How To Tell When My Furnace Filter Needs Changing


The best test to know if a furnace filter is ready to be changed is to hold it up to a bright light . If you can still see light through the filter then air is still getting through the filter. If you suspect the filter is dirty enough to restrict the air flow, then change it out to a new filter.

Nothing in this article should replace the knowledge of what the actual static pressures of your system is in relationship to the type of filter you are using. A good static pressure test should be done by a qualified service person.

Watch this short 6 minute video to understand how a static pressure test is done.

This may sound crazy, but some filters actually become better filters after they accumulate a little dust. Think about it. A little dust fills a few pores and fewer dust particles can pass.

When Does Your Furnace Filter Need ...

This image is of an Aprilaire Medial Air Cleaner Model 2000. It seems quite dirty.

This is that same dirty filter with a light behind it just to demonstrate a point. If light can pass through then so can air.

This is strictly a common sense approach to setting up a time frame for changing your filter. Don’t let the surface dust on the filter or the fact that the filter looks a little dirty indicate to you that the filter is done doing it’s job. It could be doing a better job being a little dirty.

Check the air flow before you remove your filter and then check it again with no filter in place. If you suspect the filter is dirty enough to restrict the air flow, then change it out to a new filter. It could be too small or the wrong type of filter for the amount of air flow needed.

If the filter is difficult to remove with the fan on compared to with the fan off then the filter is being restrictive. This is not a good test if your filter is just hard to remove because it’s a poor fit.

Those flimsy one inch thick filters can actually get sucked down into the system if they get too plugged up. So when the filter begins to bulge it’s past time to change.

It’s not necessary to buy a bunch of expensive equipment or pay for a service call from the local HVAC company to measure your air flow. Tape a simple ribbon onto a register in a manner that the ribbon blows so you recognize the amount of flutter.

Several things can indicated when a furnace filter is ready for the garbage can. But too many home owners throw them away way too early. Using a time schedule like changing furnace filters every three months regardless of how dirty the filter is can waste your time and money.

Every heating system is so different in it ability to filter the air. If your filter is in the grill mounted in your ceiling then the air flow has to be strong enough to pull the dust up into the filter.

With return ducts near the floor then it’s much easier for the filter to collect debris since particles that normally end up in filters are heavier than air and tend to float downward anyway.

These Are Some Signs Your Furnace Filter Needs To Be Changed

  1. The debris on the filter falls off when you remove it from the furnace.
  2. There is a bulge in the filter that makes it difficult to remove.
  3. You feel more air flow with the filter out versus when the filter is in the furnace.

Any of these three items represent a filter that has been in the system for too long. Find a time frame to change the filter that is shorter because filters that get this full of matter can be damaging to your system.

Filters that get that dirty can be compared to high blood pressure and will cause things such things as cracked heat exchangers and frozen air conditioning coils which can result in failed compressors.

So Just How Often Should I Change My Furnace Filter?

By now you should be understanding the problems with changing furnace filter too early versus when it’s too late and the HVAC system suffers a stroke.

Nothing like comparing a plugged filter to something we all fear to drive home a point.

The answer goes back to using common sense. In my 40 years of doing heating and air conditioning work this has been one of the most misunderstood concepts that home owners have about there forced air systems. Don’t throw the filter away too early and don’t replace it too late.

There are two classes of home owners when it comes to there heating and cooling systems.

  1. Those who throw away the filter too soon
  2. Those who don’t throw it away soon enough

Does Furnace Filter Thickness Matter?

The English language is sometimes funny how words relate, or is it don’t relate?

Yes, the thickness of a filter will matter because a thicker (pleated) filter can collect more matter in a longer period of time and would require replacement less often.

Thicker filters of the pleated type have considerably more surface area to allow more air flow and collect more dust and debris.

What’s on your filter under a microscope

Perhaps after seeing what the junk on your filter looks like under a microscope you don’t want to leave a filter in the furnace for as long as recommended.

Back to common sense and judgement calls. That ugly stuff is okay when it stays on the filter and if you’ve done your research and chosen a good filter then that ugly stuff is in the proper place, on the filter, and not in your furnace and duct work.

“Common sense ain’t common.” 

Will Rogers

What Is The Best Furnace Filter To Use?

Sometimes it helps when we exaggerate to show a principle or a point of discussion.

We’ve all blown through a straw and understand how the velocity of air out the other end can seem quite strong. Now imagine blowing through a piece of pipe about one inch in diameter and about the same length as a straw.

The volume is the same but the velocity of the air out the other end is much less with the piece of pipe.

Now try to picture your furnace blower moving air through a filter. A very small filter. And all that air has to get through that filter to supply air to your home.

The velocity or speed of the air through that very small filter would be quite fast. In fact, that high velocity or sheer force of air could even be allowing a lot of dust through that filter into your blower, duct work, and into your home for you to breath.

The opposite example in this exaggeration would be a very large thick pleated filter. That same amount of air flow is now moving much slower (and quieter) through that ultimate dust collector of a filter.

So where is the happy medium? With the concerns that home owners have today about indoor air quality, many people have selected higher efficiency pleated filters available at most hardware stores.

If all you’re filtering is an electric or gas furnace that may be okay. But, if your system includes an air conditioner or heat pump, you might be going out on a limb.

Not too many years ago, filter manufacturers graded their own filters for efficiency. There was no common mark to judge who’s filter was the most efficient.

Today we have a common judge. The MERV rating. Minimum Efficiency Reporting Value. The values range from 1 to 16 with 16 being the best. The higher numbers are more for commercial applications such as clean rooms. Residential MERV ratings are more in the middle of that 1 to 16 range.

Be aware that if you shop at Home Depot they have a different rating system. One of their own called Filter Performance Rating or FPR. Why would they do that? There are some claims that it’s associated with some price manipulation to get you to think that more efficient filters cost less. Not gonna try to prove that one, just be aware.

This is when a good service company needs to measure the static pressure of your system to determine the type and size of filter that will adequately let your blower deliver that correct amount of air.

Will It Help My Furnace Stay Clean If I Use Two Filters?

Using two filter would not likely be recommended by any professional HVAC person. I remember a job when a filter got so dirty it got sucked into a system and then the home owner added another filter thinking he forgot to replace the filter after removing it. The system suffered for air flow for quite some time.

Using two filters could easily allow you to fall into the stuck filter trap depending on the type of filter brackets your system has. Two filter can also run the chance of high pressures in a heat pump or air conditioning system.

The cost of two filters exceeds the cost of one thicker filter and one filter is simply easier to manage.

Can I Vacuum My Filter?

In a pinch you could vacuum your filter. Be sure to vacuum it back out in the direction that the dust and debris entered the filter. In other word don’t try to suck the dust through the filter. I’d say it was a common sense thing. Vacuum the filter if you think it will help get you by another day or so until you can get a new filter.

A good gauge to tell if your filter is doing a good job is to look inside the furnace. Look at the blower motor and wheel. The fan is a giant magnet for dust. If the fan assembly looks coated with dust. You need to change your habits or your design of filtering.

If you’ve been vacuuming your filter because a new filter is too expensive and you look at your blower and it’s coated with dust, then vacuuming the filter is a bad idea.

Can I Temporarily Run My Furnace Without A Filter?

Occasionally we all fall behind on our responsibilities such as changing our furnace filters. When you look at that dirty bugger and it just creeps you out, you hate to leave it in especially knowing what you’ve learned here.

Gas furnaces can be especially receptive to cracked heat exchangers when filter get plugged. If you want to see what a cracked heat exchanger looks like (caused by a plugged filter) go here.

So, just pull it out and get a new one on the way home from work tomorrow. You could kind of divide the amount of dust on the filter by the number of days it’s had residence in your furnace and that’s the amount of dust your blower and duct work will eat in 24 hours. Not very much.

Are There Furnaces That Don’t Have Filters?

Surprisingly yes. Williams makes a wall furnace that has no filter. There is no duct work attached. I have one in my shop. I took it out of a house where the home owner had another wall furnace he’d acquired and wanted the old one gone.

I took it home and dismantled it down to the last screw. There was so much dirt, dust, and crap in that thing I don’t know why it didn’t catch fire. It makes a darn good shop heater for me. But it’s getting dirty as it runs. Having a good compressor though helps blast out the dust on occasion. Something you wouldn’t want to do in a home.

Which Way Is The Arrow On My Furnace Filter Supposed To Point?

The arrow is an indicator of air flow. Air, flows in your furnace and duct system from the return ducts to the supply ducts which deliver the conditioned air to your rooms.

Take a look at the filter you’re using. Do you see a wire mesh on one side? This is meant to hold the together and keep it from bulging due to the pressure of the air flow. The arrow should point to the side of the filter with the mesh.

No matter where the filter mounts or is installed in your system, the arrow should point at the furnace. Point the arrow towards the furnace and not into the room or away from the furnace.

There are just a few ways that filters are incorporated into a duct system. .

  • Filter grills are mounted on walls or ceilings in or about the rooms of the house
  • Some filters are cut into the duct at or near the furnace with perhaps a door that slides or is hinged for filter removal.
  • There are some filters that come in cans mounted on the side of the furnace with a door.
  • And then there’s the filter where you remove the door of the furnace and the furnace manufacturer built a nice little slot to install your filter.

The last example is the least desirable. Equipment manufacturers provide this slot in there furnaces just in case the installer didn’t think enough of his work to provide something better.

If you have filter grills mounted in a ceiling or wall the arrow points into the duct. With the other examples of filters in duct work or near the furnace the arrow points at the furnace.

What If I Put My Furnace Filter In Backwards?

Nothing to loose any sleep over. I see them in backwards all the time. Different filters react differently to being installed in the wrong direction. It would not be a good idea to turn it around. The result of reversing the filter after it being in backwards for any time at all would be that the collected dust would be blown out of the filter into the furnace and duct work.

If you notice no significant loss of air flow with the filter in that way I’d probably leave it for some time and get some value out of the thing. It has something to do with how particular you may be. I can be a little obsessive compulsive over stuff at times.

Can I Vacuum The Dust Off My Furnace Filter?

Whether you vacuum off the dust or get out your Tim the Tool Man air compressor and blow the heck out of it, the integrity of the filter has been diminished. It’s not going to filter like it should. Filters are meant to set in a certain direction and collect dust and particulates for a certain amount of time and then be thrown in the garbage.

Despite how much of a tight wade we can sometimes be there is something to be said about the air quality in a home. We spend so much of our time eating, cleaning and living in our homes and the cost of doing the filtering the right way is so small compared to the health of our families.

Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

Recent Posts