HVACopediaAllied · Furnace troubleshooting
Sizing intake and exhaust vent pipe
What is going on
Use the tables and flowchart to size pipe. Both intake and exhaust must use the same pipe diameter.
Check in this order
- Confirm the furnace capacity 045, 070, 090, 110 or 135.
- Confirm the altitude.
- Choose the termination type standard or concentric (table 3).
- Identify whether intake or exhaust has the most elbows, and count them.
- Choose the desired pipe size (2 in., 2-1/2 in., 3 in. or 4 in.).
- Use table 5 to find the maximum intake or exhaust length. Size intake and exhaust separately, and check the minimum length in table 4.
- The exhaust collar accepts 2 in. schedule 40 pipe. In horizontal runs, step up to a larger pipe only in vertical sections, after a 2 in. elbow.
- Do not transition from smaller to larger pipe in horizontal exhaust runs. Keep any 2 in. step-up PVC as short as possible.
Applies to: Non-direct vent and direct vent installations
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Source: Allied 92G1UH installation manual, p. 21-22 (PDF p. 21-22). Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.