# Picking a filter for existing 2.5 Ton HVAC

| Posted in Mechanicals on

I’m stuck for now with a 2.5 ton rooftop HVAC package unit (Single Zone for 2 stories and oversized for 1550 SF no doubt) and a 14X30X1 filter slot. How do I calculate to select the highest MERV filter that won’t interfere with air flow? (Climate Zone 3C)

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### Replies

1. | | #1

With a thermometer measure the supply and return air temps and calculate the difference between them if your unit is working correctly the number will be more than 15 and less than 20. If you are not moving enough air the number will get over 20.

Walta

1. Expert Member
| | #2

I'm not sure how reliable it is to guess at airflow based on air temperatures. You really need a differential manometer to measure pressure drop across the filter. The usual recommendation is for the filter to not exceed about 20% of the equipment's rated max pressure, which will be measured in inches of water column (inWC or "WC). That probably means you want somewhere under about 0.2"WC or so, which isn't all that much.

Keep in mind that air filters will show more back pressure as they start clogging up with gunk too, so try not to run at max allowable pressure drop with a new filter.

BTW, you'll be limited as to how high up in terms of MERV level you can go with space for only a 1" thick filter. The usual way to get lower pressure drop with a high-MERV filter is to use a thicker filter, 2" or up to 4" or more. I use a MERV-13 filter in my own home, and I use one of the very thick Aprilaire filters that go in their own housing. I don't see much pressure drop across these, but you have to install the housing into your ductwork, so the initial installation is non-trivial (but future filter changes are pretty easy).

Bill

2. | | #5

Ductwork? Have a look at the photo showing the return "plenum" below the rooftop HVAC package unit (In fairness, this is an early 21st C. retrofit of a 1974 heat-only door-grille-return-closet-FAU with dedicated soffit supply) Anyway, thanks! I may have to settle for better flow with less filtration until I can replace the whole system with Mini-Splits.

2. | | #3

Is the 1inch filter slot because the filter is in a return air filter grille?
Is it is, there is a decent option; Honeywell FC40 filters have a 1 inch stepped flange and are 3 inches total depth. I think they're MERV 10, and they're a big improvement in pressure drop over a 1inch filter.
It looks like you'd want FC40R1169, looking at the spec sheet (I have it bookmarked).
The downside is that they're becoming really expensive; last time I bought them they were \$50 for the 20x20 size, which is almost double the cost from a few years ago.

If it's a filter grill, it's also possible to replace the grill/frame with a grill/frame for a deeper filter. Not as common, but most of the big grill brands make a 2 inch or 4 inch filter frame option in some of their models (including Aprilaire, as mentioned above). This may or may not work well depending on the duct geometry and whether there is enough room between the back of the filter and the opposite duct wall (if in a right angle duct, etc).

1. | | #6

The FC looks like a clever fix. Will look into it. Thanks! (Not gonna mess with the frame since I'm looking forward to ripping out the whole system in a bigger remodel to come)

3. | | #4

You might find these articles interesting
https://www.energyvanguard.com/blog/unintended-consequences-high-merv-filters/
https://www.energyvanguard.com/blog/unintended-consequences-high-merv-filters/

In the end if you want a very high Merv filter you need to make room for 5 square feet of filter area with 2.5 tons of equipment. It doesnâ€™t matter if is taller, wider or deeper find the space or you risk restricting the air flow increasing the temperature drop and the chance that the coil could ice over and damage the compressor

Walta

4. | | #7

Thanks for the links. I'm actually working my way through Bailes's book right now. As I mentioned in my reply to BILL WICHERS I may have to compromise on my filter aspirations for now.

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