DUCT DESIGN FOR THE TIME CONSTRAINED WORLD

Transcription

1 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 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 Step 3: Size the total gross return area using the following chart SYSTEM CFM Gross Area Return Size (Sq.Inches) Duct Size Flex Duct Sheet Metal Maximum airflow Maximum airflow for duct size for duct size 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 8X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X30 Step 5: Write the size of each duct on the duct sketch

2 REQUIREMENT ONE Room-by-room air flow requirements 2

3 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 Natural Gas Output 36,000 56,000 56,000 72,000 72,000 92, ,000 LP Gas Output 32,000 48,000 48,000 64,000 64,000 80,000 96,000 AFUE Tons 0.5 ESP

4 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

5 REQUIREMENT FOUR The filter pressure drop at total system air cannot exceed.10 IWC. 5

6 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

7 REQUIREMENT SIX Pick registers with a minimum throw of five feet. 7

8 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

9 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 Gas Input BTUs/hr System CFM

10 STEP THREE Size the Return Grille(s) SYSTEM CFM Gross Area Return Size (Sq. Inches) 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

11 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

12 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 8X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X30

13 STEP 5: NOTE DUCT SIZES ON SKETCH