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Why Does Air Conditioner Run at the Same Time as a Furnace?

The air conditioner and furnace in your home might be meant for the opposite results; they actually team up to maintain optimal temperature, especially in warm weather. But how? Let’s find out. 

An air conditioner may run at the same time as the furnace because the AC’s cooling coil is often attached to the furnace’s blower fan. During the summer, warm air gets blown over the cooling coil and gets distributed by the furnace’s blower through ducts. So, the furnace’s blower runs to maintain a comfortable temperature.

The AC and furnace are polar opposites but can’t get the job done without each other — it’s quite poetic, actually. In this article, let’s take a deeper look at how your AC, furnace, and thermostat function to keep you comfortable all year round.

How Do AC and Furnace Work Together?

HVAC units that have a furnace and a split air conditioner rely on both of these systems to collaborate year-round. Even non-furnace heating systems like a steam heat boiler or heat pump interact with HVAC systems like furnaces. 

The furnace warms up the temperature by blowing out hot air, and the air conditioner cools it down by removing the heat and blowing out cool air. Heating and cooling systems are essential parts of an HVAC system as they work together to maintain your preferred thermostat settings.

The roles of your AC and furnace vary in importance depending upon the temperature outside. For instance, the furnace does all the work in winter while your AC becomes the main character in the summers. 

However, even in summers, the AC’s cooling coil can cool down warm air, but it relies on the furnace’s blower fan to distribute it around the house through shared duct lines. Sure, the furnace doesn’t have to burn or produce any heat in summers, but the blower motor keeps running to disseminate cool air from the AC around the house

A lot of science goes into making sure your indoor temperature stays just right throughout the year. You’ve probably never given it much thought because everything works so seamlessly, and that’s a great sign for your HVAC system’s performance!

Read: What Is The Difference Between A Heat Pump And A Furnace?

All You Need To Know About Furnaces and ACs

As I said initially, this article takes a deeper look at the systems that regulate the temperature inside your home. In this section, let’s individually understand what ACs and furnaces are and how they function. 

Read: What Type Of HVAC System Do I Have?

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Most homes in cold areas rely on furnaces to maintain optimal thermal comfort. Furnaces come in several types: electric, oil, and natural gas.

They’re the winter counterpart of air conditioners but can work together with ACs in central air conditioning systems in the summer. 

Furnaces Functionality 

Different types of furnaces can produce heat in several different ways. All heating systems use up some sort of energy to generate heat and warm up your indoor temperature.

It’s the same with furnaces, but different furnaces can vary in the energy types they consume. 

For instance, a natural gas furnace would generate heat by burning gas. In contrast, an electric furnace might rely on pulling air through a heat exchanger warmed up using electric heating elements (like a hairdryer). 

Regardless of how they produce heat, here’s how a furnace generally functions: 

  • As your home gets cold, the temperature falls below your preferred thermostat settings. That triggers the furnace to produce heat and distribute it through the ductwork using a blower fan. 
  • The furnace starts a heating cycle, producing heat in different ways depending upon the type of furnace you have. 
  • From there, the air gets exposed to the heat, warms up, and gets blown out through vents by the furnace’s blower motor. 
  • The process goes on until the house warms up to your preferred degree. Once that reflects on the thermostat, the furnace gets another signal to stop the heating cycle. However, the blower fan keeps running for a bit longer to ensure all residual heat gets distributed through the ductwork.  

Air Conditioners

An air conditioner regulates your home temperature in summers. It intakes warm air from your room, extracts its heat and humidity, and releases cool air back in.

In this way, it cools down the home’s ambient temperature and maintains your ideal thermostat settings in summer. 

Air Conditioner Functionality 

Air conditioners have cooling coils to absorb the heat from the warm air in your room and let out cool air to lower the ambient temperature. Here’s how it works:

  • The room’s air goes into the indoor AC unit through its intake. 
  • It blows over the chiller pipes on the inside that extract its heat. 
  • Moisture also gets removed at this stage, which is why an air conditioner also works as a dehumidifier.  
  • Thanks to the liquid coolant flowing inside, the evaporator pipes remain cold despite the room’s warm air blowing on them.  
  • As the coolant absorbs heat, it evaporates into a warm gas. That’s where the AC’s compressor and condenser come into play, returning the coolant to its liquid form.  
  • Air that’s dehumidified and cooled down then gets blown back into the room, lowering the room temperature.

The Role of Thermostat

Now that we’ve discussed the collaborative and individual functioning of ACs and furnaces, let’s talk about the brains of the operation — the thermostat!

Behind the functioning of your furnace, air condition, and the entirety of your HVAC system is a component called the thermostat. It’s the brains of the operation, and it has one job: maintaining the perfect temperature at home according to your desired settings. 

Read: Why Does Home Thermostat Say HEAT ON But There Is No Heat?

For this task, the thermostat has tools like the furnace and the air conditioner. It uses them as and when they’re needed.

For instance, it detects that the room temperature has gone above your settings, it’ll use the AC to bring it back down. Similarly, when the temperature drops below your desired degree, it’ll signal the furnace to produce and disseminate heat until the just-right temperature is restored. 

Thermostats come with external switches to allow you to turn the heat/AC up, down, on, or off. You also have access to blower fan controls.

However, leaving everything on ‘auto’ is often the most convenient option as in that setting, your HVAC system will do what it takes to maintain an ideal room temperature consistently. 

Like every other tech tool, thermostats have also seen enormous advancements over the years. There are plenty of smart programmable thermostats out there, which have various useful features for added comfort.

Some of them have integrated WiFi connectivity, allowing you to control your HVAC and thermostat settings from your phone screen, no matter where you are! 

Moreover, they support specialized settings for different times of the day and even send you a mobile notification for an overdue furnace filter change. When you’re away, programmable thermostats can also help you cut back on electricity bills.

You can remotely turn down the heating or cooling when you’re away and turn it up 30 minutes before you get back. 

Overall, the thermostat acts as a team leader for your entire HVAC system, including the functioning of your furnace and air conditioner. 

Read: How Do Smart Air Conditioners Work?

Should You Replace Your Air Conditioner and Furnace Together?

HVAC systems require proper maintenance to operate efficiently with low energy costs. With its age, you also need to replace core components like its furnace and AC units.

But do you need to replace both of these together?

Replacing your furnace and AC at the same time is ideal but not always necessary. Both components should be equally effective, in good condition, and have all the features and tech you need. If one checks these boxes and the other doesn’t, you can upgrade just one for a well-matched HVAC system.

An HVAC system’s efficiency relies on the performance of both; heating and cooling systems. 

Since the furnace and the AC work hand-in-hand to maintain your desired temperature settings, you might wonder if you should upgrade or replace them together. 

In some situations, the correct answer isn’t so black and white. For instance, factors like your climate conditions, feature preferences, or the age and condition of your furnace or AC can come into play

If you get winters all year round where you live, and your furnace starts acting up, you might not have to upgrade your air conditioner as long as it still works. 

On the other hand, in places where you get all four seasons, the AC and the furnace play an equally important role in maintaining a comfortable room temperature all year round. In that case, they both need to perform equally well and be well-matched for efficient HVAC functioning. 

Lastly, in cases where the entire HVAC system is pretty old (10 to 15 years), you’re better off replacing both the AC and furnace together, even if one of them works fine at the moment. That’s because:

  • It’s probably going to start acting up or die on you soon.
  • It won’t be as modern or effective as the new replacement, leading to an ill-matched HVAC system and inefficient performance. 
  • It won’t support smart features that come with a programmable thermostat, while your new AC or furnace unit probably will. 

If your furnace is over 15 years old, it’s recommended to replace it along with your air conditioner.

The right decision also depends on your budget. Although it’s better to replace both components together, it’s costlier too.

However, if you’re willing to pay the extra premium for not having to worry about any repairs or replacements any time soon, it’s probably worth it. 


Most homeowners never even give a thought to the science behind a consistently maintained perfect room temperature around the year. Whether there’s scorching sun outside the house or a chilling snowstorm, the indoors is always nice and comfy.

All that is thanks to the collaborative efforts of your AC, furnace, and thermostat, and you (hopefully) now understand how all of these work.