How many air filters does my AC have?

Most residential AC units use a single air filter, but some configurations include two or more filters spread across the system. This article explains how to determine the number of filters you have and what to look for.

Common configurations

The following list outlines typical filter counts by common AC installations to help you identify what you might have.

• Window air conditioners: usually have a single replaceable filter inside the unit, behind the front grille.


• Portable room air conditioners: generally use one filter located behind a service panel or grille.


• Central air conditioning with a return-air filter: commonly one filter in the return-air path, either in a slot at the air handler/furnace or in a return duct. Some setups also include a second filter for a secondary filtration stage or a separate filtration component, leading to 2 or more filters.


• Multi-filter or filter-bank systems: some air handlers or HVAC setups include two to four filter slots in a dedicated filter bank or within the blower compartment to provide higher filtration.


• Systems with humidifiers or electronic air cleaners: in addition to the main filter, these may add one or more filters specific to that equipment, increasing the total count.

In practice, the number of filters you have depends on the system’s design and any added filtration components. If you’re unsure, check the unit's manual or consult an HVAC professional for a precise count and replacement schedule.

How to identify your unit’s exact filter count

Use these steps to locate and count the filters in your system.

1. Identify the indoor equipment: for central AC this is the furnace or air handler inside the utility area; window and portable units are self-contained.


2. Open the access panel or front grille to access the filter area and look for filter slots.


3. Count how many filters are installed at once. If there are multiple slots, there may be more than one filter in the system.


4. Note the size and thickness of each filter (for example 1-inch, 2-inch, or 4-inch) since replacement needs depend on these dimensions.


5. Check for additional filtration components (humidifier, electronic air cleaner) that may have their own filters.


6. Consult the unit’s manual or search the model number online to confirm the exact number of filters and the recommended replacement interval.

If you’re not comfortable inspecting your system yourself, or if you suspect multiple filters but can’t locate them, contact a licensed HVAC technician for an accurate assessment and replacement guidance.

Summary

In most homes, the AC uses one filter. However, some setups—especially central systems with additional filtration stages, filter banks, or auxiliary components—can have two to four filters or more. To know your exact count, inspect the return-air path in the indoor unit, check for additional filtration components, and consult the owner’s manual or an HVAC professional for confirmation and replacement guidance.

How many filters does my AC unit have?

Most homeowners expect to find one filter, but sometimes there are two or more. The number of air filters your AC system has depends on the layout of your house and the way the system was designed.

Where is my AC air filter located?

The AC filter is most commonly found at the central air handler unit or behind a return air vent on a wall or ceiling. The central unit is usually in a basement, attic, garage, or utility closet, and the filter is often in a slot near where the return duct connects to the unit. In some cases, you may need to remove screws to access the filter behind a service door on the air handler. 
Common filter locations

• Return air vent: Look for a large vent on a wall or ceiling in a main room or hallway. You may need to open a hinged cover or remove the grill to access the filter. 
• HVAC unit (air handler): Check the central unit, which is often in a basement, attic, garage, or utility closet. The filter is typically in a slot on the side, top, or bottom, where the return duct meets the unit. 
• Multiple locations: Some homes have multiple filters, with one at each return air vent or one near the main HVAC unit. 

Tips for finding the filter

• Turn the system off: Before searching, set your thermostat to "off" to prevent air from blowing while you look. 
• Look for a slot: The filter will be in a slot that may have a hinged or removable cover. 
• Check the manual: If you have trouble, consult your HVAC system's owner's manual for the exact location of the filter. 
• Remember the car cabin filter: If you are looking for a car's AC filter (also called a cabin air filter), it is usually located behind the glove box. 

Why does my central air have two filters?

Why does my central air system have two filters? Your central air system has two filters to enhance air quality and protect the system. One filter is usually located in the furnace to protect the internal components, while the other is in the return vent to trap larger particles before the air enters the HVAC system.

What is the $5000 AC rule?

The "AC 5000 rule" is an HVAC industry guideline that suggests replacing an air conditioner if the result of multiplying its age by the estimated repair cost is over $5,000. If the result is under $5,000, a repair is likely the more cost-effective option for now. This is a simple starting point, and other factors like energy efficiency, repair history, and comfort should also be considered before making a final decision.
 
You can watch this video to learn more about the 5K rule: 28sHVAC Guide for HomeownersYouTube · Feb 19, 2025
How to apply the rule

• Multiply: Take the age of your air conditioning unit in years and multiply it by the estimated cost of the repair. 
• Analyze the result:
  • If the total is greater than $5,000: Consider replacing the unit, as it may be more financially sound in the long run. 
  • If the total is less than $5,000: A repair is likely the better option, especially if the system has been reliable up to this point. 

Example

• A 12-year-old unit with a repair estimate of $600: 12×600=$7,20012 cross 600 equals $ 7 comma 20012×600=$7,200. Because this is over $5,000, replacement might be a better long-term investment. 
• A 5-year-old unit with a repair estimate of $500: 5×500=$2,5005 cross 500 equals $ 2 comma 5005×500=$2,500. Because this is under $5,000, a repair is probably the more economical choice. 

Other factors to consider

• Energy efficiency: A new system with a higher SEER rating can lead to significant savings on energy bills. 
• Frequency of repairs: If you are constantly needing repairs, the cost of individual repairs can add up to more than a new system. 
• Refrigerant type: Older units may use refrigerants like R-22, which are becoming more expensive due to being phased out. 
• Warranties: Check for any warranties on new systems or parts before making a decision. 

This video explains how to use the 5K rule for deciding whether to repair or replace your HVAC system: 58sServiceMark Heating Cooling & PlumbingYouTube · Jun 20, 2016