General Assist Technical Reference

Cool Air Prevention

Indoor fan control during heating operation
Purpose: During heating operation, the indoor unit limits fan speed while the indoor heat exchanger is still cold. This prevents cool or uncomfortable air being blown into the room while the coil is warming up.
Key point: The indoor heat exchanger temperature must rise to approximately 37°C before the indoor fan is permitted to progress toward the selected operating fan condition. The actual maximum fan speed continues to change as the coil temperature rises.

How the control works

The maximum indoor fan speed is determined from the temperature detected by the indoor heat exchanger thermistor. Different thresholds are used as the coil warms and cools, creating hysteresis so that fan speed does not continuously change around a single temperature point.

Normal operation - initial heating period

Indoor heat exchanger temperature risingMaximum fan conditionTemperature when dropping
42°C and aboveHi or selected fan mode*37°C
39°C to below 42°CMe+ or selected fan mode*34°C
37°C to below 39°CLo or selected fan mode*32°C
30°C to below 37°CCool air prevention control28°C
Below 30°CS-Lo-

* Where the selected fan speed is lower than the permitted maximum, the lower selected speed is used.

Normal operation - after approximately 7 minutes

After approximately seven minutes of heating operation, the control relaxes the cold-air prevention limitation. The fan can operate at a higher minimum speed while still responding to indoor heat exchanger temperature.

Indoor heat exchanger temperature risingMaximum fan conditionTemperature when dropping
42°C and aboveHi or selected fan mode*37°C
39°C to below 42°CMe+ or selected fan mode*34°C
37°C to below 39°CLo or selected fan mode*32°C
30°C to below 37°CLo or selected fan mode*28°C
Below 30°CLo or selected fan mode*-

Powerful operation

When Powerful operation is selected, the same heat exchanger temperature thresholds are used, but the permitted fan stages are increased as the coil temperature rises.

Indoor heat exchanger temperature risingInitial maximum fan conditionTemperature when dropping
42°C and abovePowerful37°C
39°C to below 42°CHi34°C
37°C to below 39°CLo32°C
30°C to below 37°CCool air prevention control28°C
Below 30°CS-Lo-
After approximately 7 minutes in Powerful operation: the lower stages are relaxed so that the fan can remain at Lo rather than S-Lo/cool-air-prevention operation, while the higher temperature stages remain Powerful, Hi and Lo as applicable.

What this means during diagnosis

  • A low or delayed indoor fan speed immediately after heating starts can be normal operation.
  • Do not diagnose the indoor fan motor as faulty solely because the customer-selected fan speed is not reached while the indoor coil is cold.
  • Check the indoor heat exchanger thermistor value and actual coil temperature if cool-air-prevention operation appears to continue for an abnormal length of time.
  • If the coil temperature is above the relevant threshold but fan speed does not increase as expected, continue diagnosis of the fan motor, fan control circuit and thermistor input.
Service note: Temperature thresholds and available fan-speed labels can vary by product family. Use the applicable service manual where model-specific values are required.
Technical reference: Cool Air Prevention Control - Heating mode.