Checking a shared vent after this furnace is removed or replaced

What is going on

If this furnace was on a common vent with other gas appliances, the vent may now be too large for the remaining appliances. Test each remaining appliance on its own while the others stay off. Carbon monoxide poisoning can result if this test is skipped.

Likely causes

• Vent sized for the old furnace plus other appliances
• Blockage, restriction, leakage, corrosion or poor horizontal pitch in the vent

Check in this order

1. Seal any unused openings in the common vent.
2. Inspect the vent for correct size, horizontal pitch, blockage, leakage and corrosion.
3. Close building doors, windows and doors to other spaces. Run clothes dryers and non-vented appliances, and run bathroom and range hood exhausts at full speed. Do not run a summer exhaust fan. Close fireplace dampers.
4. Light the appliance being checked and set the thermostat so it runs continuously.
5. After the main burner has run 5 minutes, check for flue gas leaks at the draft hood relief opening with a match or candle flame.
6. Return doors, windows, fans, dampers and other appliances to their previous operation.
7. If a venting problem is found, resize the common vent to the minimum size from the tables in Appendix G of ANSI Z223.1 (National Fuel Gas Code).

More furnace guides

• Annual planned service inspection
• Annual service checklist
• Blower access panel and carbon monoxide hazard
• Burner assembly cleaning
• Change blower speed tap
• Check temperature rise and adjust blower speed