Whole Floor Air Plenum

Plumdog_2

and check with your local code authority. You can't use wood frame cavities for air distribution. There are a lot of reasons why it is not code; and lots more reasons besides just code. Temperature, fire, dust, humidity, mold, come to mind. But if you are building in one of the unregulated areas; then you can do pretty much anything you want. The mountains of the west are full of crumbling, decaying, distressed, fire hazards that people sell over and over as "homes". Buyer beware.

Art M.

Whole Floor Air Plenum

I'm building a 2 story house and plan on using the floors as the air plenum. The floor trusses are of an open web design. Does anyone see any problems? Also, heating will be with a Baxi and air handlers.

Thanks

GMcD

Check your local codes

Most Building Codes do not allow any combustibles inside a return air plenum, depending on type of construction. Lined wood joist bays are one thing, but an open wood frame plenum may be another animal depending on your local Code definitions. Also, from a practical point of view, you have to insure a near zero air pressure drop path back to the air unit to insure that you get anywhere near proper return air movement. The biggest problem with most return air ceiling plenums is that most of the return air short circuits back from the closest grilles, and the furthest points get very little return air movement -very tough to balance. Run-outs of return air ducts to get a return inlet in the ceiling plenum near the middle of open ceilings may help insure more even return air flows.

John T_3

Floor Plenum

I don't think you'd want to have such an open system in the event of a fire. A round ducted system may be worth consideration. IMHO

John T.

Art M.

Whole Floor Air Plenum

The whole floor air plenum would supply the warm air. Ducts would be used for the return.

Ted_4

Floor Plenum

We have been doing a lot of blower door and duct leakage testing of new construction that uses open-web floor trusses. These buildings are turning out to be very leaky. Air-sealing is extremely tricky and must be carefully done on the perimeter AND at any framing cavity connections within the building. In our experience it is almost impossible to create a 100% seal.

Therefore, we do not recommend the use of any framing cavities for return or supply air. A typical furnace fan creates a high pressure differential. If you use the floor cavity for supply, you will pressurize it and most certainly be sending a signifcant percentage of the supply air right outdoors. If you use it as a return plenum, you will depressurize it and be drawing some return air from the outside.

Our rules-of-thumb are: fully ducted supply and return with no duct runs outside conditioned space; liberal application of mastic on all joints and seams; and full air-sealing of the building from sill plate to top plate.

clammy

open pluemun

I have worked on these types of systems in older office building that where built in the mid 60's and let me tell you they stunk then and will probalby stink now .It is in my opion a waste of cooling and heat either way open supply or a open return system are a waste .Have a good time balancing either one is a PIA .The other fact is that all your wiring in either case should be telfon coated and rated for pluemum use same goes for electric to also the other great thing all the dust and crap that will in time cover every thing in your pluemun and lets not forget about the attic or roof heat load which should make you have to increase your a/c tonnage .Also if either steel or wood roof decking will be exposed to cooler air may start to form condensation.I have recentally seen a open return system 5 of them installed in a new building i just shook my head and left .These type of systems only benefit could be a cheaper job because youyr not using all the metal but operating costs will be high and comfort level low .Even with most or alot of commerical system using pleumium return they are hard at best to get balanced with insulation of both supply and return sized to each work space and load i have mananged to get alot of my commerical office jobs within 2 degrees of the sytem sta temp .Not bad for 1 unit and about 12 to 14 offices .open supply or return system is only saving money inthe beging just do it right peace and good luck clammy