Changing your HVAC air filter is one of the easiest ways to protect your heating and cooling system. A clean filter helps air move through your system, supports indoor air quality, and can reduce strain on your furnace or air conditioner.
Most homeowners can replace an AC or furnace filter in just a few minutes. The key is knowing what size filter you need, where the filter is located, which way the air filter goes, and how often to change it.
Below, we’ll walk through how to change the filter on your AC or furnace, plus signs it may be time for HVAC maintenance from Deljo Heating, Cooling & Plumbing.
Step-by-Step HVAC Air Filter Replacement
Replacing your HVAC air filter is usually simple once you know where it goes and what size you need.
1. Check the Filter Size
Before you remove the old filter, make sure you have the right replacement. HVAC filters come in many sizes, and the wrong size can allow air to bypass the filter instead of passing through it.
You can usually find the size printed on the edge or frame of the old filter. Common sizes include:
- 16 x 20 x 1
- 20 x 25 x 1
- 16 x 25 x 4
These numbers refer to the filter’s length, width, and depth. If the size is not printed on the filter, measure it with a tape measure and round up to the nearest inch.
Once you know your filter size, you can order replacement filters through Deljo’s filter shop so you have the right filter ready before your next change.
2. Locate the Air Filter
If you are asking, “Where is the air filter in my house?” start by looking near the return air grille or the indoor HVAC equipment. Depending on your system, the filter may be behind a large wall or ceiling grille, inside a slot next to the furnace or air handler, or in a cabinet connected to the blower area.
Return grilles are usually larger than supply vents because they pull air back toward the HVAC system. Larger homes may also have more than one filter, so check each return area if you are not sure.
3. Shut the System Off
Set the thermostat to off before you pull out the old filter. This helps keep loose dust from being drawn into the equipment while the filter slot or return grille is open.
4. Take Out the Used Filter
Open the return grille, filter slot, or filter cabinet. Carefully slide the used filter out and avoid shaking it, since dust and debris may fall from the filter.
5. Line Up the Airflow Arrow
Before installing the replacement filter, look for the arrow printed on the filter frame. That arrow should face the direction air moves back into the HVAC equipment. For many systems, this means the arrow points toward the indoor unit.
6. Slide the Replacement Filter Into Place
Put the new filter into the opening with the arrow facing the correct direction. Make sure the filter sits flat and does not bend, crush, or leave large gaps around the edges.
7. Secure the Cover and Restart the System
Close the return grille, filter slot, or filter cabinet. Once everything is secure, turn your HVAC system back on at the thermostat.
How Often Should You Change Your HVAC Air Filter?
Most homeowners should change an HVAC air filter every 1 to 3 months. However, the right schedule depends on your home, your system, and how quickly the filter gets dirty.
You may need to replace your air filter more often if:
- You have pets
- Someone in your home has allergies or asthma
- Your HVAC system runs often
- You live near heavy traffic, construction, or high outdoor dust
- You recently completed a home renovation project
- Your filter looks gray, dusty, or clogged
If you are wondering how often to change an HVAC air filter, start by checking it once a month. If it still looks clean, you may be able to wait. If it is covered in dust or debris, replace it.
Signs Your Air Filter Needs to Be Changed
You do not always have to wait for a set date to replace your filter. Your system and your home may give you signs that the filter is dirty.
Common signs include:
- Dust buildup around vents
- Weak airflow from registers
- Higher energy bills
- More allergy or dust concerns indoors
- Uneven heating or cooling
- HVAC system running longer than usual
- Filter looks gray, dusty, or clogged
- Furnace or AC seems to be working harder
A dirty air filter can affect comfort, equipment performance, and indoor air quality. Replacing the filter is a simple first step, but ongoing airflow problems may point to a larger HVAC issue.
Tips for Changing Your HVAC Air Filter
A few simple habits can make HVAC filter replacement easier to manage.
Check Your Filter Monthly
Your filter may not need to be replaced every month, but it should be checked regularly. A monthly reminder can help you stay on schedule.
Write the Date on the Filter
Before installing a new filter, write the date on the edge of the frame. This makes it easier to track how long the filter has been in place.
Use the Right Filter for Your System
Some filters have higher MERV ratings, which means they can capture smaller particles. However, not every HVAC system is designed for a very restrictive filter. A filter that is too dense may reduce airflow.
If indoor air quality is a major concern, Deljo can help you choose the right filter or recommend whole-home air quality options that work with your HVAC system.
When to Call Deljo for HVAC Maintenance
Replacing your air filter is a great maintenance habit, but it does not replace a professional tune-up. Your furnace and air conditioner also need regular inspections, cleaning, and performance checks to run safely and efficiently.
Call Deljo if:
- You are not sure where your filter is located
- You do not know what filter size or type to use
- Your system still has weak airflow after a filter change
- Your AC is freezing, leaking, or blowing warm air
- Your furnace is making noises or not heating evenly
- Your energy bills keep rising
- Your home has ongoing indoor air quality concerns
Deljo has been helping Chicago homeowners with heating, cooling, and indoor comfort since 1922. Our licensed, NATE-certified technicians can inspect your system, replace the filter if needed, and help keep your equipment running safely and efficiently.
Before scheduling service, you can also check Deljo’s current specials and coupons to see if there are available offers for HVAC maintenance, repairs, or system service.
Call Deljo today at 773-829-4295 or schedule your HVAC maintenance online.
HVAC Air Filter FAQs
Which way does the air filter go?
Look for the arrow on the side of the filter. It should face the direction air travels through the system, which is usually toward the indoor HVAC equipment rather than back toward the room.
What happens if you do not change your air filter?
A dirty air filter can limit airflow, increase energy use, reduce comfort, and add stress to your furnace or AC. It can also contribute to weak airflow, frozen AC coils, furnace overheating, and extra dust in the home.
Is there a difference between an AC filter and a furnace filter?
In many central HVAC systems, the same filter helps protect both the cooling and heating equipment because the AC and furnace use the same indoor airflow system. Some homes are set up differently, so check your equipment or ask a Deljo technician if you are unsure.
What filter should I use for better indoor air quality?
The best filter depends on your HVAC system. Higher-rated filters may capture smaller particles, but they can also restrict airflow if your system is not designed for them. Deljo can help you choose a filter that supports indoor air quality without hurting system performance.
