Min Floor Truss sizing for HVAC Returns?
I’m need some help to plan out my house. It’s a 1.5 story, 1500 SF Bungalow (actually similar to the Houses Issue cover) on a slab. I know HVAC’ers need way more info, but how big a return do you normally need for about 800 SF of floor space? I believe this will be my limiting factor in sizing the trusses (not the span).
Am I right/wrong in assuming you can fit a duct 2″ smaller than the truss (a 12″ return in a 14″ truss)? Trusses would be 24″ OC w/ no double top/bottom cords. The unit will probably go in a 1st floor insulated bump-out. I’m also considering using some of the advanced framing techniques (2×6) from http://www.buildingscience.com/housesthatwork/advancedframing/default.htm , so that may affect the load calcs.
Replies
bump
"I can't say I was ever lost, but I was bewildered once for three days."
Mark,
You're right in that we (HVAC'ers) would need much more info. BUT, some rules that apply to common construction throughout the middle of the country, when a single- forced-air system is to providing heating and cooling.
Air flow during cooling is the limiting "mode" that all the ducts should be sized to accomodate. 1 ton of cooling nominally is "equal" to 400 cfm of air flow.
Obviously in Houston, a different ton/sf would be used than in Chicago as an ESTIMATE. Guessing and assuming is good for budget and scoping purposes. Details are improtant.
1/2 to 3/4 cfm per sq/ft of air flow or 800 sf/ton to 500 sf/ton are a good range "rule of thumb's" to look at for this. If you live in Atlanta, I'd say 600 cfm supply and return. If you live in Cleveland, I'd say 400 cfm supply and return. Returns ducts, properly sized, move air, during cooling operation, at no more than 500 feet/minute. A 12x8 rect duct is good for up to 320 cfm return (or up to 440 supply).
You should take 4" off of the O.C. distance for rectangular ducts to fit between joists. 12" in 16 O.C.
You're PARTIALLY correct in saying you can get a 12" tall duct in a 14" tall truss. But ONLY in the center of the span in a duct opening. You can't run a duct that tall through a truss where the webs are.
As a rule of thumb, I tell folks to figure on 18" deep (or deeper) floor trusses if they want to get everything in the truss space. Don't forget you'll also have plumbing runs and electrical to work with.
There are a lot of options with floor trusses and HVAC. The HVAC guy can run 2 smaller returns in some cases. Or simply attach metal to the webs of the truss and form duct runs.
Have your truss guy and HVAC guy get together BEFORE anything is built and work out the details. I can't tell you how many times I've gotten calls asking if they could cut out some of the webs to put duct runs in. (You generally can't)
You can't build the house, the bring the HVAC guy in later and tell him to stick everything up in the trusses. If it isn't planned in advance, you can end up with a VERY unhappy HVAC guy.
.
Just a side note -
I had an HVAC guy ask me once if he could cut the entire bottom chord of the trusses out where the return went. That way he figured he could assemble it on the floor, then lift the whole thing in place and tack the bottom chords back together.
The answer was an emphatic "NO", just in case you're wondering.