We use 2″ MERV-13 filters. The depth does help, but it’s not as straightforward as you might think. See this article:

Do High-MERV Filters Always Reduce Air Flow?

All other things being equal (size of return air grill, make, model and MERV rating of filter), doesn’t a deeper 2″ filter result in more airflow/lower pressure drop than a 1″ filter, just because there is more surface area available through which air can flow? Same for a 4″ vs a 2″.
So does that mean if all other variables are equal, a deeper filter is better?

I am sending a response to my
I am sending a response to my own statement above. I don’t think I ever got a clear, concise interpretable response. I bought a MERV 11 or something like that from a local hardware store, which I can use when I clean my electronic air cleaner and metal filters. I don’t think this does a better job on the very small sooty particulates that come with wildfires. We have been fortunate to not have much smoke here this yr but it would be nice to know what I can get that might work better, basically (I believe) a MERV 13….?

No talk of ASHRAE APPDX J ?
No talk of ASHRAE APPDX J ? STATIC CHARGED MEDIA IS NOT TRUE EFFICIENCY MERV-A RATED FILTERS ARE THE TRUE EFFICIENCY WITHOUT A STATIC CHARGED MEDIA TGAT DISSIPATES

It’s size of the face of the
It’s size of the face of the filter, not the filter media, so the 5″ depth is not factored in. A 24×24 is simply 4 sq ft.

I’m building an air filter for my shop/garage to filter out fine dust from wood and metal working as well as sanding and sand blasting dust. I bought two 24x24x5 Merv 13 filters and plan to build a box running them side by side for a large surface area. I have a 2300cfm fan I will build into the air tight box and mount the system in a corner of the 28x54ft garage about 8 feet up (15 foot ceilings).

After hours of reading and searching I found your measurement of 2 square feet (sf) or more for each 400 cfm of air flow. Based on this, I’ll need 11.5 SF of filter area.

Can someone help with the math of my filters? I can’t seem to find this info anywhere.. How do I calculate the SF of a 24x24x5 air filter?

Thanks!

I just looked up the specs
I just looked up the specs for the Flanders Pre-Pleat you are using.. Interestingly they list them the exact same way as Honeywell did for my filters. .3 in w. c. at 500 fpm for the 2″ thick one. They offer the same filter in a 4″ thick and the resistance drops to .2 in w.c. at 500 fpm, a 50% reduction in resistance for moving from a 2″ to 4″ thick filter. I thought that was interesting.

Since your recommendations result in a much lower filter face velocity than how the manufacturers are rating their filters, I suspect the reduction in resistance is less for the thicker filter at those lower flow rates, but it’s hard to tell from the tiny line chart they provide.

You have to download the Flanders spec sheet, but you can get it which includes a line graph showing resistance at increasing filter face velocities as well as full specs and available sizes here:
https://www.aafintl.com/-/media/files/aaf/commercial-and-industrial/us-products/pleated-filters/prepleat-m13/prepleat-m13_prod_mark_sht_afp-1-384.pdf

It is interesting to note the significant increase in filter media area when going from their 1″ version to their 2″ like you are using, typically over a 100% increase. While the increase in media area when moving from the 2″ to the 4″ is much less impressive (typically more like a 30% increase).

So using the 2″ filters like you are doing is definitely the sweet spot and on future projects I will look to put them in the return grills as you suggest instead of in the plenum as I have in the past since it would appear almost impossible to get sufficient filter face area in a plenum install to allow the lower pressure drop you recommend.

Allison, I know you keep
Allison, I know you keep referencing the 2009 study for filter depth showing little difference between 1″ vs 4″, but what do you think about this lennox video?

https://youtu.be/3kJrKVujkv8

It clearly shows that not all 4″ filters are created equally, while comfirming the 2009 study about the filtrete brand 4″ not being much better than 1″, but showing that other 4″ filters (lennox) can be substantially lower, mainly due to having more sq ft of media compared to others.

The lennox merv 16 filter appears to bust all theories about high merv filters and pressure drop by actually having one of the lower pressure drops of any filter despite being such a high merv rating.

Russell. So here are the two
Russell. So here are the two problems with the specification you got from Honeywell. Number 1 a pressure drop of 0.3 IWC is way way way too much. Most equipment is designed for a maximum of 0.50 IWC. So your duct system would have to almost not exist for a filter with that much pressure drop. As stated above and as shown in the graph of different filter types in the ASHRAE document I put the link in for (Figure 3) a good design would be for 0.05 IWC. That is 1/6th the pressure drop Honeywell is talking about. Number 2 of course is that they are giving you feet per minute. That means you have to convert to cubic feet per minute by multiplying by the “face area” 400 sq inches in your case or 2.78 sq. ft. So to translate their numbers to what we need they are saying your 2.78 sq ft face area filter will flow 500 FT per Minute X 2.78 Sq Ft = 1389 Cu. Ft per minute. That is 463 CFM per ton for your 3 ton air conditioner which would be an ok flow if you are not in a particularly wet or dry part of the country. HOWEVER your air conditioner would never move that amount of air since the total static pressure will be way way way too high since it also has a duct system, not just a filter.
Next yes a 4 inch pleat will give you more filter surface area with more filtration and less pressure drop than a 2 inch pleat (if you have the same MERV filter media and the same number of pleats).