All About Central Air Filters
Why Does My Central Air Have Two Filters?

Let’s say you just moved into a new house 3 months ago, and it is time to change out your air filters. You go to look in the usual spots for air filters, and to your surprise, you find filters in multiple locations! Confused?

How Central Air Systems Work

If you have a central furnace and/or AC system in your home (as opposed to ductless systems), then your system is composed of multiple parts:

• The air handler – this metal box contains the fan and fan motor
• The heating/cooling system – this system cools or heats the air by passing it through a set of hot or cold coils, thereby changing the temperature to the desired range set by your thermostat
• The duct system – this duct system does two things:
  • Air from the outside the home is drawn into the unit, chilled or heated to the right temperature, then sent into the home
  • Air from the home is pulled back through the return ducts, chilled or heated to the right temperature, then sent back through the duct system into the home. This helps keep air flowing throughout the home and keeps your temperature stable

Understanding the parts matters because it can explain cases of multiple air filters being used

When You Need Two Air Filters

There are multiple cases when multiple air filters are needed:

• Air filters are placed in both the return duct and in the air handler – this setup exists for a few reasons
  • The lifespan of filters differs – The amount of times you need to replace filters in the return duct vs the air handler varies because they are sized differently. For example, the most common size filter for return vents have a depth of one inch – by contrast, if you look at our filters specifically designed for the air handler, they have a depth of four or five inches. Because the air handler filters have more filter material by virtue of being larger, they can collect and hold more dust, pollen and smoke before they need to be changed.
  • Your HVAC system has multiple return ducts – if you live in a big-enough home, condo or apartment, you’ll have multiple return ducts to effectively recirculate air across the entire home – after all, nobody likes stuffy rooms and irregular temperatures. Having multiple return vents solves this problem, but it also requires filters at each vent to properly filter the air and prevent particulates from getting into your fan motor.
  • You have multiple central air systems, and each requires separate filters – this is especially common in two-story homes, where it is easier to regulate temperatures across individual floors. This will mean you have more than one air handler filter, and you will likely have multiple return vents – at least one for each duct system per HVAC system

When Two Air Filters Is a Bad Idea

Lastly, while this is not a common scenario, it’s worth calling out this piece of advice from a Houzz forum discussion – if you see a return duct that can accommodate a 2 inch or 4 inch air filter, but has multiple 1 inch filters stacked on top of one another to fit the slot, get rid of this setup immediately. It is questionable if it works the same way as one 2-inch or one 4-inch air filter in terms of filtration effectiveness in working with your central air system. Additionally, its more expensive to buy Air filters this way. For example, a 16x25x1 air filter costs $22.66* if you buy two filters – by contrast, buying one 16x25x2 air filter costs $17.58* - a much better deal.

* pricing is current as of 8/16/20 – prices may have changed since the publishing of this price

Changing Your Air Filters

Now that you know where you must look to find all your air filter replacements, you are on your way to breathing freely in your home. If you find you need multiple air filters, you don’t need to buy from multiple stores – simply find your sizes and your central air system brand on our site to buy the type and quantity you need. If it’s your first time installing air filters, make sure to read up on these resources to buy the right products and get it working correctly:

• How to Measure Your Air Filter
• How to Change Your Furnace Filter