Vent and inlet terminal spacing

What is going on

Terminal spacing for the vent and inlet is given in the venting figures. The numbers for each dimension depend on which termination you are building, and the reference point for each one should be confirmed against the manual drawing before you cut pipe. What I usually find going wrong is people mixing dimensions from different termination styles. Pipe supports and insulation also matter here, since a sagging or frozen pipe will cause a pressure switch trip the first time it gets cold.

Likely causes

• Exhaust and inlet ends too close, recirculating flue gas into the intake
• Dimensions mixed up between different termination styles
• Pipe unsupported, sagging and collecting condensate
• Fitting in an unheated space left uninsulated and freezing up

Check in this order

1. Keep at least 9 in between the end of the exhaust pipe and the end of the inlet pipe.
2. Check the 6 in minimum and 24 in maximum dimension (and the 12 in minimum and 15 in maximum dimension) against the drawing for the specific termination.
3. Support vent and inlet pipe at no more than 3 ft intervals.
4. Insulate any fitting that passes through an unheated space.
5. Look at the terminals after install to confirm nothing blocks them.
6. Run a heating cycle and confirm the pressure switch stays closed.

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)