Removing the boiler from a common vent system

What is going on

Pull a boiler off a common vent and the remaining appliance is now on a vent sized for more. Spillage and CO risk. This check confirms the remaining appliance still vents properly. Do it every time a boiler comes off a common vent, and use the NFGC / ANSI Z223.1 rules.

Likely causes

• A boiler removed from a shared flue during replacement or conversion
• Remaining appliance now on an oversized vent that may not draft well
• Existing vent damaged, blocked or corroded
• Building under negative pressure from fans and dryers

Check in this order

1. Seal any unused openings in the common vent.
2. Inspect the vent for proper size, horizontal pitch, blockage, leakage, or corrosion.
3. Close building doors and windows, close fireplace dampers, and run exhaust fans at maximum speed (not a summer exhaust fan). Run clothes dryers and other non-vented appliances.
4. Run the remaining appliance continuously and test for spillage at the draft hood relief after 5 minutes with a match, candle or smoke.
5. Return doors, windows, fans and dampers to normal after the test.
6. Correct any improper common vent operation to meet NFGC / ANSI Z223.1.
7. If you see spillage, do not leave the appliance running. Have a licensed tech resize or repair the vent.

More general guides

• Combustion air and leak protection checks during install
• Expansion tank and air elimination
• Install and test the CH and DHW relief valves
• Low water cut-off wiring (field supplied)
• Primary/secondary piping and 3-way valve for heating-only use
• Wall mounting the boiler