When your indoor air handler blower fan refuses to turn off and runs continuously twenty-four hours a day, it is usually caused by a simple thermostat setting or a fused relay switch. The most common cause is that your wall thermostat's fan switch is manually set to the "On" position rather than the recommended "Auto" setting, which forces the blower motor to run constantly. Another frequent culprit is a stuck or fused fan relay on the indoor furnace control board, which keeps sending 120V line power to the blower motor regardless of thermostat commands. Additionally, a faulty limit switch or advanced continuous circulation programming can keep the fan spinning. You can troubleshoot by checking your thermostat fan mode setting. You can verify that the issue is resolved when switching the fan mode to "Auto" causes the blower motor to shut off after a cooling cycle ends.
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