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I'd like to start using 4" thick filters on my HVAC system. The primary reason is that I'm seeing excessive static pressure on the return side (0.5-0.6 inches water column). This makes sense, as I gather 20x25" is pretty skimpy return size for a 3-ton heat pump system. The depth of the pleats on a 4" thick filter means that there is effectively more surface area, than with my existing 1"-thick filter. I also understand that they require less frequent changes.

Am I correct that this should help to mitigate my static pressure problem? I've asked about this on another forum and a respondent states:

"I seriously doubt it will reduce your static pressure, it will likely increase it due to the open area of the filter grille itself. If you were somehow able to put the filter ~2-3 feet past the grille itself in the box that the duct connects to AND the duct connecting the box to the air handler/furnace is 100% air sealed and sized appropriately then it would reduce static pressure on the return air side but any of that actually being possible is much less likely than it would be by putting the media filter cabinet at/near the air handler/furnace."

I have to say, this doesn't make a lick of sense to me. The resistance to air flow, and thus the static pressure drop for a given flow rate, should simply be the sum of the various resistances (much like an electrical circuit). It shouldn't matter where the filter is located in the return air path.

Additionally, I'm having a lot of frustration trying to source a filter grill and matching filters. You'd think this would be standardized, but you'd be wrong. For example, I've found an affordable 4" filter grill which states that the filter should be 1/4" to 1/2" undersize (below nominal). But for a common online supplier of filters, their filters are 19-3/8" x 24-3/8" (that is, 5/8" undersize). I found a major-manufacturer filter that is 19.94" x 24.86", again not meeting the 1/4" to 1/2" undersize standard. Finally, another maker of filter grill allows one to specify the size down to a 1/4"; but I have no idea what size to order, since it seems like I'd be committing to one brand or vendor of filters.

Why on earth isn't this more standardized? Am I missing something? What do the pros do in this situation?

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    Wait, are you setting up so you can do a 4 inch filter in a filter return? Interesting. I'll give you the very short answer: all other things being equal, a 4 inch filter will have something between the resistance of no filter and the resistance of a 1 inch filter. You can double-check this by measuring the two different pressures.
    – KMJ
    Jan 15 at 6:36
  • Oh and as to why it is not so standardized: I think it's because the filter boxes usually installed near the air handler usually have a pretty wide allowance for filter size variation.
    – KMJ
    Jan 15 at 7:45
  • I assume this is the normal way to do it. My house has a centrally-located grill low on a wall in a hallway. There's a louvered door you open and the filter is right there. Then there's a huge duct going from there to the air handler. My plan is to simply replace the grill with one designed to hold a 4"-thick filter instead of a 1" one, such as this one (if I can be forgiven for linkijng a specific product if it's not actually forsale in the link) xrefs0.plumbersstock.com/product/1/5/153870_siqeFH.pdf Jan 15 at 7:47
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    What do the pros do? Grab snips, a bucket of screws, and a few pieces of sheet metal and make it do whatever we want! I've never seen a 4" filter-grille but I do squeeze Aprilaire filter boxes in with a system replacement often. Maybe show us a photo of the air handler area and we can figure out how to move the filter to that location. Make note of whether there are multiple air return paths into the air handler.
    – Greg Hill
    Jan 15 at 16:22
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    You're fine posting links to products as examples in a question or answer. Just don't ask for product recommendations! :)
    – FreeMan
    Jan 15 at 18:09

1 Answer 1

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Will changing to a 4" thick HVAC filter reduce static pressure?

from a less overall dimension yes, as there is more square area of filter material which ends up becoming a reduction in restriction... because the filter is in a VVV pleat arrangement in the thickness dimension. A 5" filter would flow better than a 4" filter given the same L x W dimensions and MERV rating. The actual filter material is only like a 1/4" thick, and then that varies based on MERV rating, and brand.

I gather 20x25" is pretty skimpy return size

For a rectangular duct Take the size of the air conditioner (example: 3 tons) x 144 square inches = return air duct in square inches. For round duct: Take the diameter of the duct x 3.14 = return air duct in square inches..

your 3 ton x 144 = 432; your 20x25 return size = 500; it's not skimpy. And you want minimum ~200 sqin per ton on return grill size so air doesn't flow too fast over grill and howl, so could a slightly larger grill be optimal probably, is 20x25 skimpy, it's subjective. Sometimes given the house layout and where it needs to go that's the best you can do, better than the alternative of no AC. It'll still work just fine until you use a cheapo junk filter. A Trane 3-ton air handler for perspective, TEM4A0B36S31SB, has a L x W of 21" x 18.5" (the inner air flow dimensions will be smaller than 388 sqin. So your 20x25 is not skimpy.

I'm having a lot of frustration trying to source a filter grill and matching filters. You'd think this would be standardized...

house design and dimensions are not standardized, therefore things are built to fit when they interface with the house especially the return(s).

Am I missing something?

you didn't state the make & model of the heat pump or any other pertinent details like where it's installed, the duct work, the house layout and sq-footage, etc. And from your wording u state you have a pressure drop problem - why do you think that? Pressure drop is only one parameter, which you may be measuring incorrectly on top of it all. A more important parameter that you should look at is cfm. Higher cfm will result in a greater pressure drop over [any] filter.

What do the pros do in this situation?

some install a trion air-bear on the return side of the unit, 5" thick filter. Doing a 1" filter is loser grade, unless it's in a 500 sqft apartment and a 1-ton unit.

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    Just realized the asker commented on 0.5-0.6in WC as being too high on the return side. That's higher than it could be, but I doubt it's actually causing a problem.
    – KMJ
    Jan 16 at 2:54
  • So what about the quote from another forum in my OP ? That's BS, right ? Jan 16 at 17:54
  • i don't know, from what you quoted I can't really interpret what it's trying to get at. I kinda don't believe your 0.5" wc measurement. realistically- take the filter out run it, observe air flow, put a clean filter in, observe air flow. Is it significantly different? there's some basic things to do that should pass a sniff test (that's a metaphor). best i can tell ya given this is the internet and u didn't give much technical detail to go on. there's plenty of valid youtube vids on the subject you can look up also, in regards to duct sizing.
    – ron
    Jan 17 at 15:58
  • Well, I'm just using a $40 digital manometer from Amazon. Not a proper static probe, just sticking the rubber hose through a tight hole I drilled. That wouldn't be so far off, would it ? Jan 17 at 17:31
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