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Air filter doesn't fit properly.

Hello everyone,


I recently replaced the air filter in my home and noticed that despite my best efforts, it does not fit snugly. I started paying attention to this because my house has been getting very dusty lately. After some googling, I found that proper air filter maintenence can go a long way towards keeping the house dust-free, in addition to keeping the unit itself working properly. I've attached pictures to aid in my description:





You can see that the air filter does not cover fill the entire space in front of (what I assume to be) the blower fan. Meaning that if I run the AC or heat, dust just gets sucked in. The second shot shows how the filter is installed -- you just basically jam it into that slot:





The slot itself seems to be very poorly designed because the filter does not glide in smoothly, so you have to wrestle with it which causes damage the cardboard frame. Also, there doesn't appear to be anything that keeps it from moving.


Any suggestions for correcting this? My first thought was to buy a plastic filter, but I've read that they don't work as well as the paper ones. Being new to this, I wasn't really sure what I should do next.


Thanks for any and all replies.

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Posted 2012-10-15T21:00:49+0000  by baltien baltien

Hello Baltien and welcome to the How-to-Community.


Baltien you’re missing something called filter frame or filter slot. Filter frame is the sheet metal profile that is normally placed in between the unit and the return duct (plenum).Typically these frames are sized to match the return box (duct) and the return box is sized to match the blower size.


 filter frame.jpg


This is something that installer should have installed when original unit was replaced...I can tell, looking at the pics, that you had your unit fairly recently replaced. Correct ?


If you decide to address this issue yourself, you should know that these are relatively easy to install even for somebody with no hvac or sheet metal experience.


Basically an un-flanged (open) side of the frame is the side that gets attached to the unit (furnace) and the flanged side is the side that is connected to the return duct.


Size of the filter frame…


It is important that you purchase a correct frame size so that you don’t have to make changes to the return duct. It appears to me that your return box is 16x20, these are pretty standard and matching frames are available for this size. Please double check these sizes before you go out an order one, I’m gestimating this based on the filter size…

Here’s the link to one 16x20 filter frame that I’ve found online and that you can reference to.


Couple more “correction” candidates that I see on your unit…


Return duct is not aligned correctly; it is offset with the unit. In other words your return side of the unit is being restricted.


These L shaped corners are the cut out indicators for the filter frame.


Electrical supply is incorrectly routed; right now it’s placed in front of the filter frame and it is obstructing filter slot. Also I’m not sure what the NEc code is in your state but normally armored cable is used for the supply to the unit and there should be a visible switch installed near by.


Can you take couple more pictures of your unit? This looks like 95 + AFUE % unit and with these units it is imperative that air intake and exhaust is installed correctly.


Also is this unit in the attic?








Posted 2012-10-16T15:33:03+0000  by George_HD_CHI

1) The unit is in my laundry room, on the first floor of the house. The house was recently renovated, and this was the unit they installed (before I bought it).


2) You mentioned that the electrical supply is incorrectly routed. Aside from making filter installation a pain, is this otherwise an issue? 


More pics below.


The vent to where the air filter resides. You can see how it beat up it looks since it doesn't sit still (I got that filter two weeks ago -- waste of money):




Wider view of the entire unit:




Right side of the unit:




The switch you referenced, is on the ceiling:




I thank you immensely for your help. The funny thing is a frame of some sort was my first thought, but I didn't know what it was called, so I didn't know what I was looking for. Does Home Depot sell these? I check around on the website, but I didn't find anything. I don't mind installing it myself.


Again, thank you.

Posted 2012-10-17T03:01:27+0000  by baltien

You are welcome and I’m sorry about the delayed response.



Beltien i would advise to have your system inspected by the licensed and certified HVAC technician.


Starting with that return opening located right outside the laundry room.

We/I don’t cite codes on this forum, I’m not a code officer or building inspector I am an ex contractor, but i find this  important.


 "Return-air inlets shall not be located within 10 feet (3048 mm) of any appliance firebox or draft hood in the same room or space."


In other words there should be no return opening that close to the furnace and the water heater (I see a gas line going to the water heater, also pic3.)


There are several good reasons backing up this rule.


First, they don’t want fumes from the water heater or furnace to get sucked in in to the return system –supply air. Period


Second, your system right now is not balanced, at all; majority of your return air now comes from that big opening instead of being pulled from different sections of your home. Which means air is just getting pumped in to the rooms and not getting circulated.



Negative air pressure…etc.

Again, have somebody come out, close that return grille and inspect the return air velocity with the opening closed.


Also for time being do not close that pass thru-door going in to the laundry room.  Have technician calculate combustion air required for all of the appliances located in that room using BTU+ cubic feet rule.


Have him check the exhaust for the furnace too. Run + number of elbows + pipe diameter including termination on the roof.



Posted 2012-10-23T15:23:53+0000  by George_HD_CHI

Looks like I'm going to have to. I ordered the filter frame and that did not fit the opening (too wide). Unfortunately, this looks like the part where I step back and let the professionals handle it.


Thanks for all your help George, I really appreciate it.

Posted 2012-10-27T13:23:37+0000  by baltien

Hey baltien did you ever get the HVAC guy to come out? I'm just curios on what was his take on the setup.




Posted 2012-11-06T20:23:43+0000  by George_HD_CHI
Hey baltien,

Your AC appears to need a 16x20x1 filter.

The slide-in system on one of my units is exactly the same and the right size filter will simply slide into the slot.

The return vent should be measured for inside diameter in both directions.

Make certain the arrow on the side of the filter is pointed in the director of air flow.

If a standard filter isn't available, consider the Web Adjustable Filter.

Posted 2015-11-05T21:32:04+0000  by Pat_HD_ATL
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