There is debate about which furnace filters are better – 1” vs 4” filters – or maybe a 2” filter?
This page will assist you in three decisions:
- What level of air filtering you want
- What thickness filter you need to get the filtration you want
- Ultimately, what filter to buy
It covers 1-inch vs 2-inch vs 4-inch furnace filters regarding how well they work, how often they must be replaced, their cost and other important areas to consider.
A New Furnace Filter – Where to Start
Here are two keys to consider before we go further.
First, most furnaces and air handlers are designed with 1-inch air filter slots. Sure, you could try to jam a 2-inch thick filter in there (we don’t recommend it – and there’s no need to do it.
If you want a thicker filter, the furnace or plenum/ductwork will have to be modified at a cost of $350-$700 and maybe more. That’s the end of the story for many homeowners, especially when they hear the next point.
Secondly, is your purpose to get air that is better filtered? We assume it is.
Here’s the good news: A 1” (1-inch) filter might be all you need to get the level of air filtering you want.
MERV Rating and Air Filters: How well a furnace filter traps particles like dust, pollen, pet dander and smoke is indicated by its MERV rating. The higher the MERV rating, the more and the smaller the particles the furnace stops, traps and keeps out of the air you breathe.
How many and how small the particles are it traps – two keys.
MERV Max: MERV 12 or MERV 13 is pretty close to the highest MERV rating recommended for furnaces. Filters with higher ratings than that can restrict airflow and damage your HVAC equipment – a topic discussed later on this page.
A MERV 13 filter traps lint, dust, pollen, dust mites and their “debris,” mold spores, pet dander, smoke particles and even moisture droplets from sneezes.
And…MERV 13 filters are available in 1”, 2” and 4” versions! So, if you are using a cheap 6, 7 or 8 MERV filter and want to trap more and smaller pollution particles and allergens from the air, you can do it without the expensive modification to your airflow system/ductwork.
What does MERV stand for? Care to guess? It stands for Minimum Efficiency Reporting Value. That’s a technical way of saying “how many and how small” particles it filters out of the air you breathe.
High MERV 1 Inch Filters Get Dirty Quick
The more and smaller particles a filter traps, the faster it fills up with dust, pollen, pet hair and dander, etc. That makes sense, right?
So, if you choose a 1” filter with a high MERV rating, you will need to change it every month or two when the furnace, heat pump or air conditioner is used a lot.
A high-MERV 2” or 4” filter requires changing less frequently.
1-Inch vs 2-Inch vs 4-inch Furnace Filters
Here are overviews of the three furnace filter thicknesses we’ve been discussing – their ratings, what they trap, pros/cons and how often they must be changed.
Let’s start with a Table showing MERV ratings, what filter thicknesses they’re available in, the size of particles they trap and types of pollutants and allergens that apply.
2" & 4" Filters
Particle Sizes Trapped >
3 Microns and Larger
<.3 to 1 Microns
Dust Mite Debris
1-Inch Furnace Filters
1” or 1-inch air filters are easily the most common. As noted, most furnaces and air handlers from all the major brands including Trane, Lennox, Carrier, Rheem, Bryant and Heil come with filter slots sized for 1-inch filters.
They offer MERV effectiveness of 1-13, but 4-8 is most common.
MERV Ratings: 1-inch furnace filters range in MERV rating from 1-13. The “cheapies” you buy at the hardware store for $8 or less are about MERV 6 filters, and they only stop the “big stuff.”
1-inch air filters with MERV 8 to MERV 13 trap a lot more, as you can see by the chart.
Pleated 1-inch air filters have higher MERV ratings for a simple reason – they have more surface area to trap particles. Many of them also feature media – the stuff the trapping part is made from – that has a denser weave.
Caution! High MERV 1-Inch Filters
When you want improved air quality through better air filtering, but you don’t want the expense of modifying your equipment, a 1-inch 13 MERV filter does the job.
But – It will clog quickly. A 1-inch furnace filter with a 6-8 MERV rating will last up to 3 months even with fairly heavy furnace use.
However, a MERV 12 or MERV 13 filter that is just 1” thick will clog more quickly. If you choose a high MERV 1-inch air filter, check it every month during heating or AC months, and change it when it is too dirty to see light through – or before.
What Could Go Wrong?
Several problems can happen when a 12 or 13 MERV air filter isn’t changed soon enough. It’s an ugly list.
Airflow will be restricted, and you could end up with low airflow to some rooms. They’ll remain cooler in winter and warmer in summer than the rest of the house. This isn’t the only cause of low airflow, but it’s a major one.
Energy efficiency will drop. Low airflow causes the HVAC equipment to work harder than usual, burning more fuel and/or using more electricity. That means wasted energy and higher utility costs.
HVAC systems that work too hard break more easily. Especially vulnerable parts include the air handler or furnace blower motor that’s working overtime to push and pull air through the dirty filter and the heat pump or AC compressor tasked with cycling refrigerant between the outdoor and indoor units.
1-inch Filters Bottom Line
If you install a high-MERV 1” filter, replace it often. Think of the $300 to $600 or more you’re saving by not modifying the system – and spend some of that on changing the furnace filter more often.
Info Tip – Furnace filter or Air Filter? While we are calling them furnace filters, they are also called air filters because they are used in air handlers too. An air handler is a unit with a blower, like a furnace, but an air handler doesn’t have a burner. Instead, it circulates air that is heated and cooled with heat pump technology.
2-inch Furnace Filters
This size isn’t as common as 1” or 4” filters, but they are available.
MERV ratings are usually 8-16, with the average being MERV 10.
Pros, Cons and Cost: Pleated design traps more, so they might fill up more quickly. Check the filter every 1-2 months during heavy use, and change when needed to avoid the kinds of efficiency and mechanical trouble discussed in the section on 1-inch Furnace Filters. 2-inch filters cost $10 - $25 each based on size, quality and brand.
4-inch Furnace Filters
MERV ratings for filters with this thickness also range from 8-16, though most are around MERV 12 or MERV 13.
Pros, Cons and Cost: These thicker filters have 3+ times more surface area than 1” filters and twice as much as 2” filters. This means they can collect more dirt particles and other pollutants and allergy-causing particles before needing to be replaced. The disadvantage is cost. Expect to pay $25 to $60 each depending on the size, MERV rating, brand and any enhancements like carbon for removing odors from the air.
How Often Should I Change the Furnace Filter?
Whenever it needs it, which will be from 1-10 months. This Pick HVAC Guide “Signs the Filter Needs to be Changed” has plenty of detail. And see the chart below.
Here is how often you should change your furnace filter aka air filter.
But remember, time isn’t the only factor.
Replace the filter more frequently when:
- You’re using a high MERV filter like a MERV 10, MERV 11, MERV 12 or MERV 13
- The furnace, heat pump or air conditioning is being heavily used
- You have pets, especially shedding or molting pets
- Someone in your home has allergies, asthma or other breathing issues
- Your climate is dry and dusty
- It is pollen season