Home furnace filter - Is this right?

[Brad Adams]

The bigger filters have more surface area. The pressure drop across them is less. A 20x25x5 air filter typically has about a .15” pressure drop across it. I’ve seen 16x25x1 pleated filters with as much as .8” or pressure drop across them. Systems just aren’t efficient with a small restrictive filter.

[Alan Rutherford]

IMO the vertical portion of the return duct is too small. It should have the same effective area as the opening on the air handler, or more. Putting the filter in it adds some restriction to the air flow and makes the problem worse. They could have put a larger filter in at an angle and make a decent-looking job while giving you less restriction to the air flow. Sticking the filter into the duct like that would be OK if it fit and you could close it up, but if the return is choked by under-sized ducts, you have a bigger problem than the filter installation.

[Bill Jobe]

It looks as though your filter is hanging up when you reach the innermost channel.
I'd try pulling it out and measuring back to the farthest surface of the channels.
If that ain't it, I would consider a box filter on the bottom between the down section of duct and the furnace. That's where mine is and I have a Skuttle. It has a door that closes and latches.
I simply would not accept the way they set it up if the filter is, in fact, too large.

[Kev Williams]

This house I live and work in is on its 3rd furnace, and they've all had similar filter setups,
the exception being the filter installed in the wider, bottom section of the duct-
(excuse the quickie cheesy photoshop'n)
filter.jpg
this will give you a more sizeable 24x20 or so filter (mine's 25x20).

However, from experience, I can say that the smaller filter shown will work just fine, depending on the filter. 25-something years ago my parents had to replace the first furnace and had central air installed too--finally. The HVAC guys gave them a couple of electronic air filters, which I still have, they insisted they were necessary, just gotta keep that condenser clean. First thing I noticed after the install, was even tho the furnace was larger, there was less than half the air coming from the heater ducts than before. So I pulled the electronic filter out and replaced it with a cheap 4 for a buck 30 day filter, and I could fly a kite above the ducts now. I probably vacuumed those filters out a couple times a year, changed them maybe once a year. About 10 years ago that furnace (the 2nd one) cracked a heat exchanger, and I had a Goodman installed. I asked the guy who installed it, how's the AC condenser, covered in guck? Nope, clean as a whistle. And in 2016 the old AC quit, and I had it replaced-- and after still using 25c filters, I asked the installer about the condenser... Nary a plugged fin or dust bunny...

Now, 25c filters might not trap viruses & such, but they did keep my AC condenser clean for over 20 years, and the air in your house will actually move around.

[Pat Barry]

Then by your logic, shouldn't every filter be as small as possible?

If you have the same amount of air moving through a larger surface area, the volume of air per unit area is less.

See this chart:
http://www.homeenergy.org/UserFiles/...-6_p36bbig.jpg

The surface area of the input and output is the same. There is more material inside, therefore, everything being equal inside, there would be 4x the restriction. Yes, less filter. Ie no filter, is less restriction in this universe. What I don't know is what the internal differences are. Maybe that 4 in filter has lots more low density material that is inherently less restrictive.

[Pat Barry]

Also, note that the chart refers to face area, thus doubling the face area of a 12x24 filter would be using a 24x24 filter. Of course that is typically not possible to do since it means making the duct work bigger also. Bigger duct work means lower air velocity (faster flow in a smaller pipe).

[Brad Adams]

but they did keep my AC condenser clean for over 20 years, and the air in your house will actually move around.

That must be one awesome filter. It kept the condenser clean all the way outside!

[Bill Jobe]

Directly above your filter is what appears to be a section of track used to hold filters. What is it doing there if it is not a door that closes after installing the filter?

[Kev Williams]

That must be one awesome filter. It kept the condenser clean all the way outside!

Ok, sorry, the EVAPORATOR...

[Mark Paavola]

After 40 years of servicing HVAC equipment and also doing a lot of airflow testing Brad is correct. Although without knowing the size of the furnace this is still a best guess. Also that is not a good way to install a filter, there should be some type of door or cover. Did the install company provide any type of commissioning report? That would include airflow and providing static pressure tests at various points of the system. I have seen thousands of units with filters that are too small or too restrictive for the system. Unfortunately the homeowner for the most part has never had a system that works like the manufacturer designs them. High static pressure drop across that filter is a given. As an example I have a 4 ton heat pump system that runs at 1500 cfm. I have four return air filters that equal approx. 7 sq ft. This allows a little over 200 FPM flow across the filters. Ideally you should always have a flow of less than 300 fpm in a residential application.

[Alan Rutherford]

...Did the install company provide any type of commissioning report? That would include airflow...

I can't speak for the OP but won't be surprised if his experience was like mine. I had made a crude airflow tester with a computer fan in a square of cardboard attached to a meter. It was too sophisticated for our installer. He told me to feel the airflow with my hands but I was doing it wrong. He said "Use 2 hands". Now that I know I should use both hands to measure airflow, I culd be a prufeshnul.

No disrespect to the true professionals out there.