Should I Run My Furnace Fan All the Time?

Close up image of a furnace fan.

I constantly get this question from homeowners in the greater Lexington, KY, area, and the answer is almost always, “Yes!” Almost — but more about that later.

What are the benefits of running my furnace fan?

The benefit to running the furnace all the time is better circulation of your home’s heated or cooled air. This is especially true for larger homes or multi-story homes. If you have a two or three-story home with a large staircase, Mother Nature is going to try and heat the upstairs and cool the downstairs, and obviously, we want our home to be the same temperature in every room.

Combating the stack effect & keeping your home comfortable

Because hot air is lighter than cold air, hot air will naturally rise and cold air will naturally fall. In a closed space like your home this process is called the “stack effect” — it is the same process that allows your fireplace to draw smoke out of the chimney. In fact, a large staircase can easily be compared to a chimney pulling all of the hot air in the home to the upper floor, allowing cold air to fall to the lower floor. This is why in some houses the basement is always cold and the upstairs is always warmer.

Your home’s heating and cooling system will help to balance these issues but it does not run all the time and in milder months in may only run for a few minutes every hour. This is where running the furnace fan all the time can help by constantly pulling the cold air from the lower level and warmer air from the upper level, blending the two, and then redistributing a more stable, even temperature throughout the home.

What if I have a single story home — should I still run my fan?

Even if you have a single story home, you should still run your furnace fan. This is because different parts of your home will experience different external temperatures throughout the day.

For example, if your house faces east then the morning sun will likely warm the front rooms of your home a few degrees warmer than the rear of your home, and the evening sun will do the reverse in the afternoon. Running your furnace fan will bend the air throughout the day keeping the air in your home from having noticeable hot and cold spots.

Won’t running my furnace fan all the time raise my electricity bill?

While running your furnace fall all the time might raise your electricity bill a little bit, in the long-term it might even make it go down.

How is that possible?

Well, if you have a newer ENERGY STAR rated furnace it will have a variable speed motor that is designed to run all the time and with our electricity rates in central Kentucky it will cost the average homeowner less than $3 per month to run the fan 24 hours a day. If you have a builder’s grade or older system then it may cost you as much as $10-$12 a month.

However, running your furnace fan all the time could save you money each month because having more even temperatures in the home could result in the heat or air conditioner running less. Since the fan uses much less power than the outdoor unit you save money in the end.

Won’t running my furnace fan all the time cause it to fail prematurely?

Absolutely not. These fan systems were designed to run all the time and assuming your system is properly installed with the correct size ductwork it may even extend the life of the fan. The most stressful part of any motor’s life is that when it starts it is much easier on the motor to run constantly than to start and stop all the time.

Additional benefits of running a furnace fan constantly

By running the furnace fan all the time, you are also running air through your furnace filter constantly, so if you have an efficient filter and a UV light, you are constantly filtering and cleaning the air. This results in healthier indoor air quality for your family and less dust on your furniture.
In the cooling season running the furnace fan can help lower humidity in your home by running air through your air conditioner’s evaporator more consistently which allows the system to pull out even more moisture over time.

When is running my furnace fan all the time not a good idea?

There are a couple of reasons why you may not want to run your fan. The first is if you have ductwork in your attic and if in the dead of summer your attic can reach temperatures of as much as 140 degrees. Even if your ductwork is insulated it only does so much good against those kinds of temperatures, so running the fan when the air conditioner is not on could actually pull the heat out of the attic into the home — and obviously that would be counterproductive.

The other situation that may make running the furnace fan a bad idea is if you have leaky ductwork. Homes built after 2015 are required to have all ductwork sealed and tested for airtightness. However, unfortunately, homes built before 2015 may have very leaky ducts. In fact, according to a study by the Kentucky Department of Energy (DEDI) homes with ductwork in the attic or crawl space in Kentucky had an average of 44 percent leakage! That means over 20 percent of the air in your home is passing through your attic or crawl space before it gets to you — YUCK! If you have an older home with a lot of duct leakage it may not be a good idea to run the fan all the time as it may draw in dirty air from places you don’t want. This can easily be fixed by having a good air conditioning contractor come in and seal your ductwork — it’s not very expensive and is usually done in less than a day.

In short, for most of us running the fan all the time is a good idea. If you don’t have an ENERGY STAR rated furnace make sure the next one you buy is and has a variable speed motor. In my opinion, the government should mandate all new furnaces have a variable speed motor. It’s not a very expensive upgrade and it makes both heating and cooling more efficient which ultimately saves homeowners money while also making them more comfortable. When shopping for your next heating or cooling system this should not even be an option.

If you have an older home you should have your ducts sealed even if you are not planning to replace any equipment anytime soon. If you do have equipment replaced, make sure your contractor adds duct sealing to the job. Also adding indoor air quality products like high-efficiency filters or ultraviolet light systems will greatly improve the air in your home while running the furnace fan, so that you can turn your furnace into a whole-house air cleaner!

Contact Synergy Home today at 859-687-0553 or request service online for information on these or any other heating and cooling issues.

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