Flex Duct Sizing
Can anyone give me advice on the proper size flex duct I should use in my attic? (diameter size). I have a 1600 sq ft. house with the coil approx. centrally located in the attic. I have 2 supply registers in my 336 sq ft. living room, one register is approx. 4 ft. away from the coil, the other register is approx. 15 ft. away from the coil. I have 1 supply register in my 182 sq ft. MBedroom that's approx. 14 ft from the coil. The second bedroom is 130 sq ft. and the register is approx. 19 ft. from the coil. In my kitchen I have 1 register that's approx. 17 ft. from the coil. Any advice would be greatly appreciated! airman.1994 Let me send him a PM maybe wake him up. :-) Originally Posted by ssporty21228 Can anyone give me advice on the proper size flex duct I should use in my attic? (diameter size). I have a 1600 sq ft. house with the coil approx. centrally located in the attic. I have 2 supply registers in my 336 sq ft. living room, one register is approx. 4 ft. away from the coil, the other register is approx. 15 ft. away from the coil. I have 1 supply register in my 182 sq ft. MBedroom that's approx. 14 ft from the coil. The second bedroom is 130 sq ft. and the register is approx. 19 ft. from the coil. In my kitchen I have 1 register that's approx. 17 ft. from the coil. Any advice would be greatly appreciated! There is a proper sequence when doing initial or retro work on HVAC systems. First, a heat-gain heat-loss calc for each room should be done. Second, if possible renovate to further reduce high heat-gain areas reduce air infiltration. Then you can properly size the equipment begin to design the duct system. We don't know how much cooling BTUH needs to go to that room so you can select the duct sizes for each run. If you do enough searchers for duct sizing you will find some information that might be helpful. FIGURING ROOM CFM - DUCT SIZES from my duct installation page: Quick method: The heat gain and Btu/hr of cooling is done for each room. At 400-CFM per/ton of cooling, 12,000-BTUH / 400 = 30-Btu/hr for each (CFM) Cubic Foot per Minute of Airflow. At 450-CFM per/ton cooling use 12,000 / 450 = use 26.66-BTUH per each Cubic Feet (per) Minute airflow. Then Select Supply duct size by required cfm, 600-FPM branch duct velocity, optimal Supply Air *(FR) Friction Rate. Say a Room requires 3000-Btu/hr divided / by 30 equals 100-CFM, or around a 6 dia. RD metal duct. Formula for finding CFM Airflow If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM: CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area). I.E., 16 Rd duct 201-sq.ins. X's 0.00694 = 1.39494-sq.ft. X's Velocity of 800-fpm = 1116-CFM Times 1000-FPM = 1395-CFM. Branch ducts: 7 Rd duct 38.48-sq. ins. X's 0.00694 = 0.2670512-sq.ft. X's 500-fpm=133.5-cfm It is better to use a ductulator to enable the use of the appropriate Velocity Friction Rate balance to achieve the correct CFM on each Branch Run. The Edit: Flex Duct use one size larger than metal ducts must be stretched tight with no bends, strap supported with minimal sagging. You need a pro! HVAC RETIRED - udarrell I'm awake now! I was watching the 10th of an inch of rain I just got! 9 more inches to be normal. Like said you will have to do a heat load (manuel J $50) on the home so you can determine how much air you will need per room. With out that it is just a guess. One box of 8inch flex will cost you that so if you run the duct with out it and it is the wrong size you will still be out the $50. spend the money and do it right the first time!
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