When your HVAC thermostat’s display screen goes blank, you expect all systems to stop working. Usually, the air conditioner and the furnace can’t keep on working because the thermostat cannot control them anymore. But you are experiencing that while the display screen is blank the HVAC blower fan is still working. How is that possible?

The main reason why your blower fan can keep on working when the thermostat’s display screen is blank is that the fan has its own power wire and circuit breaker. It works independently from the rest of the HVAC system’s power source.

We’ll briefly look at how the blower fan normally works, list the main reasons for a thermostat’s screen to go blank, and then see why the malfunctioning of the thermostat doesn’t necessarily affect the fan. 

How your HVAC fan normally works

It is a separate electrical system but necessary 

Although the HVAC blower fan works together with your heating and cooling units, it’s a separate electrical system.

Your air conditioner and furnace only cool and heat the air inside themselves. The uncomfortable air in your home is pulled out of your house by the independent fan and then sent to the air conditioner or furnace to be heated or cooled down and then distributed back into your home.

Role of the thermostat

Your thermostat allows the air conditioner, furnace, and fan to work together to guarantee a comfortable temperature in your home. 

Normally, when your thermostat is set to “auto,” it tells the blower fan and the air conditioning or furnace to turn on and off at the same time to keep the desired temperature. 

Can the fan blow when neither the air conditioning nor furnace is running?

Because the fan is a separate electrical system the fan can run without any of the other HVAC devices running.  You can also manually flip the fan switch to “on” and the fan will run without the air conditioner and furnace necessarily also running.

A broken relay switch or short circuit can cause it to keep on running 

If only the fan is running, a broken relay switch might prevent the settings you’ve entered on the thermostat to be sent to the fan. A short circuit or other wiring problems can also prevent the on/off commands from the thermostat to be sent to the fan.

The fan keeps on running to cool down an overheated system

The fan may continue to run to help cool an overheated system off to prevent the system to stop functioning until it is cooled off.

Why can the fan keep on working when the thermostat’s display screen is blank?

To answer this question let’s first list the most important reasons why a thermostat’s display screen can go blank.

The most important reasons include the following:

  • A tripped circuit breaker, 
  • worn-out batteries,
  • a blown fuse,
  • an open furnace door,
  • dirty air filter, and
  • the thermostat being old.

Reasons why a blank screen does not necessarily affect the fan

Unless your home is experiencing a complete electrical outage or the fan and the thermostat are sharing the same circuit breaker, none of the reasons why the thermostat’s screen is blank influences the basic running of the fan.  

Your thermostat’s display controls the air conditioning and furnace but not the fan unless it is in the “auto” position. If the fan is in the “on” position it will turn on without the display.  It will keep on working if it has been on when the thermostat’s screen has gone blank. 


Because the fan is a separate system and normally has its own circuit breaker and power wire, it can keep on working even if the thermostat goes blank – as long as there is still electrical power flowing to the fan.  


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