Appears I might have the unevenness of the end faces dealt with, picked up a big honking belt sander over the weekend and that appears to do the trick. It does tend to melt the plastic a bit so I need a wire brush to reopen all the tubules. I hope it's that easy. I was alternatively thinking of using a power door planer to finish the ends, shouldn't have the melty bits that way.

I was thinking about where to get a brush assembly like they use on some 19" rack cable entry holes to block air but allow inserting cables with ease... but those were conceptually right but not quite the size needed. I had bought a $1 broom at the dollar store I was planning to cut off bristles and "glue" using 3D printer "ABS glue" acetone/plastic mix to bind the bristles, but I do imagine that could have become a mess, and the bristles would need to be frayed to make them less stiff.... then I notice the door where I work....

So now I have a source for long, fairly cheap brushes that should be great. At about $3.35 per foot, available as 3, 4, or 6ft lengths from Zoro.com, a couple of brands: "Value Brand" A626A-36 for 3ft, $10.01, Zoro P/N G0373642. Another option there is PEMCO 45061CNB36, similarly priced. I want to see some CAD drawings for this to be sure I know what I'm getting, but they look promising.

The brush fits in the midline of the heat exchanger, separating the top (outdoor air) from the bottom (indoor air exiting) and preventing most of the leakage from happening. The brush allows the cylinder to spin freely. The aluminum frame holding it should allow slightly bending also, as the ideal shape allows for the air to "purge" before the blower reverses the direction of the air as the Coroplast tubules cross over from hot to cold side.

Still looking for good ideas on brushless motors, inexpensive and 10000 hour bearing life. I'm going to prototype with bathroom vent fans, but those are constant speed. That leaves the cylinder turning motor to wrap up. I'm going to prototype that with a stepper motor, but I imagine that will be too loud for normal used. I've seen motors & drivers optimized for quiet mode using motors with less cogging at the expensive of max torque. The drum needs to spin in the 20-60 RPM range. Air velocity will interplay with that.

On another note, I'm thinking of creating another, similar project for clothes dryers. A mechanical/manufacturing engineer I used to work with talked about a venting scheme he had for his dryer (I'm thinking his was gas) that draws in outdoor air to the laundry to prevent negatively pressurizing your house and creating cold drafts. Ever consider the negative pressure issue? For electric dryers, you don't get to burn the incoming air so you need a heat exchanger to recover exhaust heat to make a real positive impact on the electric bill. Moisture condensation becomes the key to the design, so to solve this I'm thinking a cross-flow HRV pitched at 45 degrees will allow a gravity-flow condensation path, mounted above the washer drain for a pump-less solution. Auto turn-on based on temperature in the exhaust manifold. One ingress booster fan since the exhaust has a powerful blower already. Intake air could feast on preheated attic air (my laundry is upstairs, realizing most are not, YMMV) assuming that isn't a major building code problem (yes, filtration will be considered).

Of course, you COULD avoid the hassle by buying one of the new ventless, condensing dryers... essentially my goal here is to make a condensing add-on for existing dryers. Admittedly, the safety considerations here are considerable and static electricity will be present in copious amounts....