Heating, Air Conditioning, Fridge, HVAC/Furnance Air Filter Question


Hi Fred:

I've been checking out wide 20x20 air filters.   I know that the different manufactures testing can be somewhat unreliable, but perhaps (for my own knowledge) you can help me out on a couple of questions.

When is comes to comparing different filters, what might the better filter in terms of air resistance.   Of course everything else (size, Merv...etc) would be the same.

Air Resistance vs FPM

Filter A:  0.14 @ 300
Filter B:  0.25 @ 492
Filter C:  0.11 @ 300

I'm wondering here if there is a way to compare the air pressure drop with a difference in FPM of air (300 vs 492).

Also, beside the reported specs on 2-4" pleated filters, what else might be an indicator of how well the filter will perform with regard to air resistance ?   Number of pleats for example ?

Thanks for your thoughts and ideas on this.

Excellent question!

My website has a page on filtration, so you might want to check that (see note at bottom).

Generally, you want the lowest "PD" (pressure drop) for a given MERV rating. PD is proportional to face velocity, but the only way to compare different brands and/or types is to rely on manufacturers' published data. Typically, PD's are given at 300 and 500 FPM.

Limit velocity to no more than 300 FPM. Keep the initial PD no more than 0.10", which is about all most residential systems can handle. Make sure to provide properly-designed inlet and outlet transition fittings where the air mover inlet and/or duct sizes are smaller than the filter. Also, there's a Code requirement to provide uniform flow across the face of the filter (You'll need turning vanes if there's a change of direction at the filter).

This next will sound like a "DUH", but you'd be surprised at the number of bad jobs I see: If you're investing in a premium filter, you're throwing your money away if anything downstream of it is contaminated with construction debris. So if you're serious about improving IAQ, you may need to clean your blower compartment, blower and housing, cooling coil and supply ductwork. Systems using floor outlets are especially troublesome: the openings are routinely used as trash receptacles and - sad but true - occasionally as urinals by construction workers.

If you're cleaning metal ducts, use "Contact Vacuum" as opposed to "RotoBrush" methods; it'll cost more, but you do get what you pay for in this life. Don't attempt to clean fibrous glass or flexible ducts; replace the former with metal, and replace the latter with metal or new flex.

If you're building a new home, insist on ductwork and equipment that is "free of non-adhered substances and debris" (Love that phrase), and reference "NADCA ACR2006" in your contract (and you might want to search that as part of your study).

Finally, I don't get too excited about high MERV: The minimum required is MERV 6 (ASHRAE 62.2), and going to 10 or 11 is more than enough; we're not living in Clean Rooms.

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Fred Weldin


I can't help you if your whatsis won't work, people (Especially if it's a refrigerator); I have no experience with appliances, and I haven't been involved with H&AC service and repair since March 08, 1996 (Thank the Lord); I always send a "standard" reply to appliance questions and H&AC "Service and Repair" questions (about 20 a month), so if you want to know why your whatsis isn't working like it used to, ask someone else. A lot of my answers arenít read by the questioners; in that event, I always send reminders to read the answers for a month or more (the word "idiots" comes to mind - actually there's another more appropriate word, but discretion prevents its use here). If you have questions about how big a unit you need, if one room's warmer than the others, if you want an opinion oas to scope of work on bids received, etc. just ask. Do visit www.askweldin.com, there's a lot of good information on sizing, ductwork, efficiency, as well as some techniques accomplished DIY-ers can use to troubleshoot and improve their H&AC. Finally, I want to thank all of the kind, serious people whom I meet here: I enjoy working with you. My wife says I'm "snarky" from time to time; as for you others, please use your imagination as to the responses swirling on the snarky side of my brain when I read your questions.


53 years in the business. See www.askweldin.com


B of ME U of D 1965, numerous classes, seminars and a lot of "Hands On" learning since.

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