Why Does My Furnace Run With My AC?
If you’ve noticed your furnace running while your air conditioner is on, you may be wondering if something is wrong with your HVAC system. This situation can be confusing, especially when it’s warm outside, and you’re actively trying to cool your home. Understanding why this happens can offer clarity and even help you make small adjustments for better efficiency and comfort. In this guide, we’ll explore why your furnace might be running alongside your AC, the possible causes behind it, and practical steps you can take to avoid unnecessary operation of both units.
Common Reasons Why Your Furnace Runs with Your AC
There are a few primary reasons why your furnace might run with your air conditioner. Here are some of the most common causes:
- Thermostat Settings
Thermostat settings often play a big role in how your HVAC system behaves. If your thermostat is set to “Auto” mode, it will automatically switch between heating and cooling to maintain your desired temperature, which might mean both the furnace blower and AC are engaged simultaneously. Alternatively, setting the thermostat to “On” could cause the system’s blower fan to run continuously, circulating air whether heating or cooling is necessary, so the blower might run even when the temperature is comfortable. - Blower Motor Activation
In many homes, the furnace blower motor is responsible for moving both warm and cool air throughout your home. Even if you’re not actively using the furnace for heating, the blower motor could be triggered to move cooled air when the AC is on. This connection can sometimes give the appearance that both the furnace and AC are running simultaneously. - Ductwork Design
Some HVAC setups have ducts and components so closely intertwined that air from both the furnace and AC can circulate through the system. If your ductwork is aging or has not been properly sealed or cleaned, warm air could mix with cool air, making it seem like both systems are active. Leaky ducts can also lead to the furnace compensating for lost or unbalanced air, prompting both systems to run. - Incorrectly Sized HVAC Units
If your HVAC units are oversized or undersized for your home, they may cycle more often than necessary. An improperly sized system will struggle to maintain comfortable temperatures efficiently, sometimes causing both the AC and furnace to operate concurrently. This inefficiency could increase wear and tear, as both units are working harder to reach the desired temperature. - Seasonal Shifts and Temperature Fluctuations
During transitional seasons like spring or fall, where mornings are cool but afternoons warm up significantly, the thermostat may call for both heating and cooling. This can lead to brief moments where the furnace kicks on to keep the temperature stable as your AC is cooling, even if only the cooling function is technically needed. - Control Board or Wiring Issues
The control board or wiring in your HVAC system could cause simultaneous running if it’s malfunctioning. If the control board sends conflicting signals or the wiring is incorrectly set, you might find your furnace running alongside your AC even when one should be inactive. Professional troubleshooting is typically required to identify these kinds of problems.
The Role of Thermostats in Your HVAC System
Your thermostat acts as the brain of your HVAC system, coordinating how and when each component runs. Because of this, incorrect thermostat settings are one of the easiest and most common reasons for unintended furnace and AC operation. Here’s how you can optimize your thermostat settings for efficiency:
- Set the Fan to “Auto”
The “Auto” setting allows the fan to run only when heating or cooling is actively required, which can help you avoid overuse. Using “On” can keep the fan running continuously, which will circulate air but it may also keep the furnace blower on when it’s unnecessary. - Program Temperature Setbacks
If you have a programmable or smart thermostat, take advantage of its settings to adjust your home’s temperature throughout the day. By setting higher temperatures during the day and cooler ones at night, you can minimize the demand on both your furnace and AC, allowing them to operate more efficiently. - Avoid Extreme Temperature Settings
Setting a very high or low temperature can cause the system to overwork and possibly lead to both heating and cooling systems running as they try to meet the demand. Set your thermostat at a comfortable temperature and adjust it in small increments for a stable environment.
HVAC System Maintenance: Keep Your System Running Smoothly
Regular maintenance can prevent many issues related to unnecessary HVAC operation. Here’s why it’s essential:
- Routine HVAC Service
Annual or bi-annual maintenance checks can ensure that your furnace and AC are working at their best. During these checkups, technicians inspect critical parts, clean components, and address minor issues before they become major, reducing the chance of unnecessary furnace and AC operation. - Filter Replacements
Changing air filters regularly is crucial. A dirty or clogged filter restricts airflow, causing the blower to work harder, which can lead to both systems running in an attempt to reach the desired temperature. Clean filters help maintain airflow and improve energy efficiency. - Duct Sealing and Cleaning
Well-sealed and clean ducts reduce the likelihood of air leakage, which can cause the furnace to work overtime. If your ducts have leaks or are dirty, air from both systems may mix, creating additional strain and leading to the simultaneous operation of both the furnace and AC.
HVAC Upgrades: Improve Efficiency and Reduce Unnecessary Operation
In some cases, an upgrade or improvement to your HVAC system can prevent the furnace and AC from running together. Consider these options:
- Upgrade to a Zoned HVAC System
A zoned system divides your home into areas or “zones” that can be heated or cooled independently. This setup can reduce the chances of both units running unnecessarily, as only the areas that need heating or cooling will engage the HVAC system. - Consider Smart Thermostats
Smart thermostats provide precise control over your home’s climate. They allow you to create detailed schedules, monitor temperature variations, and control settings remotely. This advanced control can prevent unnecessary operation and keep your HVAC system working optimally. - Energy-Efficient Equipment
If your HVAC system is outdated or oversized, it may not function efficiently. Upgrading to a more energy-efficient model that is properly sized for your home can reduce the chance of both systems running simultaneously, saving on energy bills and maintenance costs.
When to Call in a Professional
If your furnace and AC continue to run together despite adjusting thermostat settings and performing maintenance, it may be time to consult a professional. Here are some signs that it’s best to reach out to an HVAC technician:
- Unusual Noises or Smells
Strange noises from your HVAC system or unusual smells from the vents could indicate a mechanical issue that requires professional attention. - Frequent Cycling
If your furnace and AC frequently cycle on and off, it could mean they are not working efficiently together. A professional technician can check for wiring issues or recommend upgrades. - Inconsistent Temperatures or High Bills
Uneven temperatures and high energy costs are clear signs that your HVAC system is not operating correctly. A professional can inspect the system for any underlying issues and ensure it’s calibrated correctly.
Thornton Plumbing offers comprehensive HVAC maintenance and repair services to keep your home comfortable all year. Our experienced technicians in Noblesville, IN, can troubleshoot and fix any issues with your HVAC system, ensuring your furnace and AC run smoothly—without unnecessary overlap.
Contact Thornton Plumbing Today for Expert HVAC Support
If your furnace is running while your AC is on, don’t hesitate to reach out to us. At Thornton Plumbing, we’re here to help you understand and resolve any HVAC issues to ensure you stay comfortable year-round. Whether it’s thermostat adjustments, routine maintenance, or advice on upgrades, our team has you covered.
Contact us today at 317-697-9265 to schedule a consultation or service appointment and get your HVAC system in top shape for every season.