[ham789]

I'll try to express some of my experience relevant to recent posts.
Be aware that I'm a firm believer in the 80-20 rule
and the concept of diminishing returns. It's fun to discuss
100% efficiency, but you can get a LOT of return for relatively
little investment. I try to apply as much cleverness as I can
muster to a minimalistic approach.
My requirements are lower than many people would accept.

Filters:
I use the fiberglass from the cheapest A/C filters on mine.
Just enough to keep out the (larger) bugs.
I did build an electrostatic filter for the inside. I recently
acquired an air particle counter. Will be interesting to see
the numbers when hay-fever season arrives.
Filtration is very expensive in terms of cost of moving air.
Use as little as possible.

Post heating the air:
Air coming in is colder than ambient. But it gets reheated
by whatever you use for a primary heat source. If you
can add heat more efficiently than that, you should work
on your primary heat source. I can imagine a situation
where it makes one room too cold and might require
remediation.

Cleaning:
I think it's critical to be able to clean your HRV. The thing
is wet and dirty. Perfect environment for stuff to grow.
I decommission mine in summer and soak it in bleach.
Since it's completely self-contained, I have no duct work
to worry about.

Bent metal:
I experimented using aluminum flashing. I bent a thin
lip on each side. I intended to stack them up and seal
the stack with tape along the lips. Looked good on paper,
but I discovered that the slightest imperfection in the bend
warped the whole thing way beyond usable. I'd need a much
better bending brake.
Then I decided I'd use flat flashing and aluminum bars to separate them.
But the cost of the bar stock and my lack of confidence that
I could drill hundreds of holes that all lined up killed that idea.

Then, there's the jelly roll approach.
Imagine two long strips of flashing. Glue a rubber hose
down each edge between the strips and another on the top
side. Roll the thing up using the rubber hose as spacer and seal.
If you cut the hose short on the inside and the other side
on the outside, you can make the flows exit opposite sides
of the roll. Or just make end caps with spiral grooves
to hold it in place. Stick a fan on each end cap.
Since you have only one channel, the spacing has to be
large, so the thing gets big really fast. There'll be lots
of resistance due to the changing direction, but that may
improve the boundary layer problem???
I never got around to trying it.

If coroplast is just as good, seems silly not to use it.

Fans:
I'm not convinced that super-duper fans are much benefit.
For a given air flow, pressure, duct size, etc.
there's an optimum blade/fan design. Just do that and be done
with it. Just turn the thing off when not needed and run
at optimum when it is.
Sophisticated fan controls help when you have a wide
range of operating conditions...don't do that...80-20.

Fan Balance:
I gave up on trying to balance the fans on my small coroplast
unit.
It's poked thru the wall and both fans are on the inside.
I have a decent hot-wire anemometer, but cleaning up the
air flow to get a repeatable reading has been problematic.
I have a differential pressure gauge with 2Pa resolution.
I just tweak the fans for zero inside/outside pressure differential. The slightest breeze causes more differential
than that, so I just pick what looks like average. Might not
be optimal efficiency, but I have only one variable to
adjust.

I tried poking a hole and using the anemometer to set the
air flow thru the hole to zero. Much more sensitive than the
pressure measurement, but when you're already swamped
by noise, better resolution isn't much help.

Another experiment I did was to modify a cheapo HVAC
pressure sensor. It's a UEI EM150 I picked up at the junk
dealer. Has 25Pa resolution. I cranked the gain up by a
factor of 10. Was 10x more sensitive, but the op-amps
were too crummy to keep it centered in the dynamic range.
If I didn't alredy have a better unit, I'd build a better amplifier for it.

You'd think that equal flows would be maximum efficiency.
In a symmetrical unit, maybe. Not so sure about the stuff
we throw together. Mine has narrow coroplast channels
on the output side and wide-open-spaces on the input side.
The idea of equalizing temperature
differences at the inside/outside ports sounds rational,
but there are many variables in the real world. It's hard
to measure precisely.
Just turning on the range vent hood can produce a 5Pa
pressure differential that completely swamps my HRV.

Leakage:
I just used a soldering iron to stitch-weld my coroplast
plates together. I made no attempt to seal the entire joint.
Would be a problem with air pressure, but my pressure is so
low that I don't worry about it. I'm pushing air out
and sucking it in, so heat leakage comes right back in.
80-20 in action ;-)


My problem statement was: House stuffy in winter.
$20 worth of fans and coroplast cured the problem.
And I get back about 75% of the energy lost.
I like discussing the subject, but I'm not strongly motivated
to improve things.

While I'm rambling...
I did an experiment using heat-seal tubing and popsicle sticks. Stack 'em up and the air pressure inflates them.
Totally unmanageable in practice. Might work better with
full plastic frames that could be bolted together.
I attached a picture of the gasbag HRV for fun.