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.
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. 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 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
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 DESIGN
CFM 4 Inch Design CFM 6 Inch Design CFM 8 Inch Design CFM 10Inch Design CFM Inch 12 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 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.
Step 3: Size the total gross return area using the following chart
Step 5: Write the
size of each duct on the duct sketch
REQUIREMENT ONE
Room-by-room air flow requirements
3
Select the smallest capacity equipment that
meets the whole house heating and
cooling loads.
REQUIREMENT TWO
GMS8 Specifications
GMS8
0453AN
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
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 THREE
The filter pressure drop at total system air
cannot exceed .10 IWC.
5
REQUIREMENT FOUR
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 FIVE
Pick registers with a minimum throw of
five feet.
7
REQUIREMENT SIX
8
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.
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
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.
9
