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Adding better filters to my AC/heat mini split system is one of those win-win projects. Two boxes checked. First, the air in the shop will be cleaner which means I will be breathing less harmful small particles. And second, the amount of dust that makes its way onto the internals of the mini splits will be reduced and therefore reduce environmental maintenance. Prior to this article and video I completed two of the three filter boxes.
Before I start building the third box I want to briefly touch on why I’m positive this is a good thing to do. This is my air cleaner cart. It’s incredibly simple. Just a plywood top, plywood bottom, and four corner 2×4 posts to connect them. Each post has rabbets cut to allow filters to be installed and the old furnace blower motor sits on the inside. Each of these filters is rated at MERV 13. The higher the MERV rating the smaller particles that they will catch. I’ve measured the effectiveness with my Dylos air quality monitor, comparing the results with other environments like outside on a clear breezy day and inside my house. In my last shop, this cart easily kept the particle count well below outside and inside my house. Unfortunately, it just doesn’t move enough air to quickly clean the air in this shop. Because this shop is about 4 times the amount of air volume it takes about 4 times as long to reach low values.
These are the two filter boxes for the first floor mini splits (one more mini split in the loft above). Each box has four 20x20x1 MERV 13 filters. The reason for four of them is to increase surface area. Generally speaking, the higher the filtration rating the lower the air flow of the filter. The decrease in air flow needs to be compensated by adding more surface area. In this case, my hope is that the four filters provide enough surface area to not decrease air flow.
I didn’t record the first two filter boxes being made but I did take pictures. It’s a very simple process starting by building around the filters. First the four filters were laid across the table and a plywood box was made to fit snug around them. Then some 1-1/2” wide strips were added to the inside as back supports for the filters.
The bottom piece of plywood was 1/4” thick to reduce weight so I added a scrap strip of plywood to the back to make it a little more rigid along it’s length. The french cleat strip was also added to the inside back top of the box.
I sealed all of the interior seams with a bead of glue.
Then the matching french cleat can be added to the wall. This cleat is sized to allow a little wiggle room to drop the box on top of it.
Then the box is set in place for a test fit.
To connect the box and the mini split I used a couple piece of plywood to make a three sided sleeve to fit snug against both the box and the mini split.
And here’s the completed test fit. This confirms that the box fits the space, I don’t see any obstructions with the design, and that all of the remaining seams are small enough to seal with tape.
After tracing the shape of the mini split to the bottom of the box the box is removed from the wall and the opening is cut. This opening is larger than the open intake on the top of the mini split.
The wall mounted french cleat is not as long as the box mounted french cleat so that it is easier to hang. For consistency sake I made a note on the wall mounted french cleat to push the box to the right so it is in the same location every time.
This is the washable filter that comes with the mini split. I vacuumed the right side so you can see how much dust was caught in the filter. This is after running for two months in the shop. This is another indication that my air cleaner cart isn’t as efficient at keeping up with the dust in this larger space compared to the smaller space of my last shop.
All of the seams where the box meets the wall are sealed with a strip of tape.
All of the exterior seams on the mini split are sealed with electrical tape being sure to not stretch the tape. If you stretch electrical tape it will greatly reduce it’s hold to the surface because it’s going to want to retract back. If you don’t stretch it during install the tape will do a great job of sealing and it’s quick and easy to replace for maintenance to the unit.
Here’s the completed right side. Yes, the tape and three sided sleeve needs to be removed to access the internal filters. And, yes, the filter box above needs to be removed if the entire plastic cover needs to be removed from the mini split. Neither of those tasks are difficult to do. Basically, yes, I made it 5-10 minutes less convenient to perform maintenance to the mini split. But in my opinion the pros far outweigh the cons in this case. This modification should keep the inside of the unit cleaner and therefore reduce the amount of maintenance required on an already low maintenance unit. And it also takes my shop from one air cleaning device to four once the loft box is completed.
The right side was the prototype and after confirming that it was a decent plan I repeated everything with the left side.
The loft filter box needs to be narrow due to less space being available. Just like the lower two filter box the process starts with the filters on the table and measuring for a box to fit around it.
Then some 3/4” plywood is cut to size at the table saw.
And then at the miter saw.
I used a marking gauge to transfer the filter thickness to the outside of all of the box sides. This lets me know where the filters stop and where the 1-1/2” wide backing strips start when it comes time to install them.
The loft mini split will be just a little bit inside the filter box so I decided to cut the rectangle opening before assembly. Cutting the opening on the CNC probably took the same amount of time compared to drilling holes at the corners and cutting the rest with a jigsaw but the benefit is a super clean cut that requires zero cleanup.
Assembly can begin with wood glue and brad nails at every joint. Followed by pilot holes and screws.
I had some extra 1-1/2” wide strips left over from the first two boxes so I didn’t have to rip any for this box. Just cut them to length at the miter saw.
Just like the rest of the box, these are secured first with brad nails and then with screws. Starting with the long pieces. Then the short pieces. Rinse and repeat for the back side.
A single middle brace is needed right where the filters meet. This is secured with glue and a single pocket hole screw top and bottom.
The long filter supports weren’t quite long enough so I decided to fill the gaps with caulk. And while I was at it I went ahead and hit the rest of the internal seams that weren’t glued.
The last piece to add was this top horn looking piece. The purpose of this is to prevent the box from falling once installed.
This is the view from my computer desk towards the back of the loft. The mini split is mounted on a 2×10 that spans between two 6×6 posts.
Here you can see it in relation to the stairs.
And here you can see the gap between it and the back wall. This gap will allow me to get the same four filter capacity as the longer boxes below.
Once the internal filter was cleaned and the seams taped the filter box is set into place. This one is not secured in any way. It just fits over the mini split and is supported by the horizontal 2×10 in back. The little horn piece on top contacts the truss and prevents the box from rolling forward. Super simple.
The back two filters are slid into place first.
Followed by the front two filters.
Lastly, the rest of the seams can be sealed off with electrical tape.
I don’t think I’ve shown my office space yet so here we go with it. This is from the back wall looking forward. I have a couple LED light strips firing up for an even amount of reflected light while I’m working.
My desk is at the far end so I can look over the edge.
Here’s the view over the railing.
And I have a perfect view of the front door from my computer.
Back to the filter boxes, you can see that they don’t interfere with travel up and down the stairs.
And the loft filter box tucks away neatly into the space.
That’s it for this one. Have a great day and I’ll talk to you later.