What Happens if You Forget to Replace Your Furnace Air Filter?

You forgot to replace your furnace air filter for several months and are wondering what could happen. I have asked myself that question before because I have forgotten to change my furnace filters twice with two different houses.

If you forget to regularly replace your furnace filter, your heating, ventilation, and air conditioning (HVAC) system will work less efficiently. Without a new filter, eventually, your furnace will overheat, stress the system, and possibly stop working. Also, your indoor air quality will be worse because a clogged filter can not effectively prevent contaminants, such as pollen, dirt, dust, and pet dander, from entering your home.

To convince myself that it is essential to regularly replace my furnace filter, I have compiled a list of possible consequences of forgetting to change the furnace filter.

Possible Consequences of Forgetting to Change Your Furnace Filter

You may think that I was kidding when I said that I forgot to change my furnace filter. We have been renting a house for nine months now and will be moving to a new house in Lewes, Delaware, in July.

Lately, I have been thinking a lot about the many things that we have to do with our new house. Today I walked up at 5:00 am with a big worry: Have we changed the furnace filter in the house we are renting? The answer was a big NO.

I took our corgi to the customary morning walk. I went down to my basement, located the filter, and ran to my local Lowes to buy a new filter. You can take a look at the old vs. new filter picture on the blog post feature image. I am not proud. I need to improve if I want to keep our new house in good condition.

Below is a list of what could happen if you forget to replace your furnace filter. There are two types of costs you may incur, economic costs and health and comfort-related costs.

Poor air quality in roomsHigher energy bills
Poor temperature regulationRepairs due to furnace failures
Worse allergy symtomsShorter lifespan of the furnace
Dirtier homeCarbon footprint increase

Where is My Furnace Filter?

Answering this question can be tricky. The furnace filter is typically located between the return air duct and the furnace or air handler. And this was the case for our former Pasadena house and the house we are currently renting in Delaware.

But our new house in Lewes has three filters. One in the living room, one in the master bedroom, and one on the second floor.

You could have a different model that may require you to open or slide the door panel to get access to the furnace. The filter may be located on the side or at the bottom.

Why is There an Arrow on My Filter?

Without understanding why, when I changed the filters, I made sure the arrow on the filter was pointing in the same direction as the old filter. I was just hoping the prior person who changed it knew what he/she was doing.

It is now time for me to understand this.  Most filters have an arrow stamped on its side, which indicates the direction the filter has to face. The arrow has to point to the direction of the furnace’s air handler or blower compartment.

Below is a picture of the filter I changed yesterday at our rented house. You can see that the white arrow on the top of the filter is pointing to the furnace’s air handler.

What happens if the furnace filter is backwards? It is hard to tell by the naked eye, but one side of the filter is more porous than the other. The goal of this design is to trap pollutants more effectively while facilitating airflow through the system.

If you put the furnace filter backwards, you will add unnecessary stress to the air handler because of the loss of airflow.

What Happens if You Forget to Replace Your Furnace Air Filter? - Correct direction of arrow on filter

What Size Filter do I Need?

Furnace filter size for new house

Before you go out to the store, you need to know the filter size you need. In the picture above, you see the numbers 16x20x1. That is 16″ length, 20″ wide and 1″ thick. For this information, I would rely on the prior filter. This is the filter size I need for my rented house.

I have to be careful, though. For my new house, my filters are square, size 20x20x1, as you can see in the picture on the right.

How to Choose the Right Type of Furnace Air Filter

This video goes over the various types of filter options we have to choose from. From all the options on this video, you and I are likely to need a “pleated type filter,” like the one I bought yesterday. It is also the type I will need for my new house.

How to Choose the Effectiveness of the Furnace Air Filter

Once you are convinced that you need a pleated filter, you need to decide on the quality of the filter. The industry makes it a little difficult by using different rating types to classify the quality of the filters you buy.

There are three primary ratings we need to be aware of: MERV (Minimum Efficiency Reporting Value), MPR (Microparticle Performance Rating) and FPR (Filter Performance Rating). Each level blocks different pollutants and cost progressively more.

I put together a table below to make sense of all this.

MERVMPRFPRPollutants BlockedPrice
6300N/ADust, Lint, and Pollen$
86005Dust, Lint, Pollen, Dust Mites and Mold$$
1110007Dust, Lint, Pollen, Dust Mites, Mold, Pet Dander, Cough + Sneeze, Smog, and Smoke$$$
131500-190010Dust, Lint, Pollen, Dust Mites, Mold, Pet Dander, Cough + Sneeze, Smog, Smoke, Bacteria, and Virus$$$$

I decided to go with the highest quality, but each filter costs $20. I think I will continue buying the top quality filters for the new house, but I will try to get a better price at Amazon.

How Often do You Need to Change the Furnace Filter?

From my initial picture, I know it is best to change the filters before nine months, LOL! But seriously, it looks like the consensus out there is that you need to change this type of filters at least once every three months. My house builder told me that it is best to change them every month.

I think I will compromise and change them every two months. Since I will have three filters in the new house, it will cost me about $360 per year. A little pricey, but I think it is worth it if my family is healthier because of that and save repairs cost in the long run.

I will add this recurring item to my to-do list in my house calendar, so I don’t forget it again.

How to Dispose of the Used Filter

I want to conclude by talking about how to discard the used filters. I carried my old filter from the basement, walked through the house, holding it, and put it in the trash can.

It wasn’t the best way to dispose of it. I should have had a plastic bag with me, place the filter in the plastic bag without shaking it, seal the bag, and trash it.

This research and experience were helpful to me, and I hope it helps you as well.

House Maintenance List

I want to identify repetitive tasks that need to be done to maintain our house. I add these items to the House Calendar so that I don’t forget to complete them. Two things come to mind out of this post:

  • Replace furnace air filters: Every 2 months

Video Summary

I have created a summary video for this blog post. If you like this video, consider subscribing to the House Notebook Youtube Channel.

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