HVACopediaRheem · Furnace troubleshooting
Read target airflow from the CFM LED
What is going on
RGRM/RGTM controls have a CFM LED that blinks out the airflow the blower is aiming for. Use it as a rough check of blower setup, not as a measurement.
Likely causes
- Dip switch airflow or temperature rise settings wrong for the installation
- Cooling or heating airflow profile not set for the system size
Check in this order
- Watch the CFM LED during the call (heat or cool) and count the blinks until the pause of about one second.
- Each full blink is about 100 CFM and a half blink about 50 CFM; for example 11 and a half blinks is roughly 1150 CFM.
- Compare with the airflow the system needs and with the furnace dip switch tables (temperature rise and cooling airflow selections).
- Remember this is the TARGET the motor is aiming for; real airflow can be quite different with duct restrictions, dirty filters or high static.
- To confirm actual airflow, measure the temperature rise in heat (supply minus return) and total external static pressure, and compare with the rating plate range.
Applies to: Rheem RGRM upflow and RGTM downflow/horizontal two-stage condensing gas furnaces (90 Plus)
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Source: Rheem RGRM/RGTM Installation Instructions 92-24161-84-01 (Nature Air RGRM-04*MAES on ManualsLib), Troubleshooting - CFM LED, p.53; Table 11 Temperature Rise by Model, p.47. 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.