Allied Furnace: Exhaust pipe transitions and sizing

Exhaust pipe transitions and sizing on a Allied furnace: 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.

Homeowner with a furnace problem? Call (647) 657-2977 or book a service call — licensed technicians across Vaughan and the GTA.

Applies to: Upflow and horizontal, direct and non-direct vent exhaust and intake piping

Common causes of exhaust pipe transitions and sizing

• 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

How to fix exhaust pipe transitions and sizing: step by step

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.

If the unit shows an error code, look it up: Allied Furnace codes

Models

92G1UH92G1UH0450892G1UH0451292G1UH0700892G1UH0701292G1UH0901292G1UH0901692G1UH1101692G1UH1102092G1UH1352095G1UHA93UHA95UH