A shorted or miswired thermostat can sometimes be the single component that stops a furnace or air handler blower from running continuously.

A continuously running blower can be caused by the thermostat sensing circuit faults, an incorrect fan setting, or a control signal stuck on from the system control board. Start by setting the thermostat fan control to off and observe whether the blower stops; if it does, the thermostat is the likely source and replacing it will fix the issue. If the blower keeps running with the thermostat set to off, turn power off at the breaker and inspect the thermostat wiring for a stuck fan wire or a stray conductor touching other terminals; correct any wiring issues and restore power to test. For modern digital thermostats, check for internal shorting by disconnecting the thermostat base from the wall and confirming the blower stops; if it does, replace the thermostat following the manufacturer wiring diagram and use the common wire if required. If the system uses a dedicated fan relay or control board, verify that the relay is not welded closed by listening for click when cycling power and measuring continuity across the relay contacts with power removed; replace the relay or board if it is failed. Always turn power off before touching wiring, label wires before removal, and use a volt meter to confirm expected low voltage signals are present when testing; if you are uncomfortable with electrical testing, call a qualified technician. After replacement, calibrate the thermostat fan settings and verify proper operation through at least two heating and cooling cycles to ensure the issue is resolved.

Common mistakes include assuming the thermostat is at fault without testing which leads to unnecessary parts replacement; always perform the off test and disconnect the thermostat base before replacing it. Another issue is miswiring the replacement thermostat which can short the fan circuit and recreate the problem; label and photograph terminal connections and follow the wiring diagram for the new unit. A third common concern is overlooking a failed fan relay or control board that keeps the blower running even when the thermostat is disconnected; if the blower still runs with the thermostat removed the control board or relay is likely the culprit so test or replace that component.

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