Supply Plenum Mounted High in Attic

[Skip Erwin]

I was measuring the difference in temperature between the return air vent and the supply vents in the second floor of this house today. The difference was around nine degrees. I know that's not the best place to take the measurements, but I'm not a HVAC guy. It was 80 degrees outside. I go in the attic and it jumps out at me that the supply plenum is mounted 7 feet off the bottom of the attic floor. It’s in the top of the attic space where it's hottest. Is that a really bad choice as to where to run the supply plenum? It just struck me as odd that the temperature differential was so low and then I see the plenum mounted in the hottest part of the attic space?

[bbenjamin]

Skip,

More pictures are need to evaluate this system correctly.

[David A. Andersen]

If you want to say something about where the plenum is mounted, why not say something about the lack of ventilation in the attic?

It doesn't matter how high it is located in the attic, if it's properly ventilated for HVAC purposes.

Ventilation requirements for an attic do not pertain when you install an HVAC system there.

An efficiency loss will occur because heat transfer is the area of the exposed duct , times the resistance factor (inverse of R-) of the insulation around the duct, times the temperature difference of the attic space and the supply air. For every degree Fahrenheit of increased heat in the attic equals one multiplier to the equation. So if you drop the temperature 100° (which is feasible) you can reduce the multiplier by 100! Quite substantial.

[rmccullough]

I would recommend insulating the ducting in the attice areas. Heat loss in the winter, cooling loss in the summer.

[Greg Veal]

Skip

Here in Georgia, I see HVAC systems installed in the attic space all the time. I agree, it's not the best/optimum location to locate a system, but if designed and installed properly, they work fine and efficiently.
Your description tells me it is likely an Upflow Furnace setup, and although your PIC is limited, it does look like the plenum is sheetmetal and uninsulated. I recommend you check to see if the Plenums (S/A & R/A) are insulated on the inside. I'll assume the branch ductwork is insulated or flexduct.
Not sure what part of the country you're in, but the system should work fine.

[David A. Andersen]

The amount if insulation makes no differance in the efficency loss!

I often see insulation on water pipes in unheated spaces that appear to be installed to keep water pipe from freezing. If you don't have a heat source, in this case water is never used on an outside faucet in the wintertime, it will freeze. Insulation does not keep the heat loss from occurring, it simply slows it down.

If the attic space is 190° and you're trying to make 50° air, the temperature differential is a multiplier of 140! If you have adequate ventilation and lower the attic temperatures to 95° (outdoor air temperature), there is a multiplier of only 45. That is a 68% reduction in heat gain in the supply air!

HVAC attic ventilation is the cheapest and most efficient means of reducing power consumption and increasing efficiency. The business of HVAC means heating/ventilation/air-conditioning. No one ever discusses the ventilation part of it! Actually I see trucks running around that don't have the "V" in there! It's H/AC!

[Bruce M. Graham, III]

Not to mention the reduced heat gain to the interior of the home.