Should I Run Furnace Fan on AUTO or Circulate?

A common question among furnace users is whether to leave it on Circulate or set it to AUTO. Or, you may even be wondering whether to leave it ON or set it to FAN mode. It gets worse; a few people don’t get the difference. This makes it even more difficult to decide which setting to use and when. 

We’ve created this guide purposely to clear up those confusions. So, read on to understand the difference between ON and AUTO (and FAN) settings on the thermostat, the benefits of each setting, and when to use each setting. We also answer frequently asked furnace setting questions at the end. 

The Basics 

Let’s begin with an overview of the furnace blower fan to better understand its role, how it works, and the different settings. All furnaces have a built-in blower that draws cold air into the unit via the return air plenum, pulls it across the filter, and forces it through the heat exchanger where the cold air absorbs heat (during heating) to become warm. 

The same blower is also responsible for forcing warm air from the exchanger out of the furnace via the supply air plenum and into the ductwork (where present). It also helps distribute the warm air throughout the home for even heating with few or no cold spots. This fan is controlled directly from the thermostat, onboard or wall-installed.

You can change the blower fan speed and even turn it ON or OFF directly from the thermostat. Typically, you can set the fan to run throughout, as long as the furnace is ON and getting power, or only run during heating cycles when the furnace is not just running but also producing heat. The former is the ON setting, while the latter is AUTO. Nearly all furnaces have these two settings. 

However, most modern furnaces have two additional settings, i.e., FAN and CIRCULATE. A furnace may have one or both, depending on the model. The FAN setting allows you to run the blower motor even when you don’t need heating. It’s most valuable in the summer when you need fanning but not heating.

 You can simply turn on the furnace but set it to FAN mode so that it blows a cool breeze across your home to keep you cool during the cold weather without engaging the heating elements. 

Meanwhile, the CIRCULATE mode is a cross between the ON and AUTO settings. Like the AUTO setting, it only engages the fan continuously during the heating cycle when the furnace produces heat.

However, borrowing from the ON setting, it also occasionally runs the fan during off cycles to circulate warm air throughout your home. 

Hopefully, you can now answer most furnace fan setting questions without further trouble. Here’s what you need to know generally. 

What is the Difference Between Circulate and Fan ON?

The main difference is that the fan ON setting means that the fan is set to run as long as the furnace is switched ON. Meanwhile, the Circulate setting is a hybrid and auto that keeps the fan running throughout heating cycles and then again for a few minutes every hour even when the furnace is off. 

Another key difference is that the circulate setting is only present on a few modern furnaces, whereas the ON setting is present on nearly every furnace, new and old. 

Furnace Fan on AUTO or CIRCULATE

Keeping the fan on auto is AUTO is the most energy-efficient option. The fan only runs when the furnace is up and actually producing heat and goes off when the heater enters the off-cycle or is witched off. Thus, it also ensures excellent dehumidification. 

However, the CIRCULATE setting works better for circulating warm air throughout the home. It also eliminates cold spots better. Nevertheless, most homeowners and even HVAC experts prefer the convenience of the AUTO setting. 

Furnace Fan on AUTO or CIRCULATE in summer?

CIRCULATE, if you must choose one. Unfortunately, neither setting is great during the summer as both the AUTO and CIRCULATE fan settings only work alongside the heating process. In both cases, the fan is either only or mostly running when the heating elements are engaged. Thus, the two settings are largely impractical in the summer, with fan ON considered the best choice for the warm weather.

However, the CIRCULATE setting is slightly better than AUTO because it gets the fan to run at least a few times every hour, even when there’s no heating. Meanwhile, the fan only runs during heating cycles in AUTO, meaning it wouldn’t run at all in the summer. 

Does Running Furnace Fan Cool House?

Yes, running the furnace during the summer can help cool the house – technically. The furnace’s blower fan works like any other blowing system. It has a motor that converts electrical energy into mechanical energy.

 As the blades rotate, they move air, causing a breeze that makes people inside the house feel cooler. The “cool” feeling is because the fast-moving air displaces the warm air in direct contact with our skins. This accelerates the rate of convective heat transfer from the skins to the air, leaving you feeling cooler. The faster the rotation, the cooler it feels. 

However, if you’re wondering, fans don’t actually lower room temperatures because they don’t remove heat from the air. 

How to Run Furnace Fan Without Heat

You can run the furnace fan without heat in two main ways. The first approach is to toggle the “Summer” setting on the furnace. This applies to older furnaces with a summer toggle switch.

Locate the switch and slide the toggle from winter to summer. Doing so sets up the furnace for summer use without heating. Alternatively, you can change the setting on the thermostat from ON/AUTO to FAN. 

Furnace Fan on AUTO or CIRCULATE in Winter?

Ideally, it’s best to set your furnace fan to AUTO for the winter season. However, each setting has benefits and drawbacks. So, it comes down to what works best for each individual. However, most homeowners and even HVAC experts prefer AUTO.

Why? Because setting the fan to AUTO comes with significant energy savings, without over-compromising on comfort. The fan will run for the duration of the heating cycle, then go off during the off-cycle and come back when heating resumes. So, typically you have the fan running for 15 minutes continuously, stopping for 15, and then resuming.

Although it’s not perfect, it’s more than enough to circulate warm air throughout your home. The only downside is that the fan restarts too often, resulting in faster wear. However, a few people prefer the CIRCULATE setting because the air circulations between heating cycles ensure even better heat distribution throughout the house, meaning fewer cold spots.

Unfortunately, this fan setting comes with even more restarts. Moreover, constant blowing tends to remove warm air from the house faster, potentially shortening off cycles. In other words, you may notice that the furnace resumes heating cycles sooner. 

How Long Should my HVAC Fan Run Per Hour?

The furnace should run 10-15 minutes per cycle, 2-3 cycles per hour. This means that a standard furnace fan runs 20 minutes to 45 minutes per hour or an average of 30 minutes per hour during regular heating. However, this doesn’t mean that all furnace blowers run for 30 minutes per hour throughout.

The actual figure depends on the type and condition of the furnace, the type and condition of the fan, and weather conditions. For instance, an older furnace puts out less heat per minute, meaning the unit must run longer to maintain the thermostat setting. Thus, the fan will run longer too.

Of course, the running duration also depends on the fan setting. For instance, blower fans run round the clock when set ON and long enough in CIRCULATE mode. So the 30-minute average only applies to the AUTO setting. 

Is it Better to Have Furnace Fan on AUTO or ON?

It’s much better to set your furnace fan on AUTO. The main reason is energy efficiency. The fan draws a lot less power in AUTO mode. Meanwhile, keeping it running round the clock through the ON mode means you end up with a massive power bill. 

Pros and Cons of Running your Furnace Fan on AUTO or ON

  • AUTO Mode Pros and Cons

The main advantage of setting the furnace fan to AUTO is increased energy savings. Who doesn’t want a lower heating bill? Additionally, AUTO fanning keeps the home warmer for longer because the fan only blows warm air.

Blowing unconditioned air can make the home feel colder. On the flip side, though, setting your blower to AUTO means more stops and restarts, which can take a toll on the life of the fan. 

  • ON Mode Pros and Cons 

 The main advantage of setting the furnace fan ON is fewer restarts, thus longer fan life. Additionally, the furnace removes impurities from indoor air better when the fan runs consistently rather than ON and OFF. On the other hand, keeping the furnace ON guarantees a massive heating bill.

Moreover, you may notice lukewarm drafts because constant blowing causes warm air to lose its heat faster. 


Furnace fan not working on auto: Check whether your furnace is switched ON and producing heat as the fan only runs in AUTO mode during the heating cycles. If the unit produces heat, the fan may be stuck or faulty. 

Should I run my furnace fan on circulate? Yes, during periods when temperatures drop substantially at night. A good bet is when the outside temperature falls below 85°F. 

Is it ok to run the furnace fan constantly? Yes, it’s ok to let the fan run continuously if it’s a model designed to run continuously. However, make sure it’s properly installed by a professional and has the ductwork is correctly sized.

What are the pros and cons of running your furnace continuously? The main benefit is a longer fan life. Additionally, running the fan continuously improves the quality of air through constant filtration. Unfortunately, running the fan continuously also guarantees a massive power bill. 

What’s the cost of running a furnace fan continuously? The average furnace fan uses about 400kWh per month in the ON mode. This translates to about $41.68/month, assuming the current rate of 10.42 cents per kilowatt-hour. So, about $500.16 in a year. 

Does running your furnace fan continuously save energy? Yes, running the furnace fan all the time could save you money each month because of more even temperatures that mean the air furnace or air conditioner runs less. However, you can save even more with Circulate and Auto modes. 

What do I do if the furnace fan is constantly running? First, check the thermostat to determine whether the fan is set to ON mode. If so, change it to AUTO. If that’s not the cause, perhaps the temperature limit switch is broken or stuck ON. You need to fix or replace it. 

When is running your furnace fan not a good idea? Two instances when running your furnace fan isn’t a good idea are if your ductwork is in the attic or you have leaky ductwork. Running the fan in the summer if your ductwork is in the attic can pull warm air back into the home, negating all the gains of air conditioning. Meanwhile, leaky ductwork will lose all the conditioned air. So, it’s better not to use the blower until the ductwork is fixed. 


That’s all. Now you know everything you need to about furnace fan settings. You also know when to use the different settings. Don’t hesitate to consult an HVAC professional if you encounter problems beyond your DIY knowledge.