Forum / THE MAIN WALL / Duct Sizing

## Duct Sizing (7 Posts)

• ### Duct sizing

Anyone know where I can get a good chart for sizing duct work for residential hvac systems?
• ### I'm guessing it's more complicated than that.

It was a real eye-opener for me when I took a course in forced air heating system design, there's a lot more involved than I had thought, and it took a lot of time to figure out. The Manual D method involves calculating your heat loss, and the amount of cubic feet per minute of air to deliver that heat, After determining the longest run, with all the fittings involved, you can determine static pressure, which drives which blower you select. Air flow needed for other runs drives duct size for those areas. Hope I'm not belaboring the obvious, I just didn't want anyone to think designing a duct system is as simple as using a chart. I like ACCA's duct sizing wheel, but you need to do all that other stuff first.
• ### Duct Sizing Calculators

Probably the most common duct sizing calulator everyone seems to use in my experience is the "Ductulator" from Trane. It is a manual rotary slide calculator. Similar in concept to the pipe sizing "wheel" the B&G Pumps offers. Most designers size duct using equal friction method, sizing at .08 inches of water column per 100 equivelant feet. SMACNA has a more sophisticated and pricey "HVAC Duct Design Calculator". This baby sports typical duct loss co-efficients, using velocity pressure, to nail down actual pressure losses for each fitting. The Trane unit is fine 99% of the time. For total pressure loss, once the required cfm is determined, I never use the duct software programs. Hand calcs work fine for most jobs. A common quickie rule of thumb is to multiply total duct footage by 1.5 to account for elbows, etc. For a real twisty, convoluted duct layout, use a 2.0 multiplier. These are for general estimates, and usually, especially for small jobs, will work fine. Hope this helps..... Tom Anderson
• ### duct sizing

Now duncan you know that not a guess,for a guess you sure hit the nail on the head.Duct calculator is only one tool to help get the job done.With most residential you have a low pressure system,velocities less than 2000 fpm and duct static pressure less than 2 inches wg. Pressure rise/drop=airflow/cfm =air demand=rpm=velocity. there are many variations of air distribution system and if you are willing to learn proper air distribution design and balancing you will be on top of your game and way ahead of most.Anybody can do ductwork, so they say, Good luck don
• ### duct is a PIPE

for air , and needs to be sized for proper flow justlike steam or water pipe. Guestimates and rules of thumb are just that a guess. Take 4 identical houses on a culdesacand chances are the loads will be different and the ducts for any given room will be different size. Things like solar gain wind exposure and north south orriantation do make a difference