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Today I want to show you how to quickly correct an issue that is extremely common in most home heating systems that many people are not even aware of. If you have a central air system or just a furnace with a return dock where the air goes through a filter prior to being heated or cooled, this problem lies at the furnace filter slot and can greatly affect the air quality in your home.
The issue is an open furnace filter slot. In other words, the slot has no cover and you can see the spine of the filter. A quick look at your furnace and you’ll know if you have this issue. This is not one of those problems that screams at you to fix it but it is extremely important, especially if there are young children or the elderly in your home, or anyone with allergies or respiratory problems. It’s also important to the proper functioning of your heating and cooling system and overall comfort in your home.
Easy Fix for an Open Furnace Filter Slot: Step-by-Step Guide
In this step-by-step guide, the steps to fix the issue of an open furnace filter slot are actually quite minimal. I’ll talk at some length about the main types of furnace filters and touch on how to install them, but mostly I want to emphasize why this situation is important to address before showing you how to easily fix it.
I’m going to demonstrate this problem using some incense. The smoke will show us the path of airflow so that you can really see what otherwise you can’t see.
First, let’s go through a few furnace basics to make sure we’re all on the same page.
Rather watch than read? Check out this 10-minute video.
DISCLAIMER: This video and description contain affiliate links, which means that if you click on one of the product links, I’ll receive a small commission.
Easy Fix for an Open Furnace Filter Slot: Supplies, Tools, and Costs
Supplies
Filter Lock Magnetic Filter Seal
3M Filtrete 1085 (MERV 11)
3M Filtrete 1900 (MERV 13)
Tools
No tools needed.
Costs
The Filter Lock Magnetic Filter Seal costs about $35 on Amazon.
The 3M 1085 (MERV 11) filter will run you about $10 each; the 3M 1900 (MERV 13) about twice that.
How Often Should You Change Your Furnace Filter?
Most manufacturers recommend that you replace your furnace filter every three months. I took a little poll of our viewers and was surprised to learn that almost half of those that responded said they changed their furnace filter with the recommended frequency of three months. If you’re doing that, awesome!
Here’s a little tip to make remembering easier: Many HVAC companies and home improvement centers as well as Amazon have programs where you can schedule filter delivery according to a set schedule. Usually, there’s no additional cost involved. It’s one of those things you can set and forget. I think it’s a great, no-brainer way to ensure that the furnace filter gets changed regularly.
The Best Type of Furnace Filter for Your Home
There are tons of different furnace filters on the market. I’ve got a few here that demonstrate the general options available.
Low-Cost Furnace Filters
First up is the extreme low end of furnace filters. These cost about $2.
Note that you can actually see through the filter.
Now, if you can see through the filter, you can probably guess that it’s not removing a whole lot from the air.
There are a couple of advantages to this type of furnace filter, however. One obviously is cost. The other is that it creates very little resistance. The flow of air through your heating/cooling system isn’t restricted by this type of filter, which means your unit doesn’t have to work so hard to do its job of heating and cooling your home and maintaining humidity.
Choosing a furnace filter is a matter of finding balance between removing particles from the air, which can be important if you have allergies or other respiratory issues, and the strain put on your unit by more restrictive fibers.
EM 1085 (MERV 11) Furnace Filters
Next is the 3M 1085, which has a MERV (Minimum Efficiency Reporting Value) rating of 11.
The MERV rating measures the effectiveness of a filter with regard to removing dust and contaminants from the air. The higher the rating, the more efficient the filter.
This is a classic filter, right in the middle in terms of particle removal. It’s going to do a decent job of filtering the air. For most people, this is going to be sufficient to maintain healthy air quality, especially if you’re replacing it every three months.
You can expect to pay about $10 per filter for these.
3M 1900 (MERV 13) Furnace Filters
Our last example is one that I’m actually stepping up to in my own home. It’s the 3M 1900 MERV 13.
This furnace filter is going to capture a larger quantity of particles as well as smaller particles due to the higher concentration of pleats. So even though the dimensions of this filter are 20 x 25 inches, just like the others I’ve discussed, the surface area of the filter material is 2-3 times greater than the 3M 1085.
Despite having more abundant filter material and superior filtering capacity, this MERV 13 filter will not put a lot of strain on your heating/cooling system due to air-flow restriction. It will also last longer than the other types.
I think the 3M 1900 MERV 13 is the best filter for your home but it does come with a commensurate price tag — about $20 each. That’s considerably more than the other types of filters but when you look at the cost over a year, you’ll spend $80 if you change out the filter every three months. In my book, $80 a year is worth it for the improved air quality in my home.
How to Install a Furnace Filter
When installing your furnace filter, look for the arrow on the filter housing itself. This needs to match the direction of airflow. Air comes into the system through the return duct.
As the name implies, the return duct returns air to the system to be conditioned again and then recirculated through your vents. The filter sits between the return duct and the furnace itself.
The wide end of the arrow should be towards the return duct so that the arrow is pointing in the direction of the furnace or central air unit. It’s important to get this orientation right for proper functioning of the filter.
Why Is an Open Furnace Slot an Issue?
If you can see the spine of your furnace filter, you have an issue to address. But why is it an issue? I’ll explain the concepts and then illustrate as promised using some incense.
The basic premise of a properly functioning furnace is that air returns to the system through the return ducts in your home, passing through the filter. If you have an opening in the filter housing, air is going to come into the system from places you don’t want.
Now I’ll demonstrate with the incense. I’ll light it and set it by the filter.
When the blower motor kicks in, the smoke from the incense gets completely sucked into the unit.
If you have a gap between the return duct and the filter, air from the furnace environment is going to go through the filter, as shown by our incense, which is a good thing. So what’s the big deal?
The big deal is that the air coming into the system is from the area around the furnace, which is likely your basement or utility closet — not the cleanest areas of a home. It means your filters will get dirtier faster and your unit will have to work harder to condition the air due. It’s also going to reduce the overall airflow coming through the return ducts, which will impede the system from working as it was designed to do and can result in uneven temperatures and humidity throughout your home.
If your gap is on the furnace side of the filter, it’s a double whammy. Air will be entering from the environment where the heating/cooling unit is located AND it will be bypassing the filter. Not only that, but as the filter gets clogged, the flow of air from the environment will only increase. If you’re bypassing the furnace filter, you can see how that’s going to impact your system’s health and also your health and that of your family or housemates.
The good news is that this is an easy problem to fix.
How to Fix an Open Furnace Slot
If you have an HVAC professional maintaining your heating and cooling unit or installing a new one, just request that they make a cover for the furnace filter slot.
For existing systems, if you’re like me and maintain the system yourself, there is an easy solution. It’s a product made by Filterlock, a magnetic filter seal. It’s adaptable to many different scenarios.
Let me show you how to install it. After it’s installed, we’ll get out the incense again and do a test.
The Filter Lock Magnetic Seal kit comes with two pieces. This is what makes it so adaptable.
Each piece has magnetic strips along all its sides.
I’ll apply the bottom piece first, tucking it under the filter slot before letting the sides attach.
Once the piece is in place, I’ll adjust it so that it’s laying flat across the filter slot opening.
Then I’ll do basically the same thing for the other piece, snipping the magnetic piece at the top so that it will lay flat across the upper part of the duct.
Then I’ll stretch out this piece as I did with the other one to lay flat across the filter opening.
And that’s all there is to it!
Now let’s see if it actually works.
Using my incense again, the smoke just continues rising when the blower fan of the furnace turns on. Problem solved.
Easy Fix for an Open Furnace Filter Slot: Conclusion
It is nice when every once in a while you find a product such as this Filter Lock Magnetic Filter Seal that is both inexpensive and extremely effective at resolving a problem.
I highly recommend the Filter Lock. Again, it is adaptable to all types of systems and ductwork configurations.
I wasn’t able to find it at Home Depot or Lowe’s so I ordered it from Amazon.
To Leave Comments and Questions
If something doesn’t make sense or if your situation is a little different, please leave a comment under the YouTube video. I check the comments on a daily basis and am happy to help out.
Don’t forget to subscribe to our YouTube channel as we have weekly videos coming out to help you with your everyday home repairs.
Other Furnace-Related Repairs
Upgrading a manual thermostat or an older, programmable one to a modern, smart thermostat and facing the dilemma of a missing C-wire? Our step-by-step guide, How to Fix a Missing C-Wire, will walk you through how to resolve this common obstacle.
Got some doors around the house that don’t latch?
While you’re on a roll fixing things around the house, I’ll bet you have a door or two that isn’t latching somewhere! Check out our video on How To Fix A Door That Won’t Latch where I show you three easy fixes for this issue.
