Exhaust pipe transitions and sizing

What is going on

Pipe size comes from table 5, and exhaust and intake must be the same diameter. Step-ups should be kept as short as possible. Getting this wrong shows up as pressure switch trips or poor combustion.

Likely causes

• Pipe diameter not matching table 5 for the model and venting length
• Intake and exhaust of different diameters
• Step-up in pipe size placed in a horizontal exhaust run, where condensate can pool
• Long step-up transition section
• Loose intake pipe at the inlet air connector

Check in this order

1. Size the exhaust pipe using table 5 for the model and venting length, because undersized pipe restricts flow.
2. Use the same diameter for intake and exhaust pipe.
3. Do not step up from a smaller to a larger pipe in a horizontal run of exhaust pipe, since condensate collects at the step.
4. If the pipe must step up in size, use the shortest possible length of 2 in. PVC for that transition.
5. Use transition solvent cement or a sheet metal screw to secure the intake pipe to the inlet air connector.
6. After assembly, check every joint for gaps and confirm the run slopes back toward the furnace as the manual requires.

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