Need Duct Sizing Help w/ “Ductulator”
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Hi everyone,
I’m trying to find the simple, or “rule of thumb” method of determining the cfm requirements for a forced air, heat-only system. My unit is a new 175K btu furnace for heating a new 3,000 sf, 2-story in So. CA. The actual area that needs to be heated is 2,200 when places like closets are subtracted.
I bought one of those “Ductulator” slide rules (actually it’s a rotating rule) and through other research, I ‘ve determined that the friction loss setting for flex duct is 0.1. The other variable I need is the cfm requirement per room to be heated. Is there a per square foot basis that can be applied here to determine the cfm’s?
The furnace is capable of 2,270 cfm’s @ 0.5″ w.c. and 1,760 cfm’ @ 1.0″w.c.
Or, is it as simple as determining the percentage that each room occupies and multiply that by the furnace’s cfm?
Again, this is new construction with insulation everywhere, low-e dbl. panes, and a very mild So. California climate. Heck, if it wasn’t for my wife, kids and codes I’d do without the whole mess!
Thanks!
Jim
Replies
Greetings Rock,
This post, in response to your question, will bump the thread through the 'recent discussion' listing again.
Perhaps it will catch someone's attention that can help you with advice.
Cheers
half of good living is staying out of bad situations
Fisrt, you have to include ALL spaces withing the envelope like closets and hallways, etc. Second, 0.1"/100' is out of the range that I use for good duct design. Supply side use 0.08"/100' for the maximum and design the return for 0.05"/100'. I would recommend against using flex duct except in very limited amounts.
As far as the "cfm requirement", there is no such a determination for heating. AFTER you have done a complete heat loss calcuation and determine the design heat loss for each room and the entire space, you would select the smallest furncae that meets that maximum demand. That furncace manufacturer will provide you with the heat input, the heat output and the air flow/static pressure data. I use the air flow at 0.5 inches of static pressure, account for a dirty filter (about 0.2") and a wet coil and size the ducts as required to give the required air flow for cooling. For heating, you use a ratio of the the heat required to the space/the heat required to the entire building x the rated air flow of the unit at the conditions that apply.
"Rules of Thumb" are useful only in quick, rough estimates, not in actual system selection or design.