Another issue is “formicary corrosion” causing many evaporator coils to leak. Wondering what percentage of the ductless heads with poor filtering are developing corrosion. Fujitsu acknowledges the issue of corrosion and also claims to have improved their coils with “ProCore” copper from 2019 resulting in better resistance to corrosion. Intriguingly Fujitsu and many other minisplit manufacturers tout “all-aluminum” coils in their multi-position air handlers but somehow don’t offer “all-aluminum” coils in their ductless systems; would be grateful if anyone can explain possible reasons for sticking to copper in this case.

Also, Fujitsu mentions the below FROM: https://www.fujitsugeneral.com/shared/us/pdf-fcus-residential-technologys-procore-catalog-01.pdf

“Coil Cleaning

In extreme environments, it is recommended that a contractor or installer follows industry preventative maintenance procedures and clean the coils of the indoor unit on an annual or bi-annual basis using a spray cleaner. Fujitsu recommends specially formulated alkaline detergent with the strength to dissolve grease, oil, dirt, tar, lint, nicotine, etc. without attacking aluminum fins. When mixed properly and when used in conjunction with a chemical neutralizer, this combination will ensure your system is properly cleaned. Mini-Split Neutralizer is used to neutralize the alkalinity of mini-split cleaner after cleaning and to prevent possible damage to aluminum fins from improper rinsing.”

Wondering if anyone has any suggestions on what would be examples of off-the-shelf coil cleaners that might fit the above description. Some also mention not to use municipal water with high flouride content to clean the aluminum fins.

Correction – After everything dried, the blower wheel was still pretty dirty. The pressure washer is probably necessary to fully clean it…

Personally, I think just adding in some additional free-standing filtration elsewhere should lower the overall dust “burden” that might find its way into the mini-split. Strategically place in areas where particulates might be generated or near the mini-split.

I have tentative plans to do this with my workshop unit, but I haven’t yet taken the time to figure out the details. It seems obvious that such a pre-filter system would need to provide sufficient filter surface area, so as not to restrict air flow too much. If a single 1″ pleated filter is too restrictive, then the design could provide room for 2, 3 or 4 filters.

The most common pleated filters are 1″ thick, but thicker filters are available. A pleated filter that is 2″ thick will have much more filter surface area (and less airflow resistance) than a 1″ filter of the same width and height, and there also a number of products on the market that are 4″ and 5″ thick .

Filters with lower MERV numbers will pass air more freely, and cheap fiberglass filters are less restrictive than pleated filters, so those are other considerations. The fact is that any type of pre-filter should clean the air much better than the flimsy screens included with mini-split units, but care must be taken to avoid excessive restriction of air flow through the unit.

With low external static pressure blower assembly’s I would doubt descent filters would allow proper airflow. It would take quite a bit of filter area to provide adequate esp.