If your air circulation fan inside the house doesn't work when you turn it on separately (fan on mode) but works fine when the whole AC system is running, there could be a few reasons for this issue:
Separate Fan Control: Some HVAC systems have separate controls for the fan. When you turn on the AC system, both the cooling compressor and the fan automatically start working together. However, when you select the "fan on" mode, it activates only the fan without starting the cooling compressor. If the fan doesn't work in this mode, there might be a problem with the fan control circuit or the fan motor itself.
Fan Speed Setting: Your AC system may have multiple fan speed settings (low, medium, high). If the fan speed is set to "auto" or "off" when you select the "fan on" mode, the fan won't operate independently. Ensure that the fan speed is set to an appropriate level when using the "fan on" mode.
Malfunctioning Fan Motor: The fan motor may have failed or encountered an issue, preventing it from operating when you select the "fan on" mode. In this case, the AC system may still work fine in cooling mode, but the fan won't operate independently.
Faulty Fan Control Circuit: The fan control circuit, which manages the fan operation, may have a problem. It could be a wiring issue, a faulty relay, or a malfunctioning control board that is preventing the fan from turning on when using the "fan on" mode.
Thermostat Setting: Some thermostats have specific settings that allow or disallow independent fan operation. Check your thermostat settings to ensure that the fan can be operated separately.
To troubleshoot and diagnose the exact cause of the issue, you may need to contact a qualified HVAC technician. They can inspect the fan motor, control circuit, and thermostat settings to identify and resolve the problem. It's essential to fix this issue, as independent fan operation can be useful for air circulation and ventilation in your home, especially during mild weather when you may not need the cooling function of the AC system.