HVACopediaAllied · Furnace troubleshooting
Upflow 5-ton units with 1800 cfm or more airflow
What is going on
Upflow 20C and 20D (5 ton) units that need 1800 cfm or more have to be set up with one of the listed return air arrangements. Too little return opening means high velocity, noise and a poor temperature rise.
Likely causes
- Single undersized return for a 5 ton airflow
- Filter or transition too small for the cfm needed
- Return arrangement not on the approved list for 1800 cfm or more
Check in this order
- Use single side return with a transition sized for a 20 x 25 x 1 in. cleanable filter.
- Or use single side return with the optional RAB Return Air Base.
- Or use bottom return air.
- Or use return air from both sides.
- Or use bottom and one side return air.
- Check the blower performance tables for the airflow limits before you commit to an arrangement.
Applies to: 20C and 20D (5 ton) size units, upflow installations
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Source: Allied 92G1UH installation manual, p. 6. Written in our own words; check the manual for your exact model.
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