hvac question

[Parsons]

I am doing a renovation in New Orleans on a raised house ( 3' )with 11 foot ceilings . The Hvac sub is sugesting that we run our ac and heat vents up from the floor with what he calls a downdraft system. The house presently has duct work in the ceiling, but it would simplify my modifications if I could run the ductwork from below. I think the homeowner will benifit as far as heating goes but I have concerns about

the cooling. We run AC here for sometimes 8 or 9 months of the year and

I would hate to be setting the homeowner up with a system that will be a problem for him . If anyone has any advice that might help me make a decision I would appreciate it. thanks

[tbasc]

Parsons, I live in Charleston SC - hot and humid. My first floor has 12'-0" ceilings and air comes from the floor and is exhasted near the ceiling. It has worked well for 27 years. Ducts here are run both in the attic and in the crawl space. Neither system seems to have an advantage when well designed.

With a 3'-0" crawl space in N.O., I would think about moisture / water infiltration.

TB

[Stretch]

There is SOME energy savings with ducts in the crawlspace, typically duct gain (cooling) in the crawlspace would be 0% to 10%. With the ducts in the attic, the gain would be 10% to 20 % depending on duct insulation. But if the ducts are in the crawlspace, you could get humidity and mold problems down there. If you have floor registers, furniture placement can be a problem, as furniture can block registers. Not so with ceiling supplies.

Check this link on crawlspace moisture problems. Also check the Journal of Light Construction and Fine Homebuilding.

Crawlspace problems;

stretch

[HeatMan]

In a perfect world, all heat would be in or near the floor and the cooling would be in or near the ceiling.

[Yeung Fun Ho]

an A/C system is far more energy efficient if you can AVOID running the ductwork in the hot attic.

the best place for duct work, for a low power bill, is inside the air-conditioned space

the next best location is in the crawl space below the house

[Don Young]

Just remember that, unlike old time "gravity" furnace systems, blowers are now used to circulate the air and they blow equally well up, down, or sideways. It's just important that the ductwork and registers be properly designed for the space so that enough air circulates everywhere. Don Young

[nicksanspam]

Nonono. In a perfect world, all heat would be in a hot massy low-e ceiling and all cooling would be in a cold slab floor, with ceiling fans to bring down winter heat and bring up summer cooling in occupied rooms, as in the Barra system.

Nick

[HeatMan]

Okay, how come it's always warmer near the ceiling and the upstairs of a 2 story house?

[Dr. Hardcrab]

Actually, in a perfect world, we would not need heating or cooling.

Then I would have to go back to programming.

Do they still use punch cards?

[nicksanspam]

Sounds like stratification, a desirable thing in a Barra house. Hot air from a sunspace rises up and heats mass in the ceiling without overheating the room. At night and on cloudy days, the ceiling can stay hot while the room stays cold.

Nick

[Parsons]

thanks to everyone for their help

[HeatMan]

Barra house? Never heard of it.

[HeatMan]

That's right, he's the one that said something like "It's deja vu all over again."

[Dr. Hardcrab]

Yogi Barra? Used to play for the Yankees?

Come on!

;-]

[Roy Starrin]

The (Italian) Horazio Barra system is described on pages 169-171 and

181 of Baruch Givoni's Climate Considerations book (Wiley, 1998.) The basic reference is Barra, O. A., G. Artese, L. Franceschi, R. K. Joels and A. Nicoletti. 1987. "The Barra Thermosyphon Air System: Residential and Agricultural Applications in Italy, UK, and in the Sahara." International Conference of Building Energy Management. Lausanne, Switzerland.

Barra's are said to be a lot more comfortable than other passive solar houses,with more uniform north-south temp distribution. His "spancrete" ceiling slabs in single and multistory buildings let hot air-heater air thermosyphon through the slab tunnels from south to north, where it exits and travels back north through the bulk of the room to the air heater inlet near the floor. No fans, and no selective surface beneath, but the hot air store lots of heat in the slabs. Lots of successful systems were built in Europe, but Barra seems fairly unknown in the US.

courtesy your friends @ VirginiaNewsSource.com

[Dr. Hardcrab]

Yogi's real name is Harazio???

Whoduhthunk....

[nick pine]

Gee, that sounds familiar :-) I wrote it.

Nick