How Often Should Furnace Filters Be Changed?

How Often Should Furnace Filters Be Changed?
Roger Bakies
Residential Sales Professional

I have been in the residential/light commercial HVAC business for 30 years. I grew up in a sheet metal fabrication shop and have installed, serviced, sold and helped people choose new systems to best fit their needs and lifestyle. I look forward to helping you pick the best fit for your home!

About This Article

Furnace filters do a lot of good, but are seldom replaced when they should be. Dirty filters can lead to expensive repairs and bad indoor air. When should they be replaced?

The filter is perhaps the least appreciated part of an HVAC system. It does its job quietly while it remains out of sight. It rids the air of harmful particles, keeping your family safe and healthy.

Filters can also keep dust bunnies at bay, rid your house of bacteria, eliminate cigarette smoke and smog, and help get rid of the dander from your lovable cats and dogs.

This article addresses these tireless workhorses, explaining their various types and prices, how often they need to be changed, and why it’s so important to replace them when they’re dirty.

When Should You Change Your Furnace Filters?

The easy answer to the question of when to change your filters: whenever they have lost their effectiveness. Clean or replace them when they’re so dirty, they’ve stopped working. Visually you can tell when it starts to load up. It’s helpful to have a clean one to compare it to.

It’s not uncommon to find a 20-year-old furnace that’s had the same one-inch filter in it for 20 years, and the blower wheel is caked with dirt. That means there’s likely dirt on the heat exchanger, there’s dirt on the evaporator coil, and your furnace has lost efficiency.

That dust and pollen will then sail back through the ducts again, keeping that indoor air just as dirty as though there was no filter at all. In any system, cleaner is better.

You can’t change your filter too often. If you think your filters are expensive, think of the stress it might be causing to your $10,000 system.

Some of the cheapest filters need to be replaced every 60-90 days. Some others recommend twice a year, or even once a year. We recommend looking at your filters before the due date and replace/clean them when necessary. Why? Because your air ducts may be dirty (it happens to the best of us), and that dirt can wind up on your furnace quickly, for instance. A filter that is rated to be changed after six months may need to be replaced after only four months.

Buy Your Filter

Why Should You Replace Your Filters?

Changing your air filter on a regular basis will improve your indoor air quality, improve your health, reduce the amount of dust and dander in the air, and help your HVAC system be more efficient.

For allergy and asthma sufferers who would benefit the most from higher-end filtration systems, clean air can make a huge difference.

Dirty filters increase static pressure. Too much static pressure can cause strain on your system, reducing its performance, and can result in wasted energy. If left untreated, it can result in premature failure of the system. It strains every aspect of your HVAC, from the blower motor to the ducts.

A lot of the one-inch filters that you can get from Home Depot and Lowe’s are cheap filters that might be a MERV 4 or 5 or lower. But the problem is there’s not much filtering surface there, and it loads up very quickly. The filter will say it needs to be changed in 30-90 days, but in practice, 90 days is probably far too long.

The cheapest one-inch filters, the ones that say change it 90 days, well, you can use it for 90 days and it doesn’t catch very much. The pleated ones, they say to change them every 30 days. If you use a pleated filter, you should change it every 30 days when the weather is hot or cold (when your HVAC is more active), and every 60 days when it’s milder (and your HVAC isn’t working very much).

The media air cleaner is a four-inch filter that usually doesn’t need to be replaced for about 6-12 months. If you took it out of its frame and stretched it flat, it might be 20, 25 feet long, whereas a one-inch filter, even if it’s pleated, might be a foot and a half. The media filter has much more material, which means it can catch more junk.

Four Inch Filter

Can You Change Your Filters Yourself?

Generally speaking, the average homeowner is capable of changing their filter, though sometimes the system is in the attic, or in a crawl space, and the homeowner can’t reach it easily.

The standard filter most people have is a one-inch filter that you can slide in and out. Sometimes you have to take the bottom door off of your furnace to reach it. And sometimes there’s a wire that keeps the filter in place.

When you go to change your filter, the equipment should be off. You shouldn’t have the blower running. Sometimes a filter has been blown in by the return air, so when you try to pull it out, it catches. You have to slide something in there, say a table knife or a putty knife, and reach in and pull the filter away from the furnace to guide it out.

You need to make sure the arrows of the new filter are pointing toward the furnace; that’s the direction the airflow moves. A few people miss that.

How Are Filters’ Effectiveness Rated?

You’ll often see a MERV Rating (Minimum Efficiency Reporting Value) assigned to filters based on how efficiently they catch particles of varying sizes. Higher is better.

The cheapest filter is rated MERV 4 or less and can cost as little as a couple of dollars. A filter rated MERV 1 can capture some pollen, dust mites, and clothing/carpet fibers. It’s better than no filter at all, but keep in mind: you get what you pay for. A low-end filter will need to be replaced more often and doesn’t capture particles such as mold, tobacco smoke, small dust particles, etc.

One inch filter

When you hold a cheap filter up to the light, you can even read through it. Little wonder that it’s not very effective.

One step up is a pleated filter, which can be made from paper or cloth, and is enclosed in a cardboard frame. Pleated, one-inch filters generally have a MERV anywhere from 5 - 8. They cost $40 and up.

Prices for filters rated for MERV 9-12 start in the hundreds of dollars. Besides the materials already mentioned, they are good for catching auto emissions, dust that can damage lungs, smog, and even Legionella.

Air clean enough for hospitals comes from filters rated with a MERV 13-16. The CDC recommends MERV 13 filters or better for schools, buildings, and homes. A MERV rating of 16 can capture particulates between .3 and .1 microns. For comparison, a human hair is about 100 microns thick.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) states: “Our current recommendation is to use a filter with a Minimum Efficiency Reporting Value (MERV) of 13, but a MERV 14 (or better) filter is preferred. Of course, the ultimate choice needs to take the capabilities of the HVAC systems into consideration.”

A MERV 11 is pretty standard. The MERV 11 offers a mix of good airflow and capture rate. However, people with allergies that have breathing sensitivities might want to go to a filter with a higher MERV rating.

An additional measurement for filters is the Clean Air Delivery Rate, or CADR. This rating is the number of airborne particles that can be removed from the flow of a set volume of air per minute. The higher the number, the more effective the filter is at removing particles from the air.

The most expensive and most effective air cleaners are electric filters, which are a hybrid, using both filters and electronic plates. Technically they are called air cleaners. They can clean air with up to 700 CADR.

Some filters don’t need to be replaced, but they must be cleaned with a vacuum cleaner at regular intervals.

Carbon filters can also be added to an HVAC system to provide additional filtration from smoke, cooking odors, and pets. However, you’ll want to be careful the insert doesn’t restrict too much airflow. Too much restricted airflow can result in increased static pressure

Aprilaire Filters

Normally when we install a furnace, heat pump, or air conditioner, we replace your filter with a four-inch-thick Aprilaire media filter, which ranges from MERV 11-16. They allow for better airflow than many of the one-inch filters, and still maintain filtration efficiency.

The other thing we like about that Aprilaire filter is it provides the lowest static pressure on the system.

Aprilaire filters keep the air clean. Clean air keeps your entire HVAC system clean, which includes the blower that moves the air, the heat exchanger in the furnace, the evaporator coil above the furnace, as well as your ductwork.

Electronic Filters and Air Purification

Electronic filters have a pre-filter, and then there are plates and wires. It’s a high-voltage system that sets up a charge, and as the dirt particles go through, the charges make them stick to the plates. The Trane CleanEffects Air Cleaner is one example. It is especially effective at capturing allergens and other particles as small as .1 micron.

There are some washable filters; they’re billed as electrostatic air cleaners. These washable filters are usually pretty dense, and they’re usually pretty restrictive for airflow. You can wash them off or use an air compressor to clean them.

Common HVAC Air Filter FAQs

-       My thermostat tells me I need to change my filters when they don’t need it. What’s the problem?

Most thermostats have a filter change reminder in them. A timer measures how long the blower runs, and then sends an alert after a set number of hours.

Part of an installer’s job is to input what type of filter is used in the system. Then the thermostat will calculate how many hours it can go before the filter needs to be changed.

It may say you need to change your one-inch filter in sixty days, when, in fact, you have a four-inch media filter that doesn’t need to be changed yet. That setting needs to be set up properly at the time of installation. If this issue sounds familiar, you will most likely need a service tech to come out and change that setting.

-       Why Don’t All HVAC Units Use Four-Inch Filters If They’re More Effective?

Why don’t all units use four-inch filters? That media filter cabinet and the filter have a cost. The cabinet that houses it may not be built for a four-inch filter, and that filter costs more than the one-inch filter.

The other reason not to would be is there’s no room for it. You need an additional 5 ½, 6 inches to slide in a four-inch filter. If the furnace is against a wall or in the closet, you’re probably stuck with the one-inch variety.

A graphic showing how filtration works.

What’s Next

HVAC technicians would be more than happy to do the dirty work, to replace a grungy filter with a similar or more expensive one. They can also wax rhapsodic about the higher-end filters, including the very pricy HEPA varieties and air purification systems.

In the end, it’s up to you. I hope that this article answered a question or two that was on your mind about filters.

What’s most important is that filters are as essential to your HVAC as your furnace, heat pump, or air conditioner, but they are much easier to take care of. Filters are relatively cheap, especially compared to the cost of repair to your HVAC.

If you’d like to learn more about filters and indoor air quality visit our website’s filter page. There you can check out additional resources on filters and other HVAC topics. You can also order filters directly and have them delivered right to your front door.

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