Air Filtration Unit #4

Last summer, Central Oregon – along with a large swath of the western US – experienced yet another season of raging wildfires. And with wildfire comes smoke, lots of it. It got pretty severe in here in Bend, very unhealthy. In recent years, I’ve combated the problem by making air filtration units equipped with near-HEPA filters and ultra-quiet fans. So far, I’d made three such units. Along with the house forced air unit (the furnace), we kept the inside smoke to a minimum. My next door neighbor also suffered from the smoke so when the air was at peak unhealthiness, I quickly cobbled together a portable air filter to help out. It was basically just a wooden frame sized to fit a filter and then attached to a fan. It worked but was obviously a temporary solution.

After the smoke cleared, I took the filter/fan unit back and proceeded to construct a proper one. My initial thought was to make it into a bedroom end table but the space and dimensions weren’t right so option B was to see how small a piece of furniture I could squeeze the fan and filter into and go from there.

Here’s the result:

It’s almost exactly a sixteen inch cube, with the body and legs made from hemlock and a flip-up top from alder. There’s a 12″x12″x1″ filter that sits inside held by gravity. A wood frame with a screen sits on top of the filter. It serves no function other than to hide the filter from view. As with the last two units, the electrics include an outlet in case you want to set a lamp on top, or a phone charger, or whatever. Air is drawn in through the gap on three sides and exhausted out a port in the back. The gap on the fourth side is blocked with a piece of hard white oak, necessary to attach hinges.

I’m happy with the result but, like with all my projects that I build without a set design (winging it), I made mistakes – some I had to correct, some I decided to live with. I’ll mention those in the build details.

Sizing things up:

I started with the wood frame piece I built for the temporary unit, seen here sitting unattached on top of the fan (a Broan bathroom fan). It was sized to hold the filter so there was no reason not to re-use it. The plywood on the bottom was cut to the same size as the filter frame with the idea I could ‘simply’ slap four side walls around the fan. Mounting the fan to the plywood and the frame to the fan seemed trivial but actually was quite a finicky operation given that the frame had to be lined up exactly with the bottom.

This is the result (above). I first attached a piece of 2×4 to the bottom to provide a secure mounting surface for the fan’s side flanges, and also screwed it to the bottom directly. The filter frame is then attached to the flanges on the fan body – the finicky operation that required keeping the frame exactly lined up while I drove screws in from the bottom. It took like an hour to do that.

The sides:

Unlike with the living room end-table piece I made a couple years ago, for which I used nice white oak pieces, I was looking at lighter, cheaper wood for this unit, preferably available from the local big box hardware store. My choices were poplar and hemlock. I’d not made anything with hemlock before and I saw the store had 1/2″ stock available in that species, so hemlock it was. The thinner stock would cut down on weight and wouldn’t require me to plane 3/4″ pieces down to size. And hemlock is fairly cheap.

The easiest way to fashion the sides was to glue three long boards together using a spline joint and then cut the individual sides to size.

One side needed a bit more work to accommodate the exhaust port and power switch/outlet. And it’s here I made my first mistake, which I didn’t recognize until much later in the build. I think the exhaust port should have been on a panel adjacent to the power switch. That way the unit can sit against a wall hiding the switch and cord, while the air is free to flow out the side into the room. As it is, the unit will have to sit away from the wall to ensure proper air circulation or the switch and cord will have to be visible. Not a huge issue.

To hide the end grain at the top of the side pieces, I glued a thin slat of hemlock. Then a round of sanding and a couple of coats of wipe-on poly.


Before attaching the sides, I installed the switch/outlet, ran the wiring and also fashioned a short duct to channel the fan exhaust to the outlet port. There’s a screen over the port to keep critters out.


Also made from hemlock, the legs are two pieces glued in an ‘L’ shape designed to ‘capture’ the side pieces against the plywood and filter frame. Besides being lightweight, the design accommodates my goal of being able to disassemble the unit in case the fan malfunctions or the outlet fries. For additional support, I glued blocks inside the legs; the body of the unit actually sits on these blocks with long screws coming up from the bottom. Before attaching the legs, I put screws through the middle of the side panels, top and bottom. The screws are a nice-looking square drive type I found at Lowes.

We arrive at another of my mistakes; this one I had to correct. Originally, I sized the leg pieces to overlap the sides by maybe an inch. That wasn’t enough as it would mean driving the screws too close to the edge of the wood. I had to trash them and start over using wider pieces.

Here’s where we are with sides and legs attached and wiring complete:

The top:

For the top, I judged hemlock to be too soft. I wanted wood that would be at least a little impervious to scratches and dents, but not heavy white oak. A couple of years ago, I rebuilt the bay window shelves in my dining room and used alder for the top. Alder is also sort of soft but not like hemlock. It’s also a light-colored wood. So, off to the local wood supply store to fetch some alder.

A gratuitous shot of the bay window build:

I used a familiar method to join pieces to make a wider panel: biscuits. I like my biscuit joiner, although it’s probably not the choice for more accomplished woodworkers. And here I made another mistake. Not having decided on exactly how big the top would be – that is, how much it would overhang the sides – I glued up a big enough piece thinking I’d just cut it down to size. But I didn’t think about where those pesky biscuits were and ended up cutting through them on one side. It’s not an issue structurally but you can see the biscuits (that edge isn’t seen in the below photo but you can see it in the first two photos I posted).

The top is just a flip-up deal with hinges on one side. I needed a stout piece of wood to attach the hinges to so I inserted a piece of white oak on the back (what I’m calling the back – the side with the switch and outlet port).

You’ll also see in the above photo where I decided to glue blocks inside the part of the legs that stick up, thinking they would provide a nice surface to attach felt pads. I eventually removed them, which involved taking the legs off and running the table saw blade along the inside. It was that or toss them and make a (third) set of legs. The table saw did the job nicely.

Removing those upper blocks became necessary because of a design element. With the sides open to allow air in, the filter was visible and it didn’t look right. Initially, I tried to design side screens but it just didn’t work. The solution was to build a screen frame to sit on top of the filter and hide it. For added interest, I attached a cool pig knob to the screen.

So here’s the final unit:

The tale of the 4th air filtration unit is not complete without one more mistake. Not a mistake, per se, but a minor design issue. I was real proud of how the top came out – it was absolutely flat and sits on the legs perfectly. But over a couple of weeks, I noticed it was warping slightly. It still sits on the legs nicely but the front to back edges are a bit bowed now. It’s not real noticeable but might get worse. After contemplating ways to fix the issue – or at least halt further warping – I decided to leave it be. The top is not designed to be strong – you can’t sit or stand on it, so a bit of bowing is OK. Probably the reason it bows is because I failed to alternate the grain patterns of the three pieces I glued to make the top. Rookie error. Next time I’ll be sure to do that, and to incorporate some sort of strengthening pieces.

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