#Hvac duct sizing calculations how to#
#Hvac duct sizing calculations manual#

The supply branch runouts add up to 1480 cfm, which is close, but OK. The four main trunks add up to 1800 cfm, which is good. The revised layout passes this quick test. If they had used a better design methodology they might have come up with this revised version of the same plan. Their numbers are growing, but far too slowly. Kudos to those who actually do a good job. Most installers just use old rules of thumb that result in these substandard systems. (See my article about why we need a simpler Design Methodology).

#Hvac duct sizing calculations manual#

Some even take the time to use Wrightsoft or Elite Manual J/S/D software but then have some hidden setting in them that messes everything up.

#Hvac duct sizing calculations how to#

This system would run at a much higher static pressure and probably would not pass the minimum air flow and maximum fan watt draw test of CAвЂ™s Title 24 energy code (350 cfm/ton and 0.58 watts/cfm) and those are not meant to be hard numbers to beat, but far too few installers know how to properly size ducts. A 16вЂі duct should only handle about 1050 cfm вЂ“ VERY BAD. I frequently see 16вЂі returns on 3.5 and even 4 ton systems. The supply branch runouts add up to 1270 cfm вЂ“ NOT GOOD. The four main trunks add up to 1200 cfm вЂ“ NOT GOOD. LetвЂ™s do it for this sketch: The target airflow would be 3.5 x 400 =1400 cfm. This test will not tell you if the equipment is over or undersized, nor will it tell you how well the system is balanced вЂ“ whether the air is going to rooms in the right amounts, relative to other rooms, but it will quickly identify one of the most common problems: undersized ducts that impact overall airflow to the system.

Are there enough return ducts to handle this much air? Repeat step 2 for the return ducts. Are there enough supply branch runouts to handle this much air? Repeat step 2 for the supply branches (ducts that serve a single supply register).Д¤. Add up all of the flows вЂ“ they should be greater than the target flow (from #1).ДЈ. This table represents how much air a certain size duct should handle in a вЂњreasonably well-designedвЂќ system (friction rate = 0.10 iwc/100ft). Are there enough supply trunks to handle this much air? List the diameters of all the start collars coming off of the supply plenum. If the designer tells you a higher number, use that.Дў. I usually use 400 cfm per ton (condenser tonnage) as a minimum. Figure out how much air is the system is supposed to handle.