When your thermostat is set to cool but your system blows hot air, it is typically caused by a misconfigured reversing valve on heat pump systems, incorrect thermostat wiring, a stuck relay switch, or a completely inactive outdoor condenser unit. If low-voltage control wires are crossed at the wall sub-base, the reversing valve engages heating mode instead of cooling. Alternatively, if your thermostat displays "Cool" but the outdoor unit has no power due to a tripped breaker, only the indoor furnace fan runs, blowing uncooled ambient air through the ducts. Verification: Go outside to check if your outdoor condenser unit fan and compressor are spinning and actively running.
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