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Proper Vent Sizing

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].

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Proper Vent Sizing

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  1. Proper Vent Sizing Natural Draft Furnaces Fan Assisted Furnace Common Vent Systems

  2. 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]

  3. Natural Draft Vent ChartFurnace Input Rating in Thousands of BTU Per Hour

  4. Fan Assisted Furnace ChartAppliance Input Rating in Thousands of BTU Per Hour

  5. 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.

  6. 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.

  7. Common Vent Systems • The installation of two or more gas appliances. • One the same floor level. • Are connected to one common vent system.

  8. 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.

  9. Common Vent Systems

  10. 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.

  11. 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.

  12. Common Vent SystemsCombined Appliance input rating in thousands of btu/hour

  13. 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

  14. 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.

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