duct design for the time constrained world

duct design for the time constrained world

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
1.5
2
2.5
3
3.5
4
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.
System
CFM
600
800
1000
1200
1400
1600
Gas Input
BTUS/HR
40000
60000
80000
Duct Size
SYSTEM CFM
600
900
1200
Step 3: Size the total gross return area using the following chart
SYSTEM 600 700 800
CFM
Gross Area
Return Size 300 350 400
(Sq.Inches)
900 1000 1100 1200 1300 1400 1500 1600 1700
450 500
550
600
650
700 750
800
850
4
5
6
7
8
9
10
12
14
16
18
Flex Duct
Sheet Metal
Maximum airflow Maximum airflow
for duct size
for duct size
30
30
50
50
75
85
110
125
175
200
225
240
325
350
475
525
700
750
1000
1200
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
60
90
120
150
180
210
240
270
300
330
4 Inch
4X6
4X8
4X10
4X12
4X16
4X18
4X20
4X22
4X22
4X24
Design
CFM
60
110
160
215
270
325
375
425
500
550
600
650
725
800
6 Inch
6X4
6X6
6X8
6X10
6X12
6X14
6X16
6X18
6X20
6X22
6X24
6X26
6X28
6X40
Design
CFM 8 Inch
90
8X4
160
8X6
230
8X8
310
8X10
400
8X12
500
8X14
575
8X16
675
8X18
750
8X20
850
8X22
925
8X24
1025
8X26
1100
8X28
1200
8X30
Design
CFM 10Inch
125
4X10
215
6X10
310
8X10
425
10X10
550
10X12
675
10X14
800
10X16
925
10X18
1050 10X20
1200 10X22
1325 10X24
1425 10X26
1550 10X20
1675 10X30
Design
CFM
150
275
400
550
675
800
950
1100
1250
1400
1600
12
Inch
4X12
6X12
8X12
10X12
12X12
14X12
16X12
18X12
20X12
22X12
24X12
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
Heating Capacity
Input
Natural Gas Output
LP Gas Output
AFUE
Tons AC @ 0.5 ESP
GMS8 GMS8 GMS8 GMS8 GMS8
0453AN 0703AN 0704BN 0904BN 095CN
GMS8
1155CN
GMS8
1405DN
45,000
36,000
32,000
80
3
115,000
92,000
80,000
80
5
140,000
112,000
96,000
80
5
70,000
56,000
48,000
80
3
70,000
56,000
48,000
80
4
90,000
72,000
64,000
80
4
90,000
72,000
64,000
80
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
Gas Input
BTUs/hr
System
CFM
1.5
600
40000
600
2
800
60000
900
2.5
1000
80000
1200
3
1200
3.5
1400
4
1600
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 300 350 400 450 500 550 600 650 700 750
(Sq. Inches)
800
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
850
STEP FOUR
Size each duct section according to
these tables
Duct Size
4
5
6
7
8
9
10
12
14
16
18
Flex Duct
SHEET METAL
Maximum airflow Maximum airflow
for duct size
for duct size
30
30
50
50
75
85
110
125
175
200
225
240
325
350
475
525
700
750
1000
1200
1400
1600
STEP FOUR CONT.
SQUARE TO ROUND CONVERSIONS
DESIGN
CFM
60
90
120
150
180
210
240
270
300
330
4 Inch
4X6
4X8
4X10
4X12
4X16
4X18
4X20
4X22
4X22
4X24
Design
ICFM
60
110
160
215
270
325
375
425
500
550
600
650
725
800
6 Inch
6X4
6X6
6X8
6X10
6X12
6X14
6X16
6X18
6X20
6X22
6X24
6X26
6X28
6X40
Design
CFM
90
160
230
310
400
500
575
675
750
850
925
1025
1100
1200
8 Inch
8X4
8X6
8X8
8X10
8X12
8X14
8X16
8X18
8X20
8X22
8X24
8X26
8X28
8X30
Design
CFM
125
215
310
425
550
675
800
925
1050
1200
1325
1425
1550
1675
10 Inch
4X10
6X10
8X10
10X10
10X12
10X14
10X16
10X18
10X20
10X22
10X24
10X26
10X20
10X30
Design
ICFM
150
275
400
550
675
800
950
1100
1250
1400
1600
12 Inch
4X12
6X12
8X12
10X12
12X12
14X12
16X12
18X12
20X12
22X12
24X12
STEP 5: NOTE DUCT SIZES ON SKETCH