Fan ON AUTO switch settings explainedContinuous HVAC Blower Fan Operation for Optimum Indoor Air Quality

  • CONTINUOUS BLOWER FAN OPERATION - CONTENTS: Continuous HVAC Blower Fan Operation: how to improve indoor air quality by continuous blower fan operation in the heating or cooling air handler. Why & how to run the blower fan continuously indoors & warnings about proper blower fan duty cycle and fan speed to avoid blower fan motor failure
  • POST a QUESTION or READ FAQs about whether or not to run the heating or air conditioning blower fan continuously

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Continuous blower unit fan operation guide: this article explains how and why to set your air conditioning or heating system blower fan to continuous operation in order to improve indoor air quality by increased and continuous indoor air filtration.

(Naturally if there is an IAQ problem source in the building it is essential to also find and correct that condition. This website answers almost any question you might ask about air filters for heating or air conditioning systems.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Continuous Blower Fan Operation: Why Run an Air Conditioner or Heating Blower Fan Continuously?

Photograph of attic air filterFor maximum indoor air qality (IAQ) improvement in buildings: for a new blower installation we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which permits continuous operation if we wish to run the system that way.

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Article Contents

Reasons for Keeping the Blower Fan Unit On

Running the blower continuously at low speed resulted in continual air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed. Here are the details:

There are reasons to leave the fan in the ON position on a heating or cooling system, but we do not recommend that you do this without first asking for advice from your heating and service technician.

If your air conditioner or heating system fan blower unit was designed to permit the fan to run all of the time that's great and you can consider the benefits of continuous fan operation we list just below.

But if your fan motor was not intended for continuous duty and you run the fan that way, you may find the motor fails prematurely.

Here are some possible advantages of continuous blower fan operation

Here are some disadvantages of continuous air conditioner or heating system blower fan operation

How to Put the Air Conditioner or Heat Pump Blower Fan into Continuous-On Operation

furnace fan limit switch

Most heating or cooling systems have one or perhaps two methods for turning the blower fan to continuous "on": at the room wall themostat and (for heating systems) at the fan limit switch - a safety control found inside the heating appliance.

  1. The room thermostat may include a "Fan On" position - though not all versions of this control have this switch. Not all room thermostats include this switch - but most do - see our page top photo. For help finding the furnace fan control switch on your room thermostat
    see FAN ON AUTO MAN Thermostat Switch.

    Try moving the thermostat fan control to ON or MAN - this should turn the fan in the air handler for continuous operation.
  2. The fan limit switch located in the air handler's blower compartment or near the oil or gas burner for your furnace may include a "Manual Fan" Switch.

If your air handler unit provides both heating and cooling (that is not just air conditioning) then there will be a limit swtich installed inside the air handler blower compartment or burner compartment.

Look for the white push-pull switch such as the one shown in our photo at left. You will see this switch without having to remove the cover from the limit swith itself.

If your limit swith includes this fan switch, try moving the fan switch from "Auto" to "Fan On". You should hear the blower fan turn on and continue running as long as the switch is in this position.

To find the manual furnace or air handler blower fan switch, see the articles, photos, and sketches in these articles

See the white knob at lower left in our fan limit switch photo at left. That's the manual fan-on switch on a limit control. As we note, not all limit controls include this switch. The cover has been removed from the switch in this picture.


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