Downward venting

What is going on

Downward venting is when the vent has to go down before it goes out, usually through a basement wall or floor. It works, but it is limited in length, and because water naturally runs down the pipe you need a condensate trap on the vent so it does not run back into the furnace. Think of it like a kitchen sink trap. Slopes and supports are what keep the system working long term.

Likely causes

• Downward run longer than allowed
• No condensate trap on the vent, so water runs to the furnace
• Horizontal pipes without slope holding water
• Trap drain not connected to a proper drain
• Termination too low or too high above the roof

Check in this order

1. Limit downward vent length to a maximum of 15 equivalent feet.
2. Install a condensate trap of at least 6 in height on the vent pipe.
3. Slope all horizontal pipes at 1/4 in per foot and support them at no more than 3 ft intervals.
4. Connect the vent and inlet condensate trap to a condensate drain pump, an open or vented drain, or the outlet hose of the furnace condensate trap.
5. Keep 12 in minimum clearance above the highest expected snow level, not exceeding 24 in above the roof.
6. After startup, check that the trap holds water and drains freely.

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)