I recently had the following Rheem ROCB furnance installed.


It's paired with a Rheem 5 ton condenser for cooling. However, I feel the air flow is set way too high. The house cools down very quickly (typical cooling cycle only lasts 7 minutes) and the strength of the air coming out of the vents is very strong.

I was reading the manual of the furnace and there are dip switches to control the rate of air flow. Here is the manual page that discusses it.

ecm motor airflow manual

Currently, the furnace is set to 5 tons. My question is, is it truly as simple as adjusting the dip switches (per the manual settings) to lower the airflow CFM? Would doing this introduce the risk of any other adverse effects to the rest of the system?

  • Have you had a Manual J load calculation run for your house? – ThreePhaseEel Aug 4 '20 at 0:56
  • The installer did one. – Cooper Aug 4 '20 at 1:31
  • Ah, what did they get for results? (I'm glad to hear they actually went to the time and effort to work that out, by the way, way too many installers just rule-of-thumb it, which tends to lead to oversize systems) – ThreePhaseEel Aug 4 '20 at 1:32

The problem with reducing the airflow two far is the air May get much colder and cause condensation on the ducting and possibly icing on the evaporator coil. The model you linked to showed 2-4 ton so if you have a 5 ton coil and condenser you may already be pushing the limit before icing. the thermal expansion valve may need to be adjusted to prevent icing. This can reduce the efficiency but at 7 minutes per cycle it sounds like your system is oversized or was sized for the hottest days not a bad thing but it affects the efficiency (short cycle time requires more starts).

I would measure the air temp at the intake vent and the temp at the discharge vent, make your change and take the same measurements including the cycle time. A general rule of thumb is a 20 degree temp change is your target. There are other things that affects the temp change humidity but 20 degree is quite normal. If you notice a longer cycle without icing and if the air is not uncomfortably cold your lower air flow may be fine it can be a delicate balance between just right and freezing the evaporator, if you notice a lower airflow and a few minutes later again the airflow is still going down or the temp change is reduced the evaporator is probably freezing and the TXV will need to be adjusted to accommodate the lower flow without freezing up.

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