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Proper Vent Sizing. Natural Draft Furnaces Fan Assisted Furnace Common Vent Systems. Natural Draft Furnaces. Note the furnace vent connector DIAMETER. Note the furnace input BTU/H rating Note the vent HEIGHT [vertical length] Note the vent LATERAL [horizontal length]. - PowerPoint PPT Presentation
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Proper Vent Sizing
Natural Draft FurnacesFan Assisted Furnace
Common Vent Systems
Natural Draft Furnaces
Note the furnace vent connector DIAMETER.
Note the furnace input BTU/H rating Note the vent HEIGHT [vertical length] Note the vent LATERAL [horizontal
length]
HeightH
LateralL
NATMax
NATMin
6 0 46 862 36 674 34 64
10 0 53 1002 42 815 40 77
Natural Draft Vent ChartFurnace Input Rating in Thousands of BTU Per Hour
Fan Assisted Furnace ChartAppliance Input Rating in Thousands of BTU Per Hour
HeightH(ft)
LateralL(ft)
Min
3”FANMax
NATMax Min
4”FANMax
NATMax
6 0 0 78 46 0 152 86
2 13 51 36 18 97 67
4 21 49 34 30 94 64
8 0 0 84 50 0 165 94
2 12 57 40 16 109 75
8 28 49 35 39 98 66
Fan Assisted Furnace Chart
As the vent diameter increases in size, more heat is required to keep the vent dry.
Fan assisted appliances must keep the btu/h input rating between the fan minimum AND fan maximum.
– If the btu input rating is less than the FAN MIN, condensation may occur.
– If the btu input rating is more than the FAN MAX, the flu gas may backup and cause the vent to become pressurized.
Single Wall vs Double Wall
Single wall connectors permit greater heat loss.– Must restrict the use of single wall vent
connectors due to rapid condensation development.
– Building / vent damage will occur due to corrosive condensation.
Common Vent Systems
The installation of two or more gas appliances.– One the same floor level.– Are connected to one common vent system.
Common Vent Systems
Use the ‘common vented appliance tables.’
Use the vent height measured from the taller appliance.
– The taller appliance will have least total height.
Use the vertical height of the connector rise.
Common Vent Systems
VentHeightH(ft)
Conn.RiseL(ft) Min
3”FANMax
NATMax Min
4”FANMax
NATMin
6 1 22 37 26 35 66 46
2 23 41 31 37 75 55
3 24 44 35 38 81 62
8 1 22 40 27 35 72 48
2 23 44 32 36 80 57
3 24 47 36 37 87 64
Common Vent Systems
Common venting with a natural draft appliance is allowed.– Follow Min/Max range in the common vent
connector table.
– Use double wall for the ‘fan assisted’ appliance.
May use single wall vent for the natural draft appliance.– Must use double wall vent for the ‘common’ vent.
Common Vent Systems
The common vent uses the ‘common vent table’– Must use NAT / FAN column when using
Natural Draft combined with Fan Assist.– Must use FAN / FAN column when using Fan
Assist combined with Fan Assist.
Common Vent SystemsCombined Appliance input rating in thousands of btu/hour
VentHeightH(ft)
FANFAN
4”FANNAT
NATNAT
FANFAN
5”FANNAT
NATNAT
6 NR 78 64 NR 113 99
8 NR 87 71 NR 126 111
10 NR 94 76 163 137 120
15 121 108 88 189 159 140
25 131 118 98 208 177 156
30 145 132 113 236 202 180
Common Vent Systems
Vent tables take into account for two 90º elbows for the entire vent system– When more than two 90º elbows are used, the
maximum capacity listed must be reduced by 10% for each additional elbow or equivalent.
– Two 45º equal one elbow– Using 45º elbows instead of 90º is preferred
• Less restriction the better
Common Vent Systems
Always maintain 1/4” rise per foot of LATERIAL [horizontal] run-out.
Always maintain 75% less horizontal run-out of vertical height.
Terminate 2 feet higher than any structure with 10 feet of the vent.
