Jump to content

Air filter slot in return air plenum

Brandon Whitmore

Recommended Posts

I inspected a house about a month ago and had a laundry list of issues with it. Should I mention that it was pre- inspected?

For the first of my questions, only because I am wondering if I am too picky-- code aside and all.

One of the furnaces is located in the attic. One the sheet metal return air plenum there is a slot cut in the top of the return air plenum just before the blower to stick a filter in. There is nothing to hold the filter into place, just a cut in the top of the square plenum.

I wrote up the lack of a filter cover (air leak) in the plenum, and recommended a proper cover be installed, and that a proper filter slot be fabricated to hold the filter into place. On the original inspection, there was a slightly over-sized paper filter installed so that the cardboard end stuck up/ out enough to hold the filter into place.

The HVAC contractor argued and said that a filter cover was not required at this location and it would not hurt anything to leave it open. He said this is how it was built (no @#$@). I pushed back and said #$@#.

For the re- inspection, I held my breath hoping for something decent. All the contractor did was make up something that my infant may have facricated for him. He stuck in a smaller filter (any guesses what happened to that filter). Then he stuck a home made type of cover over the opening that was too long (heavy air leaks on both ends), and had some weatherstripping on it. Problem being that the square plenum was not level, so the top cover did not seal the cut in the plenum. Frigging shocking.

Here's the reply I recieved just now on an e- mail regarding my re- write up:

a. The additional metal cover with airtight seals was put in place and

wingnuts used to assure a good seal. The re-inspection pushed the filter

down into the furnace (where it was left by the inspector). I fished out the

filter and replaced it with the original size filter that does not fall into the

furnace. (The size the inspector said was incorrect)

Conclusion: This item is satisfied.

What does "the re- inspection pushed the filter down into the furnace" mean? Basically they are saying that they had to use a smaller filter that would fit into the return air opening so that they could cap the opening off. Seeing as how there was nothing to retain the filter, how did they expect to keep the filter into place? I stepped up into the attic, with one of my favorite Realtors following being me. I looked at what they did before getting to the furnace, and said "you have got to be frigging kidding me". I looked at the crappy filter cover thingy, slid it to the side and saw a filter lying sideways in the plenum.

This was on the reply to my client's from the listing agent:

5) HVAC: Please see the receipt from AAA Heating and Cooling, Inc. This receipt

details that the furnaces were serviced, proper sized air filters installed and “vent


Link to comment
Share on other sites


Why wouldnt a simple strip of foil tape have worked over the first filter? They could leave the tape there and every time they put a new filter in, cut off another strip and place it over the filter. Seems like it would have been every bit as good as anything else you'd described.



Link to comment
Share on other sites

What I usually see is one of those male sheet metal type covers installed that pushes down into the opening. I wish I would have taken a picture of what they "fabricated"

Foil tape would have been allowed, and functional, but a pain down the road. A good roll of that foil tape will set you back about $45 or so, at least that's what I paid out to my supply house when I purchased some recently. I remember telling the buyer's agent that foil tape would be fine, but it would be kind of crappy in my opinion. The addendum just said to fix it, and didn't say how. Their "fix" was a joke.

I should add this to each of my posts:

"Did I mention I can't stand doing re- inspections?"

Link to comment
Share on other sites

It sounds like you’re missing a furnace filter cabinet.

What you have is an unsupported filter that could collapse on or inside the blower not to mention return air bypassing it that could reduce furnace efficiency or worse clog a heat/cooling coil.

I've yet to see tape as a filter cover instead, I see three different styles of cabinet gates:

The ‘A’ shape slides up and down in a track and over the opening.

Download Attachment: icon_photo.gif AFilter1.JPG

86.3 KB

The ‘L’ shape uses screws to secure it in place, it slides right/left and you need a screwdriver to loosen them. Couldn't find a pic sorry.

A ‘U’ shape that snaps over the filter opening. They need more patience to fiddle with to make them fit providing the tabs haven’t been damaged.

Download Attachment: icon_photo.gif HeatingFurnaceRm2 (Medium).jpg

82.82 KB

I’ll mention a missing gate if the cabinet is missing one otherwise the older open style ones don’t make the report.

Cutting a slot in the return plenum doesn’t cut it; I’d call it out as a missing filter cabinet.

If you ask me, this is what I would recommend, something I seldom see.

Download Attachment: icon_photo.gif FilterCabinet1 (Medium).jpg

53.99 KB

Download Attachment: icon_photo.gif FilterCabinet2 (Medium).jpg

56.92 KB

Link to comment
Share on other sites

Kind of off-topic, but almost all attic furnaces I have seen have the filters in hinged return grills, in the living areas. Which seems like a very good idea. Getting a homeowner to regularly check and/or change a filter is hard enough without making them crawl into an attic to do so. "Out of sight, out of mind!"

Of course, the same holds true for those horrible, barely accessible, twin filters in a vee formation found in way too furnaces...but that's another pet peeve.

Link to comment
Share on other sites


Regarding the "open" filter slot, I've had the same problem with HVAC contractors on numerous occasions. These 2 items usually end the discussion:

  • Duct systems are to be "substantially airtight"
  • There shouldn't be any "inlet" into the return air system within 10' of any combustion appliance.
The manufacturer's installation manual may have something about properly securing the filter and not allowing air to enter behind it. If not, contact them and they will likely provide something in writing.
Link to comment
Share on other sites

Thanks for all of the responses as usual.

I tried to stay away from mentioning any codes on this one, because the system is about 15 years old. I'll have to check my photos to see if I snapped a shot of the model number of the furnace-- that will be the easiest route to take. Hopefully it'll put the bonehead HVAC guy in his place. It had so many issues, I may not have in this case. I just thought it was so wrong as to be common sense, and didn't expect to be arguing with the contractor on this one. Live and learn.

I also wrote up a cracked PVC exhaust vent right next to the furnace. Anybody want to bet as to whether that was replaced?

Link to comment
Share on other sites

When I get that kind of idiotic push back from hacks, I simply take photos of the condition, and describe it for what it is. No need or benefit for getting into a match with a moron.

I'd quote the appropriate code reference having to do with return air openings <10' from the furnace and the fact that the filter isn't functioning as intended.

I'd also say it's the contractors prerogative to be a hack; I can't force someone to think.

Link to comment
Share on other sites

I agree with the hack designation for the contractor.

But on a side note, one of the best filter doors I see is a simple strip of refrigerator magnet a couple of inches wide taped on one side to the cabinet and loose on the other three sides so that the heavy tape forms a hinge.

Peel the magnet open and hinge to the fixed side, replace the filter and close the self sealing door just like a refrigerator.

Lasts for years and always seals tight.

Simple, functional, no tools required, no screws to drop.

Beautiful in its simplicity.

Link to comment
Share on other sites

  • 1 year later...
  • 11 months later...

HACK. The guy is a hack period. Infiltration from an unconditioned space is major. Any amount of infiltration that can be easily controlled by a proper installation should not be disregarded. Guys like this give my industry a black eye.

We use" filter lock" magnetic filter slot seal. Creates air tight seal and cost about $8.


Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...