Can I do something to prevent outside cool air being drawn into the house when I turn on the exhaust fan in the bathroom?

Asked by Felix
San Antonio, TX

In winter if I turn on an exhaust fan in the bathroom, the outside cool air will be drawn into the house (reverse effect in summer) lowering the efficiency of the AC, is this normal? Or can I do something to prevent this?


Yes, any time you operate a bathroom exhaust vent fan you are removing stale (conditioned) air from inside and replacing it with outdoor air.

  • The purpose of the bathroom exhaust fan is to remove excess moisture and odors from the bathroom.
  • Heat recovery is not a function of the bath fan.

While the energy in the air that is exhaust air is lost it is critical to continue to operate bath fans to remove excess moisture/odors.

The amount of energy lost is insignificant in the overall scope of energy use when compared to the health advantages of using the fan.

As an alternative you could install a heat recovery ventilation system (HRV) that pulls air from the bathrooms, captures a percentage of the energy in the exhaust air and transfers that energy to the fresh air brought in to make up for the exhausted air.

  • The problem with an HRV is it removes moisture from the house, when needed (during winter months) and adds moisture into the home when not wanted (hot humid summer).
  • An energy recovery ventilator (ERV) would leave outdoor moisture outdoors and interior moisture indoors so it would not help remove unwanted bathroom moisture.

My advice is to continue using bathroom fans to remove moisture/odors and not worry about the loss of energy or introduction of moisture. Since the fans are located in rooms with high concentrations of moisture they will never bring in moisture to levels being exhausted.

If you want to make sure that you are operating your exhaust fans efficiently you should:

  • install Energy Star qualified fans and
  • have them controlled by humidity levels.

If humidity levels rise in the bathroom the fan activates and continues to run until the humidity levels return to normal.

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