HVACopediaAllied · Furnace troubleshooting
Intake piping for direct vent installations
What is going on
On a direct vent Allied furnace, the combustion air comes from outside and the flue goes outside, so the intake pipe has to be run to the exterior. What I usually find on a bad install is the little factory air intake screen stuck on the outdoor intake. It is not for direct vent, so leave it off. Size the intake from table 5, not from guesswork.
Likely causes
- Factory air intake screen installed on a direct vent intake
- Intake pipe not run to the outside of the structure
- Intake pipe undersized for the length and fittings used
Check in this order
- Do not install the provided air intake screen in a direct vent application.
- Route the intake pipe to the outside of the structure.
- Follow the general guidelines for terminations and the direct vent termination section.
- Size the intake piping from table 5.
- Check all intake joints for tightness after assembly.
Applies to: Upflow and horizontal direct vent installations
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Source: Allied 92G1UH installation instructions manual, p. 25. Written in our own words; check the manual for your exact model.
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