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.

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

(* 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... *)

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

-AC_Hacker

[AC_Hacker]

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

(* 2 of 4-part post continues here *)

BIG PROBLEM
Too Restrictive. Can't get enough air flow through it.
Does seem to be transferring heat though.
Needs to have bigger channels.

Heat Pipe Version

While I look for cheap aluminum plates for the flat plate exchanger,
I looked at a heat pipe.

1/4" copper pipe. The gizmo in the center is the piercing clamp that is designed
to add freon to an exixting air conditioner. ..with a R12 to R134 adapter.
Clamped the ends shut and soldered.
Added R134.
Need to learn to tell how much liquid is in there.
Seems to transfer heat quite well. But coupling hetat into the pipe needs work.

Really can't make use of the curves...radius is too large.

Vertical straight pipes look like a better solution
Not my idea. Found a similar design on a website that I can't remember.

Valves from old bicycle tube

(* to be continued... *)

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

-AC_Hacker

[AC_Hacker]

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

(* 3 of 4-part post continues here *)

Hard part was filling it. Rube Goldberg setup of all the pieces
it took to get from one thread to the Schrader valves.

I used ice on the copper to condense the gas while filling the tube. Takes several minutes.
Put in some and vent it to clear the trapped air. Then fill it.

When it's done, you can tell how full it is with a heat source and stethoscope.
Listen to the tube and move the point heat source up and down on the pipe.
Starting at the top, move the source slowly down the pipe. When you hit the liquid
level, you can hear it boil. If it's too low, add more...too high, vent some.
For a heat source, I use a tiny catalytic hot air attachment for a butane soldering iron.

CAUTION, this stuff is cold. Wear gloves and wrap in insulating material. If it vents
on your hands while you're trying to hook it up, it will give you FROSTBITE.
EYE PROTECTION IS ESSENTIAL...DON'T GO WITHOUT IT.
LIFE IS HARD ENOUGH WHEN YOU CAN SEE.
USE ADEQUATE VENTILATION.
DO NOT ATTEMPT TO DO THIS.
FOR EDUCATIONAL PURPOSES ONLY.
UNSAFE PROCEDURE.
DON'T DO IT!!!!

Next step, transferring heat from air to the pipes.

Made some fins out of .040" aluminum

The resistor is an attempt to have controlled power input. At 25W, the resistor temp
is WAY above the pipe temp. Lots of losses getting from heat source to the refrigerant.
The line about 1/4 the way between the fins is the liquid refrigerant level.

(* to be continued... *)

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

-AC_Hacker

[AC_Hacker]

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

(* 4 of 4-part post continues here *)

The heat pipe is doing its job. Getting heat in and out is problematic.

I finally did the math I should have done earlier.
Using round number approximations:
In winter, assume it's 35F outside and 65F inside. deltaT of 30 degrees.
100cfm of air exchange with outside sends 1.08X100X30 = 3240 BTU/Hr. = 950 Watts..
Assuming we can save 500W of that, the effective thermal resistance of the system
has to be 30/500 = 0.06 degreesF per watt!!!
I need MUCH bigger heat pipes ;-)


Test Jig
Best temp differential I've seen this summer is about 15 degrees F.
Inlet air temp is about half way between the inside and outside temp.
But the margin for measruement error is a significant portion of that.
Need much hotter or colder day to get the temp differential up.

Fan overlay

November 2011
BETTER HEAT PIPES

I bolted a pair of computer CPU heat sinks back to back and mounted them in the window.
I haven't done much evaluation, but the incoming air temperature seems to be midway
between the internal and external temperature. That seems high. Need to do something to separate
the airflows to keep them from mixing. Also need to close up the air leaks in the heat exchanger.

I also noticed that, in calm weather, the air close to the house is warmer than the "outside temperature".

The results here led me to experimenting with water-based heat pipes.
Some quick tests suggest that, while they may work fine for computer
chip temperatures, at HRV temperatures, the vapor pressure of
water is so low that it can't overcome the weight of the water
column. Evaporation is confined to the surface of the water
column and little heat is transferred. The computer heat sinks get
around this using a wicking structure and no standing water column at all.
I have no confidence that I can duplicate that in my garage.
Freon seems to be the best choice for DIY experiments.



ROCKS



Somebody suggested I use a reciprocating HRV using steel balls as the storage medium.
I like rocks. Sounds strange until you do the math.

Basically, you blow air out of the structure through the rocks. This heats the rocks
in winter or cools them in summer. The average temperature of the rocks is
half way between inside and outside temperature. You don't need much temperature change.
After some time, you reverse the fan and pull outside air back through the rocks.
This recovers heat from the rocks.

In winter, moisture from inside condenses on the rocks and drips out the bottom, so it
doesn't come back on the next half-cycle. This performs one of the primary functions
of the HRV, reduction in internal humidity.

You're still blowing air in and out thru cracks in the structure, so you get only half the potential
heat savings. Two units out of phase equalize the pressure and should double the efficiency.
If the units are in different rooms, you get some internal circulation.
It also solves the piping problems with routing air all over.

Major design hurdle is the fact that most fans are not reversible. This needs some
research. The rest looks simple.

Stack up six cinder blocks on top of a grate.
Some baffling to reduce intermixing of the flow.
Fill up the holes with river rock.
Wrap it with insulation.
Pipe air to each hole, run them out of phase.


MONITOR

In addition to monitoring temperatures and airflow, I also track the amount of time the furnace runs.
This gives a running commentary on how well, or badly, an experiment is working.

The numbers are :
sample number, % run time this cycle, average % runtime since start, minutes furnace ran this cycle, length of the cycle in minutes, outside temp.
The outside temperature reads zero because the sensor is not plugged in.
The graph represents one day with noon in the center. Each line segment is a furnace cycle. Length of the segment
is relative to the length of the run-time, but had to lengthen them to make 'em visible. Vertical position is the percentage
on time for the cycle. That represents the BTU/hour for that cycle.
Each percentage point represents an average heating power of 170 watts. I can watch the gas furnace output go up
when I turn off my computer.
Green blip at the baseline is the current time. Everything to the right of the blip is yesterday's data.

(* end of 4-part post *)

&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&

-AC_Hacker

[AC_Hacker]

4-part posting of:

HRV + Heat Pipe + Hillbilly + interesting monitor...

...is now complete, all in-line images are working correctly.

-AC_Hacker

[AC_Hacker]

Here's a link to a German company that makes HRVs... wheel-type, cross-flow and counter-flow.

Klingenburg GmbH - Energy Recovery

They have a program that will calculate amortization due to energy savings. It just so happens that it will approve (or not) HRV cores dimensions that will work for a given airflow, thus it can be indirectly used as a design program...

http://www.klingenburg-usa.com/pdf/Wirt.zip

The program just runs, apparently there is no install, it just runs.

-AC_Hacker

Strange that this thread has headed down a similar path that I have been working on. I agree with the issues on the heat pipe as a starting solution. I think there is some possible solution mixing fine wire mesh interconnecting several heat pipes as has been fabricated in the pictures. The fine wire will transfer more heat energy between the sources than using the fins shown in the pictures. The fine wire will create more turbulence thus mixing the air as it passes across the wires. Another problem with the heat pipe solution is that heat pipes want to function better when the hotter temperature is on the high side and the colder on the bottom. This would mean that these heat pipes would need to reverse themselves when the "delta T" in and out reverses. In other words at least fall and spring they would need to flip. In the Summer the cooler inside air needs to flow across the bottom of the heat exchanger and in the winter the cooler outside air needs to flow across the bottom of the heat exchanger. The more heat pipes the faster the transfer of heat energy. Likewise the more interlacing of fine wire to make the system work and more laborious the task.

[AC_Hacker]

Originally Posted by nrgynrd

Another problem with the heat pipe solution is that heat pipes want to function better when the hotter temperature is on the high side and the colder on the bottom.

The heat pipe that Mike showed me had liquid Freon that was more dense and was at the bottom of the pipe, and Freon vapor was filling the top of the pipe.

Heating the bottom of the pipe caused the liquid Freon at the bottom to boil and the vapors rose to the top of the pipe where they condensed and released their heat and returned as liquid, to the bottom of the pipe.

-AC_Hacker

Yes you are correct I had my logic reversed. The heat pipe would still need to flip over or the outside incoming air would need to switch locations in order to make the process function correctly.
I have also looked into a thermoelectric peltier unit to be the heat exchanger with two heat pipe fine wire systems one on top of the thermo unit and one on the bottom of the unit. You would switch polarity of the unit when the inside/outside delta t reversed. These units were $6 before christmas. They have doubled in price since christmas.
Adding this thermo component reduces efficiency drastically.

[Student 07]

Hi,
I like the "Mad Scientist's" inventions. How big is the coroplast that he is using for his heat exchanger? It seems logical that more surface area in a heat exchanger will increase efficiency; however, I think all those turns are causing the problems.

The vertical pipe heat exchanger is great. It seems very similar in principal to an evacuated tube solar collector.

I found this heat exchanger, which is kind of different, so I thought I would post it for discussion.

It is basically a heat pump which recovers heat from the exhausted stale air, and supplies warm air to the fresh air supply side. It doesn't use a "heat exchanger". I had this idea a few weeks ago, and now I found that somebody actually makes one. It was also pointed out that this kind of HRV would be more efficient if it used a heat exchanger. I couldn't find a lot of information about this HRV and everybody is out for the holidays.

I'm not sure if it posted with the picture, if not, then here is an excerpt from their advertisement.

ThermalAir is a programmable air exchanger which saves energy and dehumidifies the air. We would like to say that it keeps the heat indoors in cold climate and retains the cool inside in summer but this would be an understatement because ThermalAir is more than 100% efficient. ThermalAir will extract 9,958 btu's of heat from the outdoors in winter and blow the warm fresh air indoors. Conversely, in the summer it will blow 6,284 btu of cool fresh air indoors. ThermalAir is supper efficient at 390% in heating mode and 710% cooling mode, this means for example, for every watt required in cooling mode to run, ThermalAir returns 7.1 watt of cooling output. Now that's green and bonus: it provides 15,600 cubic feet of fresh oxygenated air per hour!