A low velocity system will accumulate dust above the ceiling. Most fire regulations would not allow this open passive air distribution between floors. Also any exposed fiberglass insulation in the ring joist could become air-bourne and lead to respiratory problems.

Check your building codes. IECC 2009 will not allow plenums in framing and has been adopted in many states.

Posted By Dteltech on 01 Jun 2010 06:48 PM
Check your building codes. IECC 2009 will not allow plenums in framing and has been adopted in many states.

Can you give the specific part of the code that states that?

to me, it sounds like you would be better off to research a radiant heat solution by way of a staple up system, and a slab system in any concrete floor. Check out radiantdirect.com look at the polaris water heater then search for pex staple up products and manifolds etc...