Check venting and combustion air on non-direct vent installs

What is going on

Test for spillage and proper draft after startup when the furnace shares a vent with other gas appliances. Carbon monoxide poisoning risk: do this for each appliance on the venting system.

Likely causes

• Vent undersized or oversized after replacing a Category I furnace
• Blocked, leaking or poorly pitched vent
• Insufficient combustion or makeup air

Check in this order

1. Seal any unused openings in the venting system.
2. Inspect vent size, horizontal pitch, blockage, leakage and corrosion.
3. Close building doors and windows and doors to other spaces as far as practical.
4. Close fireplace dampers.
5. Turn on clothes dryers, other non-vented appliances and exhaust fans at full speed. Do not run a summer exhaust fan.
6. Light the appliance and set the thermostat so it runs continuously.
7. After 5 minutes of main burner operation, hold a flame at the draft hood relief opening to check for flue spillage.
8. If spillage occurs, correct the vent or add combustion air, then resize the vent using the National Fuel Gas Code or CSA B149 tables.
9. Return doors, windows, fans and dampers to their normal condition.

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