Furnace and central air conditioning thermostats generally come with two fan settings. One setting is "auto." This causes the fan to be turned on and off when the furnace or air conditioner has heated or cooled the inside of the unit enough for the air to be distributed throughout the dwelling. The second setting is "on." In this setting, the fan will run continuously regardless of what is going on with the heating and cooling system. Leaving the fan in the "on" position all of the time can produce some positive and some negative results.
• Always consult the owners manual of your unit for the manufacturer's suggestions
Some furnaces are designed to do more than just heat your house. They have built in systems to maintain a constant humidity in the home regardless of whether the furnace or air conditioner is running. Some of these units also contain special filtration devices to remove particulate matter and allergens from the air. Leaving these fans on can have some beneficial effects.
• Filtering devices installed in the central heating system may require the fan to run
If you have a special filter built into your system, it may be wise to let the fan run. Although it will use some additional electricity, the benefit of the filter may easily outweigh any additional cost. The same is true with a humidifier/dehumidifier. These units may require a constant flow of air to monitor the humidity in your home and make appropriate adjustments.
• Some argue that the fan will last longer if it is just allowed to run continuously when your furnace is operating
The idea behind this argument is that constantly turning the fan on and off can damage the coils and mechanical parts of the blower motor. By letting the fan run, the strain of starting up and sitting idle will be eliminated. Of course, you do run the risk of wearing out the bushings or bearings on the shaft when you do this.
• There are people who like the continuous air flow in their home
When the air is in motion, it gives the feeling that fresh air is being brought into the home. However, this is not usually the case. Most of the air that goes through your furnace is recycled through your cold air return from inside your house. So, although it can be a little refreshing for some people to feel the breeze the fan creates, it is not fresh air.
• Letting the fan blow constantly will reduce the temperature in your house in cold weather
Unheated moving air loses heat. This is why a fan works to cool your room in the summer. When your furnace fan continues to run after your furnace has cooled, it will bring a flow of unheated air through your air ducts. In many cases, these ducts run through the attic or crawl space where the temperature outside of the duct is many degrees cooler than you keep the inside of your house. Letting the fan circulate this cooled air will cost you more on your heating bill. The same can be true for the air conditioner in the summer. In fact, during the summer, the fan may bring moisture back into your home from the water collection pan below your cooling coil and make your house feel warmer due to increased humidity.