DUCT DESIGN FOR THE TIME CONSTRAINED WORLD This is a simplified duct design process. It assumes the user has covered the following requirements before starting: 1: Identify room by room heat loss and heat gain with the designed CFM requirements for each room 2: Select the smallest capacity equipment that meets the whole house heating and cooling loads 3: Identify the fan curve for the equipment selected. The fan MUST be able to meet the total system airflow at no greater than.6 IWC 4: Identify the filter pressure drop at total system air flow. The pressure drop cannot exceed.10 IWC 5: Don t do dumb stuff 6: Select registers with throw in mind Step 1: Sketch the system on the floor plan with each register delivering no more than 150 cfm. It s your job to know where the beams will be so take them into account during layout. Write on the sketch how much air each register will deliver. Write on each duct how much air it will be carrying. If a trunk serves three 100 cfm duct runs, it needs to carry 300 CFM. Heat Pump Or AC Tonnage System CFM 1.5 600 2 800 2.5 1000 3 1200 3.5 1400 4 1600 Step 2: From the following charts, determine the highest system flow rate you will need. It is recommended that you size as if an AC system will be installed even if you are not presently installing one. This will be used to size the main plenum and main return duct. Gas Input BTUS/HR SYSTEM CFM 40000 600 60000 900 80000 1200 Step 3: Size the total gross return area using the following chart SYSTEM 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 CFM Gross Area Return Size 300 350 400 450 500 550 600 650 700 750 800 850 (Sq.Inches) Duct Size Flex Duct Sheet Metal Maximum airflow Maximum airflow for duct size for duct size 4 30 30 5 50 50 6 75 85 7 110 125 8 175 200 9 225 240 10 325 350 12 475 525 14 700 750 16 1000 1200 18 1400 1600 Step 4: Use the duct size chart and the square to round chart to size each section of duct work. Note these sizes on each section of duct work. If a duct run is greater than 25 feet or has more than 4 fittings, upsize the duct one size. SQUARE TO ROUND CONVERSIONS DESIGN CFM 4 Inch CFM 6 Inch CFM 8 Inch CFM 10Inch CFM 12 Inch 60 4X6 60 6X4 90 8X4 125 4X10 150 4X12 90 4X8 110 6X6 160 8X6 215 6X10 275 6X12 120 4X10 160 6X8 230 8X8 310 8X10 400 8X12 150 4X12 215 6X10 310 8X10 425 10X10 550 10X12 180 4X16 270 6X12 400 8X12 550 10X12 675 12X12 210 4X18 325 6X14 500 8X14 675 10X14 800 14X12 240 4X20 375 6X16 575 8X16 800 10X16 950 16X12 270 4X22 425 6X18 675 8X18 925 10X18 1100 18X12 300 4X22 500 6X20 750 8X20 1050 10X20 1250 20X12 330 4X24 550 6X22 850 8X22 1200 10X22 1400 22X12 600 6X24 925 8X24 1325 10X24 1600 24X12 650 6X26 1025 8X26 1425 10X26 725 6X28 1100 8X28 1550 10X20 800 6X40 1200 8X30 1675 10X30 Step 5: Write the size of each duct on the duct sketch
REQUIREMENT ONE Room-by-room air flow requirements 2
REQUIREMENT TWO Select the smallest capacity equipment that meets the whole house heating and cooling loads. GMS8 Specifications GMS8 0453AN Heating Capacity GMS8 0703AN GMS8 0704BN GMS8 0904BN GMS8 095CN GMS8 1155CN GMS8 1405DN Input 45,000 70,000 70,000 90,000 90,000 115,000 140,000 Natural Gas Output 36,000 56,000 56,000 72,000 72,000 92,000 112,000 LP Gas Output 32,000 48,000 48,000 64,000 64,000 80,000 96,000 AFUE 80 80 80 80 80 80 80 Tons AC @ 0.5 ESP 3 3 4 4 5 5 5 3
REQUIREMENT THREE The fan curve for the air handler you select MUST be able to meet the total system airflow at no greater than.5 IWC for PSC motors or.7 for ECMs 4
REQUIREMENT FOUR The filter pressure drop at total system air cannot exceed.10 IWC. 5
REQUIREMENT FIVE Don t do dumb stuff Stretch the flex Keep duct runs short Use elbows on boots Use turning vanes on the return and supply air handler fittings Don t install the flow killers (keep individual fitting s ELs below 50) 6
REQUIREMENT SIX Pick registers with a minimum throw of five feet. 7
STEP ONE Sketch the system on the floor plan with no more than 150 cfm being delivered by each register. It s your job to know where the beams and obstructions will be, so take them into account during layout. Write on the sketch how much air each register will deliver. Write on each duct how much air it will be carrying. If a trunk serves three 100 cfm duct runs, it needs to carry 300 CFM. 8
STEP TWO From the following charts, determine the highest system flow rate you will need. It s recommended that you size as if an AC system will be installed even if you are not presently installing one. This will be used to size the main plenum and main return duct. Heat Pump or AC Tonnage System CFM 1.5 600 2 800 2.5 1000 3 1200 3.5 1400 4 1600 Gas Input BTUs/hr System CFM 40000 600 60000 900 80000 1200 9
STEP THREE Size the Return Grille(s) SYSTEM CFM 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 Gross Area Return Size (Sq. Inches) 300 350 400 450 500 550 600 650 700 750 800 850 A GENERAL RULE: 2 cfm per sq. inch Examples: 3 tons: 1200 CFM/2 = 600 sq inches 2 tons: 800 CFM/2 = 400 sq inches
STEP FOUR Size each duct section according to these tables Duct Size Flex Duct SHEET METAL Maximum airflow for duct size Maximum airflow for duct size 4 30 30 5 50 50 6 75 85 7 110 125 8 175 200 9 225 240 10 325 350 12 475 525 14 700 750 16 1000 1200 18 1400 1600
STEP FOUR CONT. SQUARE TO ROUND CONVERSIONS DESIGN CFM 4 Inch ICFM 6 Inch CFM 8 Inch CFM 10 Inch ICFM 12 Inch 60 4X6 60 6X4 90 8X4 125 4X10 150 4X12 90 4X8 110 6X6 160 8X6 215 6X10 275 6X12 120 4X10 160 6X8 230 8X8 310 8X10 400 8X12 150 4X12 215 6X10 310 8X10 425 10X10 550 10X12 180 4X16 270 6X12 400 8X12 550 10X12 675 12X12 210 4X18 325 6X14 500 8X14 675 10X14 800 14X12 240 4X20 375 6X16 575 8X16 800 10X16 950 16X12 270 4X22 425 6X18 675 8X18 925 10X18 1100 18X12 300 4X22 500 6X20 750 8X20 1050 10X20 1250 20X12 330 4X24 550 6X22 850 8X22 1200 10X22 1400 22X12 600 6X24 925 8X24 1325 10X24 1600 24X12 650 6X26 1025 8X26 1425 10X26 725 6X28 1100 8X28 1550 10X20 800 6X40 1200 8X30 1675 10X30
STEP 5: NOTE DUCT SIZES ON SKETCH