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.

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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

  • Improved air filtration, lower indoor dust levels - this is particularly true if you have a good air filter system installed. In our forensic lab when we installed a new gas furnace we asked the installer to include a multi speed fan (the fan chooses the needed speed automatically), a fan rated for continuous duty cycle, and we specified that the air filter system include HEPA filtration as well as an electrostatic air cleaner. We also included a cheap disposable filter at the air returns.

    This fan plus filter design permitted us to run the blower fan continuously in our lab, regardless of whether or not the heat was running (we don't need central air). We measured airborne dust levels in the building before and after installation of the new system and found that indoor dust levels improved by at least one order of magnitude.

    This system was enormously more effective than the dust filtration capacity of any free-standing "air cleaner" found on the popular consumer product market. That's because a warm air central heating system or central air conditioning system blower and duct work move many more cubic feet of air per minute through the filter system than a free-standing room "air cleaner" or "air purifier" can handle.

  • Increased building comfort: especially if a multi-speed or variable speed fan motor is included in the system design, the continuous movement of air in the building avoids sudden hot or cold blasts when heating or air conditioning turns on; indoor air temperatures tend to be more even.

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

  • Energy use: A downside of continuous blower fan operation is higher electrical costs. On the other hand, if we needed to reduce building dust and allergen levels and/or increase comfort levels, this approach might avoid wasting money on other attempts at air cleaning that are expensive and less effective.

    More careful study is needed to really evaluate the claim that continuous blower fan operation uses more overall energy. In some cases there may be a net energy savings, depending on how hard the heating or cooling system has to work to satisfy the thermostat (and occupants) during "on" cycles.

  • Equipment life: if the blower motor is not rated for continuous duty operation, it may need replacement sooner when continuous fan operation is used. In this case be sure the new blower is rated for continuous duty.

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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