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Old 07-23-2019, 02:02 PM   #1
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A/C Return Filter Material

Got a balky A/C unit cooling this morning — just in time for the end of the heat wave. A guy helping me saw the thick mesh filter material the PO had left on the boat and said it restricted airflow too much. This material is cut to size, held in place by brackets on the inside of the return air grill and needs enough rigidity not deform and get sucked into the unit.

Most of what I see online looks like flimsy foam or stuff that looks as restrictive as what I already have. What do others use for return air filters?
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Old 07-31-2019, 08:36 AM   #2
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I use nothing at all, and never been a problem. Boat is not dusty like my house.
The evaporators have never gotten dusty.
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Old 07-31-2019, 08:57 AM   #3
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I just installed a new AC unit this winter and it came with a 1/4" thick piece of foam that slides in to a holder directly in front of the cooling fins. I installed the AC in a lower cabinet space so had to figure out how to not restrict the air flow to the unit. I ended up removing the panels out of the sliding cabinet doors and replacing the panels with noseeum screen I had in my garage. Works great. The screen acts as a filter and I am surprised at the amount of lint that I remove from the screens periodically.
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Old 07-31-2019, 09:49 AM   #4
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I went high tech and installed the cheapest least restrictive home central AC filter on the backside of the louvered cabinet door that acts as my return air source. It's amazing how much dirt, dust and debris it traps.

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Old 07-31-2019, 10:22 AM   #5
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Such a thing may trap some dust on my boat if I had one, but dust never clogs the evaporators ( I have 2), if there is any in the air. I dont have dust generators like pets and dont live on the boat so the AC is not on a lot.

I have an old 1971 model 16K btu Cruisair with 2 remote evaporators.

Small air dust is going to wash down with the condensate into the drain. I drain all the condensate into a cut open 5 gallon red gas can with a 300GPH Rule pump to pump it out. Same container collects shower water.

I think if I added a filter, I would get a cheap furnace filter and cut it to fit. Tape its edges so it wont unravel itself, and slide it on the backside of the evaporator. that would catch the big stuff and not affect airflow much.
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Old 07-31-2019, 10:46 AM   #6
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Quote:
Originally Posted by sdowney717 View Post
(snip)

I think if I added a filter, I would get a cheap furnace filter and cut it to fit. Tape its edges so it wont unravel itself, and slide it on the backside of the evaporator. that would catch the big stuff and not affect airflow much.
That's what I did. Used some siding J-Channel to make slots.

https://docs.wixstatic.com/ugd/d19c2...b06f64f330.pdf

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Old 07-31-2019, 01:10 PM   #7
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Thanks, all. (Boathealer: brilliant! Thanks.)

My A/C guy told me to avoid mesh because it was too restrictive, but the screen he was recommending looked like it might only stop medium-sized walnuts. Pretty much useless for dust.

So I may try nothing for a while and see whether the fins are staying clean.
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Old 07-31-2019, 03:54 PM   #8
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Quote:
Originally Posted by angus99 View Post
Got a balky A/C unit cooling this morning ó just in time for the end of the heat wave. A guy helping me saw the thick mesh filter material the PO had left on the boat and said it restricted airflow too much. This material is cut to size, held in place by brackets on the inside of the return air grill and needs enough rigidity not deform and get sucked into the unit.

Most of what I see online looks like flimsy foam or stuff that looks as restrictive as what I already have. What do others use for return air filters?

Your suspicion is more correct than not. Your filter media appears to be Duralast, it's available in rolls, and is very handy to cut to size. In my time in the HVAC service trade we usually kept a partial roll on each truck. It's better than throw away fiberglass filters, it can be sprayed with cleaner like simple green, hosed off and put back in service. It really doesn't need to be bone dry, the air moving across the media will quickly dry it. The 1/4" foam is more restrictive and can block with just a light coating of particulates. Pet dander is particularly hard on filters. Just as a point of reference for your larger home filters, pleated filters offer less resistance. Resistance translates into reduced air flow that drops the efficiency of the system and can cause the evap coil to freeze. If you have that happening, you need to get more air flow either by juicing up the fan speed, cleaning the fan & coil, or changing the filter more frequently & perhaps to a less restrictive media- like the Duralast. The filter's job is to protect the coil and fan from accumulation of airborne dust, it's not intended to clean the air you breathe. Those "high efficiency HEPA" filters and the like are typically pretty restrictive. Homeowners think they're making a big improvement in their air quality when the more significant impact is they're adding unnecessary restriction to their system and dropping the efficiency. If you really want to make an improvment in the air quality, you need to look into electrostatic air cleaners. Not cheap, but very minimal resistance and most effective particulate removal in the micron range.



You can prove it, clean the filter. Re-install. Run unit to get it settled out, then check the splits (entering vs leaving air temps). Record. Remove the filter, repeat. Compare results. If the split is higher with filter, it's restricting. Couple of degrees isn't a problem, but it will demonstrate if it's restricting. Filter efficiency increases as it gets dirty to where it's 100% efficient. But then NOTHING goes past, even air.
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