Which Side Of Furnace Filter Should Be Dirty?

Almost all types of furnaces, no matter their fuel or location, are equipped with air filters. Because of this, you are wondering, which side of the furnace filter should be dirty? We combine industry professional knowledge and up-to-date research to thoroughly answer your question in this post.

In all cases, the furnace filter should be dirty on the side that the air comes into the furnace from. If your air filters are dirty in some other fashion, it is likely your furnace is not working properly, or the filter was installed incorrectly.

Keep reading the rest of this post for more details on why furnaces have filters, which side of the furnace filter should be dirty, and how often to change your furnace filter. To conclude, we answer several questions related to the topic of this post.

Male Hands Change Large Pleated Furnace Air Filter. Which Side Of Furnace Filter Should Be Dirty

Why Do Furnaces Have Filters?

Almost all home heaters which rely on airflow have cleanable filters built-in or are equipped for disposable filters. These filters help keep the furnace clean, improve furnace efficiency, and help with indoor air quality.

Male hands are installing a furnace air filter. This filter is not brand new but it is relatively clean, it was removed for inspection

Furnace types with air filters include all central furnaces, mini-split ductless heat pumps, and some plug-in electric heaters. Heaters that do not use air filters include electric wall inserts and most plug-in electrically powered heaters.

Some furnaces even have two or more filters to either cover more area or to remove even more dust, dirt, and particulate from the air. The advice and facts contained in this post are applicable to furnaces with one, two, or even more filters.

According to Energystar.gov, clean air filters help to keep furnace systems running efficiently by preventing grime and dirt from building up in the system. Essentially, all the moving parts and heat exchangers in your furnace work less well and age quicker if covered in airborne dirt.

dirty air filter close up photo

Also, dirty filters force the furnace fan to work harder to pull the same amount of air. This extra work leads to higher energy bills and less efficient home heating.

Further, and according to this article from the Environmental Protection Agency, in-home air filtration is one of the best ways to help ensure clean indoor air. Furnaces, especially central furnaces, pull and push air from the entire home, making them especially useful for cleaning all your home's air.

As demonstrated here, furnace filters not only help keep you and your family healthy, but they also save you money by keeping your furnace running more efficiently and effectively.

Which Side Of Furnace Filter Should Be Dirty?

Given the importance of furnace filters and especially clean furnace filters, it becomes clear why understanding furnace filter dirtiness is important.

As mentioned above, it is most common to see furnace filters gather dirt, hair, and dust on the side of the filter that first encounters airflow. This, however, begs the question of how to tell which way the airflow is on your furnace. We answer this question in the next subsection.

Sometimes, furnace filters may appear dirty on both the leeward and windward sides. This is usually the case when a furnace filter has become so dirty that the dirt has sucked over to both sides. Other times, it is possible that the filter was flipped around purposely by someone else.

If you find a filter that is dirty on both sides, first replace or clean it (depending on the type of filter). Then once your furnace has run for a couple of weeks, you should find that only one side is dirty. If both sides are dirty again, it is time to call an HVAC technician to do a little maintenance.

How Do I Know Which Way the Airflow is on my Furnace?

Often, you can determine the airflow on your furnace through simple visual inspection. The build-up of dirt is one clue, as are marks that are labeled to indicate the way that air flows.

Usually, a furnace will come with a filter pre-installed. It is usually advisable to trust the airflow labels on these pre-installed filters as HVAC professionals are well-practiced at understanding how furnaces work. However, it is possible the filters were installed incorrectly by previous homeowners or renters.

Senior male reaching up to open filter holder for air conditioning filter in ceiling

Another technique to understanding furnace airflow is through the sense of feel. First, turn the furnace on, or turn on just the furnace fan. Then, hold your hand up to different parts of the furnace to see if you can feel the direction of airflow.

Sometimes, it is hard to tell exactly which way the air is flowing by feeling alone. In these cases, take a tissue or other light item (like yarn) and hold it near the vent or area in question with the furnace still on. The item will point in the direction of the airflow!

How Often Should I Change My Furnace Filter?

According to this article from Energystar.gov, you should replace your furnace filter every month. On the very outside, do not wait any longer than three months between furnace filter replacements or cleanings. Be sure to replace your filter with a filter of the correct dimensions.

Click here for a furnace filter pack from Amazon.

However, the actual best practice is to replace your filter when it becomes dirty enough to restrict airflow to your furnace. This, of course, depends on the type of filter, type of furnace, and cleanliness of your house, and ductwork.

To understand when to change your filter, listen to how hard your furnace motor is working and regularly check the filter. Remember, it is usually cheaper to change your filter too often than not often enough. If you have cleanable filters, you are in luck; these are generally free to clean.

Why isn't my Furnace Filter Dirty?

If your furnace filter is not dirty, it could just be a sign of a very clean house! Often, it takes months for furnace filters to begin to soil, especially in new construction or in homes that are lightly lived in.

On the other hand, if your furnace filter is suspiciously clean for a very long time, a few things might be amiss.

man inserting a new air filter in a HVAC Furnace

First, it is possible your filter is in the wrong place. In this case, all the dirt that your filter would be catching never comes into contact with the filter.

To determine where your filter should be located, take the time to read the furnace owner's manual, look the furnace up online by make and model, or ask for help from an HVAC professional.

Second, it is possible that your filter is not becoming dirty because the furnace is not functioning properly. If you are sure the filter is in the right place and it still stays clean, this might be a sign that your furnace fan is underpowered or underperforming.

Generally, you will notice an underperforming fan first because your furnace will not be heating your home properly. If this is the case, call up a local service company to get that furnace back up and running.

Why is my Furnace Filter Dirty After One Week?

If your furnace filter becomes dirty after a single week, it is because there is a lot of dirt in the air of your home. This may be because your home houses a lot of people and pets, and other dirt producers. Other times, the dirt is windblown from outside the house.

Cleaning inside heating floor vent with Vacuum Cleaner

Steps to take to reduce the speed at which your filter gets dirty are to take greater care when cleaning your home, clean out ductwork in your home, and improve the seals around your doors and windows.

Can I Spray Air Freshener on my Furnace Filter?

Yes, you can. Spraying air freshener on your furnace filter is a way to potentially help improve the scent of your home. Alternatively, consider using specialty furnace filter scented pads.

Click here for whole-home air fresheners from Amazon.

These products are designed to be installed on your fresh filter and thus provide more and longer-lasting scents as compared to simply spraying air freshener on your furnace filter.

Take note; if you find that a bad order is coming out of your furnace, the problem is likely something a couple of spritzes of air freshener will not fix. Instead, you might have something rotting in your ducts, or you might have an issue with your furnace that requires a specialist.

Additional Reading

To learn more about furnace filters and furnace cleanliness, read these great articles from HVAC Seer:

Man cleaning air ducts in home. Which Side Of Furnace Filter Should Be Dirty

In Closing

In this post, we answered the question of which side of your furnace filter should be dirty. This guide includes many helpful tips for understanding the use of furnace filters. To conclude, we answer several questions related to the topic of this post. Good luck!
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