HVACopediaAllied · Furnace troubleshooting
Exhaust transitions in upflow and horizontal runs
What is going on
Figures 22 and 23 in the Allied manual show typical exhaust connections for upflow and horizontal runs. The short version: you can go up in pipe size, but only where the manual allows it, and the exhaust and intake pipe stay the same diameter. Take a look at the figures before cutting any pipe.
Likely causes
- Pipe size transition made in the wrong place or run direction
- Long 2 in. section or step-up that traps condensate
- Exhaust and intake pipe of different diameters
Check in this order
- In upflow direct or non-direct vent, make the transition in vertical runs only.
- In horizontal runs, keep any 2 in. step-up as short as possible and keep the bend within 45 degrees.
- Keep the maximum straight 2 in. section at the exhaust connection at 12 in.
- Use the same diameter for the exhaust pipe and the intake pipe.
- Compare your layout to figures 22 and 23 before gluing.
Applies to: Upflow and horizontal direct or non-direct vent
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Source: Allied 92G1UH installation manual, p. 20 (PDF p. 20). Written in our own words; check the manual for your exact model.
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