DIY ventilation heat exchanger

[AC_Hacker]

Met a very interesting "mad scientist" the other day...

He is reluctant to post his work, gave me permission to do so.

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(* 4-part post begins here *)

I had my house insulation upgraded and some leaks sealed.
I'd like to do more sealing, but will need some external ventilation.

I read that, for a well-insulated house, 40% of the heat cost goes
out the window (so to speak) as fresh air infiltration.

I looked into Heat Recovery Ventilators. Cheapest I could find was
$400 on ebay. And that doesn't include shipping or all the mounting and piping
and electrical work. Quotes for installed systems are over $1000.

That ain't gonna happen, so I started looking at DIY solutions.
Found web page that described a cross-flow heat exchanger for
dryer heat recovery. It was made out of corrugated plastic sign material.

I was concerned that the thermal conductivity of the plastic would be
insufficient, but I built one anyway. Tests with a heat gun and a fan
concluded that it might be OK. I was sufficiently encouraged to embark
upon a quest for a cheap plastic HRV.

The day after an election, 24" x 18" corrugated plastic signs are free.
Finally, something useful from politicians!!
Even in off season, they're not hard to find. First sign shop I tried
was kind enough to give me a stack of old signs.

The problem was coming up with a design that could be built
with resources found in the average kitchen drawer.

Here's what I came up with:.
Has basically one part repeated multiple times.
Should be able to construct it with a linoleum knife and some
shipping tape to seal off the channels.

Cut the sign in half to make 24" x 9" plates.
Cut notches in the end and triangles in the interior.
Tape off some of the channels.
Flip and interleave them for a stack height of 3".
A 4" round duct pipe coerced into a square should fit
nicely over a 3"x 3" stack of plastic.
Depending on mounting, wrap it with fiberglass insulation
or sandwich it between two chunks of rigid insulation board.

There are issues with ducting and water drainage and mold growth that are not resolved.

To maintain the true hillbilly spirit, my first attempt will be to forgo the ducting entirely.
Cut a rectangle of 1" insulation board that fits in the window channel.
Glue this Hillbilly HRV to the outside.
Cut holes for the top air in/out and duct the air into the house with a chunk
of sign board and some duct tape.
Place an old computer fan over one of the holes to force air in or out.
Depend on the house being tight enough to make the air go the other way.
Can always add a second fan if required...or better yet, more leak sealing.
IF the wind is blowing, the hillbilly HRV can stop working, but in that case,
you've got plenty of ventilation thru leaks anyway.
Use a plastic deflector so the water that drips out will miss the side of the house.
Stick a second layer of insulation on the outside to complete the plastic sandwich
within the insulation sandwich.
Depend on baffling to reduce the mixing of the in/out air on both sides.
Tape some filter material over the outside openings to keep critters out.

This is NOT an optimal design. It's a LOW price/performance attempt to
get a significant percentage of a real HRV for almost ZERO cost.
It's also important that it can be easily constructed by the average person
(ME) with stuff found in an average kitchen drawer..
Put it on the backside of the house obscured by bushes and your neighbors
will never know your house looks like it belongs in a trailer park.
Or paint a picture of your cat on it. Your neighbors will think your cat
likes to sit in the window.
And you can remove it in seconds when you invite your boss to that dinner
party.

I can run the fan off a UL approved laptop power supply so the gumment
inspectors don't get their panties in a wad. It's not permanently attached.

It rarely stays below freezing for long here, so defrosting is not an issue.

I can improve the thermal efficiency by cutting big "non-overlapping holes in the
board, but that can seriously compromise the dimensional stability. It really needs
to hold its shape when you build the sandwich. And it needs to come apart for
cleaning without breaking the plastic.

My biggest concern is how to keep nasty mold, mildew, etc.
from growing in it.

In the spirit of "measure twice, cut once", I'd like input before I go wasting
all my free sign material on a flawed design.

Ideas?
Thanks, mike


7/22/10
Well, didn't get any input, so I built one.
All the plates are identical

Flip every other one so all the triangular holes get covered by plates on either side to form
the serpentine air path.

It's a folded axial-flow heat exchanger
with three parallel air paths...per sheet. Every other sheet
carries air in through the plastic channels that make up the corrugations.
Alternate sheets carry air out.
Heat is exchanged thru the flat sides of the plastic sheets.


Haven't put on the tape to Block the relevant channels...yet.

First Prototype:

(* to be continued... *)

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-AC_Hacker