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tiny air to air heat exchanger

 
paul wheaton
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So I have my short term digs in Missoula.    My room has a really excellent floor - some rich browns mixed into cement.  Most folks would probably think this is lame - but I want to do some experimenting with radient heat.

Okay - setting all of that drivel aside ...

This place is one, bit ziplock bag.  The walls are literally sealed.  The windows are super efficient and actually make a sealing sound when you close them. 

My first thought it that I would like to open the window an inch and put something in that inch that would be a sort of tiny air-to-air heat exchanger.  Something where a tiny bit of air would go outside and a tiny bit of air would come inside and the heat in the outgoing air would pop over the to incoming air. 

Has anybody ever heard of anything remotely like this?

 
Erica Wisner
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paul wheaton wrote:
... in Missoula....

This place is one, bit ziplock bag.  The walls are literally sealed.  The windows are super efficient and actually make a sealing sound when you close them. 

My first thought it that I would like to open the window an inch and put something in that inch that would be a sort of tiny air-to-air heat exchanger.  Something where a tiny bit of air would go outside and a tiny bit of air would come inside and the heat in the outgoing air would pop over the to incoming air. 

Has anybody ever heard of anything remotely like this?


Yes.  Do an Internet search.  I think I saw some papers out of Britain with stuff like this that they put in walls - honeycomb of tubes.

Want to try an experiment for me? 

You could make something with industrial tin foil: cut a piece a little taller than your window.  Fold it in half lengthwise, then roll it in a spiral, so it's still long enough to match the window height.  Tape or seal the sides to the window and frame, with the top hanging over slightly to the inside, and the bottom hanging slightly outside.

Air should move up the inside, and down the outside, exchanging heat as it goes.  You might need to add some spacers between the two layers to maintain an even flow area - like maybe some double-sided foam sticky tape.
Insulate the outside of the device, allowing the cold air to reach the inflow point, to prevent wind chill from robbing heat from the rest of it.
You may be able to create a special separation of in- and out-flow points, for example by leaving the separating inner part of the tinfoil hanging out on both ends, and peeling back the outer walls to allow air in and out along a greater, more separated, area.


If this is all too ugly for words, see if you can find or make a nice slim box to enclose it, and protect the insulation from the weather.  Or use a document tube, with weatherproof outsides.

Other options might include twisting metal tubes past each other, or embedding inflow vent pipes alongside outflow ones in your roof.  You can also create a heat-exchanger that only pre-warms the incoming air, like a long, slow vent.  Your heating system or woodstove is probably sending plenty of stale air out to compensate.  Any set of tubes or cappilaries encased in a heat-transmitting medium could work for this. 
Again, insulate the outside except the intake/exit point, or you'll effectively have a heat-exchanger passing your home's heat to the outdoors.

But tinfoil seems easier to play with.

good luck have fun and stay warm!

-Erica
 
paul wheaton
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Yes!  I want to do the experiment!

There seems to be several elements to this that don't wanna go into my head.

1)  maybe it will help to tell you that my window is the kind where the lower half slides up and down.  And when it is up about half an inch, there is a gap at the bottom and a less detectable gap in the middle where the two panes overlap and are no longer sealed.

2)  My understanding of air-to-air heat exchangers is that you want to have the outside air intake as far as you can get from the outside air exhaust.  Similarly, you want the inside air intake as far as you can get from the inside fresh air source.  In this case, it would be something like two feet. 

3)  Industrial tin foil.  Ooooooo - that sounds useful.  Where do I get some?

4)  You are attempting to use air temps to move air up and down.  But if the heat exchange is any good, won't this end up not working? 

You mention wood stove stuff doing some air exchange.  Let me paint a clearer picture for you:  all electric heat.  They run a dehumidifier because the place is sealed up so tight that the air gets muggy from people breathing. 

Current outside temp is 7 degrees.  It's supposed to hit zero on sunday.


 
Brenda Groth
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my son put radiant in floor heating in his basement when he put it in..and now we have a fluid to fluid heat exchanger putting heat through his floor from our wood furnace..and it really works grand..

does use a lot of fuel to get it up to heat but not much to maintain it though
 
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paul wheaton wrote:
Yes!  I want to do the experiment!

There seems to be several elements to this that don't wanna go into my head.

1)  maybe it will help to tell you that my window is the kind where the lower half slides up and down.  And when it is up about half an inch, there is a gap at the bottom and a less detectable gap in the middle where the two panes overlap and are no longer sealed.

2)  My understanding of air-to-air heat exchangers is that you want to have the outside air intake as far as you can get from the outside air exhaust.  Similarly, you want the inside air intake as far as you can get from the inside fresh air source.  In this case, it would be something like two feet. 

3)  Industrial tin foil.  Ooooooo - that sounds useful.  Where do I get some?

4)  You are attempting to use air temps to move air up and down.  But if the heat exchange is any good, won't this end up not working? 

You mention wood stove stuff doing some air exchange.  Let me paint a clearer picture for you:  all electric heat.  They run a dehumidifier because the place is sealed up so tight that the air gets muggy from people breathing. 

Current outside temp is 7 degrees.  It's supposed to hit zero on sunday.


You're right; the best air-to-air heat exchanger is only going to make both flows of air the same temperature.  It's not going to make the cold air warmer than the hot air.  Seems like you'd probably get them both to average out around 30 degrees.  Is that going to work for you?

My quickie design was not supposed to create a fast flow of warm air.  It's supposed to prevent a very strong, fast flow of cold air from developing.  The air movement should be more like wafting gently in and out, a little breathing pore in the otherwised sealed envelope. 

But the horizontal window calls for a different design.
And the temperature calls for the air providing the heat-exchange to 'outnumber' the air coming in, if you want to get it up above 30 degrees.
I'm thinking... maybe a delicate maze like a metallic ant farm, where there are separate paths for incoming and outgoing air (you may not be able to control which is which, so making them both the same length would be good).
Instead of trying to put two exterior holes on each panel, how about building two and put them across the house from each other? 
The wind can decide which is which, and that way you can force any incoming air to go through a very long heat-exchange channel past warmer and warmer air, until it finally gets inside.

You could do it by embossing, or by rolling and wrinkling.  I'd try scroll patterns from Chinese, Celtic, or Greek geometric keys, where the continuous line is a incoming air channel, and the broken lines are for surrounding air.  Think vanes on a membrane, like a heat radiator for a CPU.

Again, you're not forcing the air to move quickly; you're creating a convoluted pore, and letting the wind or air pressure differences push it along.

Indistrial tin foil:
I was thinking of the thicker, wider stuff you see in restaurants.  Ask a local caterer or taco shop where they get theirs, you might be able to talk them out of the end of a roll, or a sheet long enough to play with.

Ernie sez forget the window; the temperature's too cold outside. 
Put a vent going down into the ground, below the permafrost level, and then up again under your floor.  Like the U-bend in a sink.  Air can move in and out, but cold air tends to stay trapped at the bottom.  And you get the ground temperature (55 or so) instead of outside air temp. Used in igloo entrances.

My other idea is to open the window, and stuff a pillow in it.  Might have to rotate the pillow if it gets damp.  That could be the "out" for your floor "in".

I'm guessing the damp problems are only inside the house; the outside is pretty dry, yes? 
 
paul wheaton
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It doesn't get cold enough here for permafrost.

Here is a quickie drawing of what I thought of. 

If needed, I could put a tiny laptop fan on one of the inside holes. 

The yellow-ish stuff is supposed to be insulation.

I figure I could put this in the bottom of the window and then for the upper opening, stuff in some sort of foamy stuff.

So if I blow air into one of the exposed holes, I would like to hope that fresh-yet-warm air comes in through the other hole. 

Materials can be a challenge.  I would like to get something of about pop-can thickness.  I used to have a source for the metal that was used to make metal roofs - only it was still flat.  That would work pretty good.  But I don't have that source anymore. 

Surely there is something out there that is mighty cheap and easy to find that would work.  ??

Will this design work?

air-to-air-heat-exchanger.gif
[Thumbnail for air-to-air-heat-exchanger.gif]
 
paul wheaton
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Since aluminum has such very high heat conductivity, I'm thinking that aluminum is probably the smart stuff to use. 

And now I'm thinking I would make the outer walls out of something insulative and foamy.  So there will be, on thin stick of aluminum on the inside.  Maybe it could be made from an aluminum ruler or yardstick or something.  Maybe cut from an abandoned baking sheet?



 
paul wheaton
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To enhance this idea a wee bit ...

air-to-air-heat-exchanger_2.gif
[Thumbnail for air-to-air-heat-exchanger_2.gif]
 
paul wheaton
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slight variation including how to make it. 

Flatten the tube, insert the metal strip.  Then when the tube tries to resume its shape it doesn't become fully round again.

air-to-air-heat-exchanger_3.gif
[Thumbnail for air-to-air-heat-exchanger_3.gif]
air-to-air-heat-exchanger_4.gif
[Thumbnail for air-to-air-heat-exchanger_4.gif]
 
paul wheaton
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Next up:  a tiny fan ....  on ebay I found 5 volt (DC) fans for about five bucks.  They are designed for use as parts for a laptop.  I would think that this would give me just enough air movement to make my heat exchanger work awesome. 

Surely there is someplace that has broken laptop with a little fan in it that still works.

So ... I suppose the next thing I would need would be one of those little power adapter things.  It might be tricky finding one with the right voltage.  Again, surely there is the right one out there somewhere.  I just got rid of  a box of about 20 of these things at a yard sale just before I moved.  (damn!  who knew I would actually want one just a few months later!)

 
charles c. johnson
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run it off a patatoe
 
paul wheaton
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carbonout wrote:
run it off a patatoe


I don't know what that means.
 
paul wheaton
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my latest drawing ...

air-to-air-heat-exchanger_5.gif
[Thumbnail for air-to-air-heat-exchanger_5.gif]
 
charles c. johnson
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your fan, run it off a potato. like the clock that runs off potato or lemons
 
paul wheaton
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carbonout wrote:
your fan, run it off a potato. like the clock that runs off potato or lemons


Oh!  I get it!

Not a bad idea. 

I sent an email to my friend Bob, who replied and seems to have both the fan and the AC power adapter. 

I suppose the only real issue now is what if the fan take 5v and the adapter is 9v.  Will the fan blow up?

 
charles c. johnson
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ac power adapter = dc motor so run it off  batteries or 7 potatoes
 
Joel Hollingsworth
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The potato itself doesn't supply the power: the pieces of metal you place in the potato provide the power as they dissolve in the potato. And unless your soil needs a lot of zinc, the potato becomes useless for compost in the process. The embodied energy of that same zinc would be converted to electricity more efficiently by a factory-made carbon-zinc or alkaline cell, and would stay contained in a more-easily-recycled package.

I think convection would be a more sustainable way to drive air flow. Air coming into the room could be ducted down to floor level with an anti-chimney, and the exhaust could be piped up toward the eaves of the building with a chimney. After all, the heat exchange will never be perfect: you might as well harness the remaining energy somehow.

Also, the aluminum sheet will work better the thinner it is, and the more area it has. An accordion pleat of light-duty foil will pass many times as much heat as a ruler or section of cookie sheet would. Remember, even though it has much less thermal resistance than other materials, aluminum does resist the flow of heat a little bit. It's a very bad insulator, but like any insulator, less heat flows through the thicker it gets.

Here's a sketch of a cross-section that I think might work better. You might tape or glue the accordion pleat into a half-pipe bent from flashing or re-claimed disposable bakeware, and then attach the other half of the pipe afterwards.  You might wrap this with a foam pipe sleeve, or maybe even slide it into a pool noodle.
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I like the Idea about the heat exchanger.  Remember them old long Pisces of srapped metal that looks like a VVVVVVVVVVVVVVVVVV going around the Pipe? Its easy to make from any scrap and would fit tight around the Pipe! You could even put it inside too I believe. Just a thought.


I was thing about this my self I know more serfuss of your metal the better the transfer of heat!

A friend of mine Attach A lawnmore batter and use a 30.00 solar recharger pantle he bought off the Internet to run his fan and charge his battery. he using a 12v fan from a Powersuply from a Old PC works good. BTW on your 5v Fan yes it will work at 12 volts but may burn up fast sense your pushing more to it! Some Fans say 5v to12v the more power the faster the fan goes!
 
Ernie Wisner
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you might be able to run your fan off of bias metal strip with one side in the heat and the other in the cold. like one of those little stove top fans
 
Max Kennedy
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Paul, sounds like quite a temperature differential and since the air paths are separate you shouldn't need a fan. Instead of having your inlets and outlets at the same height for the outgoing air have the inlet up high, a tube going to the ceiling say. In going through the heat exchanger that air cools and falls out. The air moving out of the room will cause a slight pressure differential between the outside and inside drawing fresh air through the exchanger into the room. Just need to ensure the 2 paths are sealed from each other. The flow rates would be determined by the size of the flow channels and you could put valves on the interior inlet/outlet to restrict movement or even stop it if both are closed should weather, travel etc mean that you don't want it open for a time.
 
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