Venting check when an existing furnace is removed from a shared vent

What is going on

If the old furnace was on a vent that also serves other gas appliances, the vent may now be too large for the remaining appliances. Test each remaining appliance on the shared vent with the others off.

Likely causes

• Vent sized for the removed furnace is oversized for the remaining appliances.
• Blockage, restriction, leakage or corrosion in the vent.

Check in this order

1. Seal any unused openings in the venting system.
2. Inspect the vent for correct size and horizontal pitch, and check for blockage, restriction, leakage, corrosion or other faults.
3. Close building doors and windows, and doors to other spaces as far as practical.
4. Close fireplace dampers.
5. Turn on clothes dryers and any appliance not on the vent. Run exhaust fans, such as range hoods and bathroom fans, at maximum speed. Do not run a summer exhaust fan.
6. Light the appliance being checked and run it continuously on high demand.
7. After 5 minutes of main burner operation, check for spillage at the draft hood relief opening using a match or candle flame.
8. If spillage or improper venting is seen, correct the vent per NFPA 54 / ANSI Z223.1 or CAN/CSA B149.1-15.
9. When the vent passes, return doors, windows, fans, dampers and other appliances to their prior condition.
10. If the vent must be resized, use the correct table in Appendix G of ANSI Z223.1 or CAN/CSA B149.1-15.

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)