If your thermostat is set to heating mode, but your cooling system turns on instead, there could be a few possible reasons for this issue:
Wiring issue: There might be a problem with the wiring connections between your thermostat and your HVAC (heating, ventilation, and air conditioning) system. It's possible that the wires are crossed or connected incorrectly, causing the cooling system to activate instead of the heating system. Checking the thermostat wiring and ensuring it is properly connected is a good first step.
Thermostat programming or settings: It's possible that the thermostat is programmed incorrectly or has incorrect settings. Double-check the thermostat's programming and make sure the desired temperature and heating mode are set correctly. Some thermostats also have a "fan" setting that determines whether the fan should run during heating cycles. Make sure it is set appropriately.
Malfunctioning thermostat: There could be an issue with the thermostat itself. It may have a faulty temperature sensor or a malfunctioning internal component that is causing it to send incorrect signals to the HVAC system. In such cases, you may need to replace the thermostat to resolve the issue.
HVAC system issue: It's also possible that there is a problem with your HVAC system itself, such as a malfunctioning relay or control board. This can cause the cooling system to activate instead of the heating system when the thermostat calls for heat. In this case, it's best to have a professional HVAC technician inspect and diagnose the problem.
If you're unsure about troubleshooting or resolving the issue, it's recommended to consult a qualified HVAC technician who can identify and fix the problem correctly.