Venting through a masonry chimney

What is going on

Venting a condensing furnace through a masonry chimney is only allowed under specific conditions, and the chimney must never be one that serves a solid-fuel appliance, wood-burning stove, oil furnace or incinerator. The manual has conflicting text on sharing a chimney with another gas appliance, so do not assume it is allowed. Confirm the chimney's condition and the code requirements with a licensed technician before you proceed, and make sure nobody touches anything until you have confirmed it is safe. Acidic condensate can destroy an unlined chimney.

Likely causes

• Chimney also serving a wood, oil or incinerator appliance
• Unsealed flue pipe joints leaking flue gas
• Condensate leaking to the chimney base from an unsealed elbow
• Pipe not centered or supported in the chimney
• Insufficient clearance to combustibles where pipe enters a wall

Check in this order

1. Do not run the vent through a chimney used for wood-burning or oil furnaces or incinerators.
2. Confirm what the chimney is serving and whether sharing with another gas appliance is permitted, since the manual conflicts on this.
3. Support the single-wall flue pipe and center it in the chimney opening with angles or an equivalent method.
4. Seal all flue pipe joints.
5. Seal the 90 degree elbow connection to the vertical pipe to prevent condensate leaking to the base of the chimney.
6. Keep 6 in minimum clearance to the joint where the pipe enters a noncombustible wall or chimney.
7. Have a licensed technician verify the vent and check for flue gas spillage before use.

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)