I'm trying to make airfilters to fit over my Fujitsu Halcyon Wall Mounted heating & air-conditioning split system. I'm talking about the indoor unit specifically. The wire mesh screens that come with the unit do not do an adequate job at filtering out dust, and combined with moisture kicked up by the centrifugal blower fan, it turns the whole thing into a breeding ground for mold, requiring me to dismantle and clean the whole thing.

So I'm trying to build a cover with an air filter, that will fit over the intake to the unit. I want to make the thing look as sleek as possible, and Lowe's offers a choice of sizes, although I'm concerned that if I choose a filter that is too small, I could choke off the system(the unit wasn't designed for this).

I found the data sheet for all Fujitsu models, and it lists CFM for my units as:

                    Mode Hig.Med.Low Quiet
2x asu12rlf(small)  CFM: 388 330 265 182
1x asu18rlf(large)  CFM: 542 436 365 324

How do I calculate the effects of my custom filter holder on air flow? And what size filter do I need? Will an 8"x20" filter work? Or do I need to use a full size 20"x25" filter? Or two 5" thick 20"x25" filters? The small unit doesn't stick further than 7" out from the wall, so I'd like to use as small a filter as possible.

Also, does MERV factor into this? What about MPR?

I'm sort of getting into engineering here, I don't want to break my unit, although it's really quite a pain to maintain, and I think all the dust is causing corrosion. If I can't fix this I think I might need a whole new system.


I found another spec sheet. Its for a slightly newer model, although perhaps it is still relevant.

It says motor output = 30 Watts.


...I'm thinking of adding a baffle and a muffin fan to boost the pressure across the filter. What size/power fan would be appropriate? I'm going to need 2 different size filters, 2x small filters(approximately 7"x30") and 1x large filter(approximately 9"x38"). I've found the wikipedia about Fan Laws. How do I use the Affinity / Fan Laws to calculate the size/type filter and the size/power fan needed? I want to maintain the designed cfm for my units.


I've bought plywood, and ordered the filters:

both filters have the following specs:

Filter Efficiency   93%
MERV Rating 12
MPR Rating  1900

How do I estimate the resistance/static-pressure without measuring?

  • going to Lowes today to get parts. still don't know what size fan to use, will figure that out later – kipbits Apr 24 at 13:08

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