If the blower fan for your HVAC system won't turn on, it could be the fault of your Honeywell thermostat, but there are also other possibilities to investigate. The thermostat is a good place to begin troubleshooting, though, because sometimes, the problem is as simple as a wrong setting or a dead battery. You may even uncover a wiring problem that is easy to correct.
When you can't find anything wrong with the thermostat or the wiring, it's time to head down to the air handler, which is the housing that contains your heating and cooling units. You might find that the unit isn't getting any power at all or that a switch somewhere needs to be reset. Some problems, such as a faulty relay or transformer, are difficult to diagnose yourself and need professional attention.
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Troubleshooting the Thermostat Controls
Before you do anything else, check the main control on the thermostat. It should be in the AUTO position, but perhaps someone inadvertently turned it to OFF, and if so, just set it back where it's supposed to be. If it's winter and the thermostat is set to COOL, switch it over to HEAT.
If your thermostat has an LED display and it's blank, it's possible that the battery needs to be replaced. If that doesn't help, check the circuit breaker in the main panel that controls the system, and if it has tripped, turn it all the way off and then on again. If it trips again, there's a short circuit that needs to be diagnosed and repaired by an electrician or an HVAC specialist.
Testing a Honeywell Thermostat
If the thermostat LED is lit, do a test by turning the thermostat to its maximum setting (in HEAT mode) or minimum setting (in COOL mode), turning the main control to AUTO or FAN and waiting for the blower to start. If it doesn't, remove the thermostat cover and check the wiring. The wire that controls the blower is connected to the G terminal — the wire itself should be green — and it's of particular interest. Make sure it's tight (turn off the breaker first) and look for corrosion that may be interfering with the connection.
When the LED display isn't lit and you're sure the thermostat battery is good, the wire connected to the Rh/Rc terminal, which should be red, may not be supplying power. Make sure this wire is tight. If nothing helps, it's possible you need a new thermostat, but there are a couple of other places to look first.
Check the System Components
Sometimes, the reason the blower won't work is as simple as a failure to close the door of the air handler all the way. Many models have a safety switch that cuts power to the entire system. You may also have to reset the blower. To do this, turn off the breaker, press the red reset button on the back of the blower motor, and turn the power back on.
A strange noise coming from the blower, especially a humming sound, indicates internal problems with the blower motor, often a burnt-out capacitor. Similarly, a malfunctioning relay switch will prevent the blower from coming on, and a bad or improperly connected transformer will prevent any signal at all from getting to the thermostat. All of these problems can be fixed, but it takes an HVAC technician to do it properly.