proper vent sizing

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.