DIY ventilation heat exchanger
 

I am thinking more and more about our upcoming renovation, a part of which will hopefully be redoing our ventilation system. One of the things that would greatly increase the efficiency of the new setup is an air-air heat exchanger (recuperator). The ones I've seen here cost the equivalent of US$2000-2500, which seems a lot. Does anyone know how easy it would be to build one at home?

I found a local forum (in Polish) where someone was planning to make one. The plan was for a 120cm long countercurrent heat exchanger, with 200mm or 250mm diameter input/output and 25m^2 of exchange area. The calculations for the number of sheets of thin steel (later copper or aluminum) needed came to a cost of 60%-70% of a factory-made unit, plus random parts, machinery, at least a whole week of time, etc. The whole thread was 60 pages long, with 50 posts/page, and a change in design every 5 posts, so I gave up after the second page and have no idea if the project ever got off the ground. No pictures in that thread (or only for registered users).

Any ideas on how to go about making a more or less efficient (60+%) heat exchanger? Pictures/drawings of the insides of different types would be nice, as would a cost vs. efficiency comparison.

It would draw a bit more energy but you could build one with an air conditioner that in the winter cools the warm outgoing air before it leaves the house and warms the incoming air, in cool weather this could be enough to keep the house warm and in warm weather it might be enough to keep the house cool.
otherwise the inside of heat exchangers that I've installed have what looks like coroplast... might even be coroplast but it should be something more conductive.

Thanks Ryland:) A small air-air heat pump (like an old A/C unit) could be hooked up after the heat exchanger, increasing the amount of heat transfered, or reducing the temp difference on both sides of the AC, depending on how you look at it. In fact, there will already be a heat pump in the system we are planning, it will transfer heat from waste air to the hot water tank. That will be the first step, the second - a heat exchanger for intake/waste air, and finally a ground source heat exchanger to preheat (precool in the summer) before the recuperator.

I looked around and found a few pics. Here is the inside of a cross-flow recuperator:

http://ecorenovator.org/forum/attach...5&d=1266778299

And here is how it works:

http://ecorenovator.org/forum/attach...7&d=1266778299

Here is a homemade plywood construction that uses 2 cross-flow recuperators in a counter-current set-up (increasing efficiency):

http://ecorenovator.org/forum/attach...8&d=1266778299

Here's the inside of a large area, counter-current heat exchanger:

http://ecorenovator.org/forum/attach...9&d=1266778299

It is made of scores of thin sheets of copper or aluminum, stacked 4-5mm apart. The space in between sheets is closed off at every other end (let's call them even) at one end and odd ends at the other. That way the two airstreams don't mix, but exchange heat over an area equal to the area of all of the sheets (minus 2). This is very efficient, but large, heavy and expensive. On the other hand, if space isn't a problem, and the cost is not more than a commercial heat exchanger, then the higher efficiency may make this option worth while.

Here is a pictures of someone's finished project:

http://ecorenovator.org/forum/attach...6&d=1266778299

You can see the smaller diameter bypass for when you do not want to lose/gain heat.

An idea I got while looking through these projects was to use a long length of thin, flexible tubing inside a larger diameter, insulated tube (like Ben's DIY Solar Hot Water Heat Exchanger). This could be a temporary solution, until a better heat exchanger could be made/installed, after which the tubes could be reused in the house's ventilation system or as a ground source heat exchanger.

I have always thought it would be a good idea to vent your bathroom exhaust and dryer through on of these. Perhaps you could vent the bathroom full time, and the the dryer only during the heating season.

Panasonic makes one that is part of a bath vent fan, but if it gets to cold of air coming in to it from the out doors it bypasses the heat exchanger to prevent freeze up, around 20F or 25F if I remember correctly.
Dryers need to have the lint removed first or they will clog it.

ERV, HRV, de-humidifier hack...
 

Piwoslaw,

Here's a URL for a DIY Heat Recovery Ventilator:

Make - Volume 18

* * *

The photos I see of Heat Recovery Ventilators usually use a cross-flow cell, with the air paths crossing at a 90 degree angle. They usually have an efficiency of about 60% to 70%. I have seen write-ups of experiments done on cells that had a 180 degree cross-flow and the efficiency was increased to some degree (maybe 5% to 8%). Sorry, but I didn't book mark the studies.

There's also a wheel type that claims efficiency up to 90%. Here's an example:

Heat Exchangers Energy Recovery Ventilator Air

Additionally, there's a heat pipe HRV, but I don't see very many Internet references to that type, but here is one:

Computer Environment, Inc. Website

Here's a page with a pretty good overview of the topic:

SustainableSources.com: Energy Recovery Ventilator

It looks like the need for and success of a HRV/ERV are based on having a very tight house...

* * *

Regarding the 90 degree cross-flow types, if my understanding is correct, there are Heat Recovery Ventilators (HRV) and then there are Energy Recovery Ventilators (ERV), the difference being that the ERV uses a water permeable membrane to allow outgoing air condensed moisture to be used to re-humidify the incoming air.

I have seen a page that represented various Chinese companies, that listed just the cross-flow cells of various sizes. I tried to locate that URL for this post but was not able to find the page I remembered. But I'm pretty sure that the cells for most, if not all of the ERVs made in the US and probably also Europe are manufactured in China, and put into locally made boxes. I have seen the completed units coming from China:

Here

Here also

* * *


In this photo is the de-humidifier someone left on my front porch, minus the plastic & steel covers.

The way it works is air is drawn through the evaporator coils, where it is chilled and frost forms and then passes through the condenser coils, where the air is warmed back up. There is a sensor on the evaporator coil and when it senses that the evaporator is likely to be full of frost, the compressor stops for a few minutes, the frost melts to water that goes into a bucket.

But the evaporator core and the condenser core are made separately, and are attached by sheet metal screws. With a little bit of luck and some very gentle handling, the cores can be separated, with the refrigeration lines intact. Then incoming air could be made to pass through an air filter and then through the condenser (hot) and into the house, while outgoing air would be made to pass through an air filter and then through the evaporator (cold) and out to the world.

Thus, the hacked de-humidifier would extract heat from the air that is exiting the house, and then return that heat to the air that is entering the house.

This is a hack that is begging to be done!

Regards,

-AC_Hacker

Thanks for all the info, AC:)
I'm on the lookout for an old A/C (window) unit already, and thanks to you I realized that a dehumidifier would also work (I actually know how a dehumidifier works, but somehow the think processes didn't get there yet), so I'll look for one of those too.

Now I'll sift through all those links...

Those are some GREAT links AC Hacker! Thanks. I really like that Make article. Although, I do think it would be more efficient if it had flow going like the commercial units that use 90 degree cross flow. I'd imagine that it just makes better use of the sheet metal between the intake/exhaust. Do you have any examples of the 180 degree flowing design? With only a small amount of modification I think you'd really have a nice heat exchanger, and it could be made CHEAP too.

All Roads Lead To Rome...
 

Daox,

I tried to find the links when I did the post, but couldn't find the web site again.

Sometimes I find material that is very interesting but not relevant to an original search. I try to bookmark as much of these surprises as I can, but some just get away. I think that's how it is with the Internet.

As I recall, the increase in efficiency for the 180 degree counter flow was small (5% to 8%). The 90 degree design seems to dominate the market, my guess is that it is pretty efficient and easy to make.

For me, the lesson to be learned is that China is making the cross-flow cells and selling them to OEMs. If we can discover the source of the cells, we can build the box ourselves.

But effectively utilizing an energy recovering ventilating system is predicated on having a home that is very tightly sealed...

This gets us back to insulation and sealing... Talk about 'all roads lead to Rome', here we are again.

Regards,

-AC_Hacker

P.S.: BTW, don't know if you saw my post on Grey Water Energy Recovery, that one is not dependent on having a tight house.

From what I read somewhere, the cross flow's efficiency is around 60%-70%, while the counter flow can get over 90%, so that may be worth fighting for. I think the differences in our sources come from different designs being tested.

I'm all for finding a source of cheap cross flow cells. Using 2 or 3 to make a counter flow set up (like in the picture in my second post) would give killer efficiency. I tried googling in both English and Polish for just the guts, but no luck, only "recuperators with alu cell". I'll ask at a local builder's forum.

Re heat pump: In my climate it's very hard to find an old A/C unit, air conditioning is just starting to be mainstream. Old refrigerators, now that's a different story. I can find one in any town, or even in any roadside ditch or forest:/ On the other hand, taking a fridge apart is illegal in Europe (environmental protection laws), so hush-hush;)

EDIT: I found this page on different types of heat exchangers. It's in Polish, but maybe Google Translate will help. It confirms that the cross flow exchanger's efficiency is 60%-70%, and adds that using 2 of them can be 95% efficient. Also, there is a spiral heat exchanger, which appears to be just aluminum foil rolled up into a spiral tube. The benefits are 85% efficiency and frost resistance (frost is the main enemy of cross- and counter-flow cells, requiring heating elements to keep the airflow from getting restricted).

I was reading on the Canadian Passive House Institute website and found this:

Sounds like the US/CAD models are just too small! 92% is amazing. The ones I was looking at the other day you were lucky to see much more than 60%! Sounds like a bigger DIY option would be far superior.

Those things do look large. But, it seems logical that a smaller unit would work
for smaller houses, if you used a slower air-flow.
Since slower air will get more of a chance to give up it's heat before leaving the unit.

I believe it was in that MAKE article which AC_Hacker linked (I can't get it to open this morning) it was said that the lowest speed setting for the fans was not the most efficient, that it seemed to exchange more heat with a higher speed. Maybe with more air coming through there is a larger DeltaT, increasing the amount of heat absorbed?

I've noticed that the main two issues when constructing recuperators are

1. Cross-section, which limits the amount of air that can be pushed through the unit,
2. Surface area, which limits the amount of heat that can be transfered from one airflow to the other.

If the unit is very small, then either the surface area is puny, or it can't handle a large volume of air (pumping losses), or both. The big DIY counter-current unit in one of the pictures I posted previously had its largest dimension probably around 150cm, the other two below 100cm, and its exchange surface area is about 25m2.

Yeah the fan speed thing is odd. I'd tend to think that slower would be better as Xringer said, but their findings show otherwise. In any case, it definitely looks like larger is the way to go.

Laminar Flow & Turbulent Flow
 

In Fluid Mechanics, there is the issue of laminar vs. turbulent flow.


This graphic illustrates a couple of aspects of the difference:

In laminar flow mode, the friction of the surface of the pipe or in our case, the surface of the heat exchanger, causes the flow rate to become slower at the boundary between pipe and fluid. The maximum flow rate is at the center of the pipe and it decreases toward the pipe surface. This means that if there is a temperature difference between the fluid and the pipe, the slower moving layers near the pipe edge change temperature more readily than the layers near the center.

If the flow rate is increased, there comes a point where the fluid no longer flows in regular stratas, but changes flow mode and the flow becomes chaotic, the fluid tumbles as it flows.

In turbulent flow mode, the layers are continuously and chaotically changing, The graphic suggests that some of the fluid flows backward, which it does not, but some flows forward more slowly.

Since heat transfer happens as a function of temperature difference, the turbulent flow mode is more efficient than laminar flow, because the 'particles' of fluid get exposed to particles of pipe and are more subject to temperature change.

Also, the idea that slowing down the the fluid flow rate will result in increased heat transfer doesn't hold up either. While it is true that if the fluid flows more slowly, it's temperature will be higher when it exits, there is less total heat exiting with a lower speed.

To appeal to intuition, if you had been working in the hot sun and had gotten dangerously overheated and needed to cool down, would you rather sit by a window through which the air was barely moving, or would you rather sit in front of a window where there was a fan turned to high speed?


That's the difference that increased flow rate, and turbulent flow makes.

Regards,

-AC_Hacker

Thanks for the explanation AC Hacker. I am familiar with laminar and turbulent flow. I just didn't expect that a higher speed created that much more turbulence. Very interesting!

I think this forum needs a Thanks:thumbup: button!

I recently read that an airflow speed of at least 5m/s in ventilation ducts creates unacceptable noise. This may be the speed at which laminar turns to turbulent flow.

Turbulent Flow Noise
 

That's it exactly.

-AC_Hacker

I'd give you a thanks, but... We'll get on it. Its a nice feature. :)


Thanks for that tidbit of info too!

I found a (Swedish?) company that is a global supplier of heat exchangers:
Heatex AB

No idea of prices, but 'm sure that if we group-ordered a few hundred, then we could probably negotiate a deal :thumbup:

Yeah, it would be interesting to see what the European makers have done besides increase the size to get the extra efficiency.

Free ERV Design Software...
 

Piwoslaw,

Don't know if you saw it, but the company has some interesting ERV design software (more properly, sizing software). So if one were going to try to fabricate their own Energy Recovery Ventilator, You could get a good idea where to begin.

The link is here:

Heatex AB - The No.1 Air-to-Air Heat Exchanger Company: Thanks!

... If this doesn't work for you,maybe go to the root URL and look for "software".

Regards,

-AC_Hacker

Thanx AC:)
Downloaded the program, now I'll have to find some time to play with it.

A very home-made exchanger
 

These are fascinating posts. Somebody asked about plans for 180 degree counter-flow exchangers so I thought I'd post.

I'm making a counter-flow air to air exchanger out of plywood and aluminum foil as an experiment. The design is very simple: imagine a piece of plywood as wide as the foil and as long as you want. Along the long edges are 1x2's to support the foil. Then the foil is laid on them. On top of the foil at its edges are another pair of 1x2's, with another sheet of plywood on top. Imagine folding the whole thing back on itself every couple of feet giving you two channels for the air, separated by foil (obviously you can't fold the plywood, that's just to give the concept). Add two small fans to move the air, some duct work at each end, and there you have it!

Those of you with far more experience than I have can improve on materials and dimensions, but if the foil is 18 inches wide and maybe 30 feet long, it should do a pretty good job. Humidity is not much of a factor where I live, but if condensation becomes a problem I'll clearly have to modify things a bit. This version is just for my own fun.

Sounds like an interesting experiment. Be sure to post some pics.. :)

And welcome to the group..

Cheers,
Rich

Hello 1blueox.
What kind of foil are you planning on using? I'm afraid that typical kitchen-grade aluminum foil is to flimsy and may not be able to handle the pressure differences between the channels it's separating. Aluminum that's slightly thicker than the kitchen stuff should be OK, you may also look around for thin copper foil, which is also a good heat conductor.

As for condensation, you'll get it anywhere that there is a temperature difference and at least some moisture in the air. The incoming air may have more moisture in the morning or during rain, the outgoing air will be carrying moisture from the house (showers, cooking, breathing). You'll have to make a small drain for both channels.

Welcome to the site 1blueox. Sounds like a very interesting project. I too would love to see pictures as it progresses. :)

1bluebox, how is the project progressing?
 

1bluebox,

This sounds like a worthy project.

From my Internet research, it learned that there are two types of air energy recovery schemes. One uses non-porous membrane material, like aluminum foil, and the other uses a porous material that will allow condensing water to re-humidify incoming air. This might suggest other materials for you to try.

Also, it would seem to me that you'd want to go much thinner than 1x2 for your membrane separator. I'd think that membrane spacing on the order of 1/8 inch would be more in the ball park.

I also came across a DIY ERV discussion wherein someone was considering using a material like plastic food wrap for the membrane material. Seems like such a material would be cheap, easy to work with, quiet, and unaffected by condensation.

As my home insulation and infiltration project progresses, I'm beginning to notice that the controlled admission of new air is now required keep the comfort level at an acceptable level.

Regards,

-AC_Hacker

I saw some HRV units on Ebay in the $450 range. Looked up the maker and found this
site: HRV Airiva Heat Recovery Ventilator with a video.. http://www.suncourt.com/Flash/Suncourt%20Video.swf

Here's an installed review.. Amazon.com: Suncourt AIRIVA HE150 Heat Recovery Ventilation System: Everything Else

I've been thinking about air quality a lot lately. My wife has been experiencing
a hacking cough for years and it seems to be getting worse.
Doctors don't have a clue, so I'm wondering if it's airborne irritants.?.
Besides, I kinda like the idea of getting some fresh air during the winter,
when everything is shut up tight.

Yeah, that looks about as efficient as the other ones I've seen. I'd like to see something ~90% efficient! :)

I'd try to open up the house when you can, try to get fresh air in. If that seems to help her you may be onto something.

We may have reached 'lift-off'...
 

Regarding whether to use HRV (sensible heat recovery only) or ERV (sensible + latent heat recovery) I came across the above graphic that spells it out...

Interesting that it uses a Polypropylene core...


They show the core in the video, it looks to be the same kind of material that political yard signs are made of, cut into 10 to 12 inch squares, and glued together with the air channels in an 'alternating' orientation.


Since building the core seems to be the biggest snag in making a DIY HRV, we may have reached 'lift-off'.

DaoX,
A 90% or greater HRV is very difficult to achieve, the only one I have seen is an 'enthalpy wheel' design.

http://www.drirotors.com/pdf/article..._ac_wheels.pdf

http://platinum.ts.odu.edu/Apps/FAAU...enFileResource

This ERV?s For You - Feature Articles - Extra Edition - Air Conditioning, Heating & Refrigeration NEWS

Also, I came across a Ventilating Industry Test Proceedure used for certifying ERVs. May supply some interesting ideas for makers:

http://web.archive.org/web/200708201...on+30sep05.pdf

...and here's a Certified Product Directory, with efficiency ratings of various tested HRVs:

http://www.hvi.org/assets/pdfs/CPD/H...01June2010.pdf


-AC_Hacker

Coroplas sheets? Coroplast Is there enough heat transfer??
It's plastic.. Or is it some special heat-conductive coroplas?

I guess that more research and/or some experimenting would be in order here.

-AC_Hacker

I guess it must have some thermal conductivity, since people have made solar collectors and PV coolers out of it.. Since it's hard to glue water connections on the ends, it's more trouble than it's worth.. But for air? That sure looks easy.. :)

I'll try some DIYer testing. :)

It's not a great conductor, but it works. I held some Blue coroplas in the sun for about 45 seconds.
It got up to 126 F on top, and the bottom side zoomed right up to 113 F.
It's about 86 is the shade outside and my cedar deck is about 140!

It cools off very quickly in the shade, so it's got some good heat transfer characteristics..

My test strip was about 2"x15" with the holes running the long 15" path.
I'll bet it would conduct heat (top to bottom) better if I sealed the end holes
off with tape. Trapping the hot air (140?) inside.

I put those coroplast core pics up in the heat exchanger thread on the local builders' forum and they said that it can be done, but in order to extract a useful amount of heat it would have to be very big. Instead of glueing sheets of perpendicular coroplast it would be better to have them go in one direction with spacers in between, then there is only one layer of plastic for the heat to go through, and it could be made into a countercurrent set-up (one direction inside the coroplast sheets, the other between them). But it would still have to be big.

IMO the ribs in the metal core help transfer heat by increasing area, but this probably doesn't work for plastic. It will however increase the strength, so there won't be a problem if the two air streams are at different pressures.

If only they made "corocopper" at a similar price...

EDIT: Someone proposed stacking a few sheet of coroplast parallel-wise and somehow plumbing it so that intake air and exhaust air go in different directions in every other tunnel. Then the ribs also transfer heat and get a much larger (at least 2x) area. Getting it hooked up would be terribly tedious, but it may be worth a try.

I've got some scrap coroplast, but it's 5mm thick. To maximize surface area I'd need the 3mm, or even 2mm, variety. That's going to be hard to work with. Then find some heat sink glue that will stick to plastic?

I've used Coloplas sheets to make RC model airplanes and I've read there is a coating
on the surface that makes if very-very difficult to glue to.. (Or even paint).

SPA3D pictures by Xringer - Photobucket

I've never even tried to glue anything to it. It's slick stuff..
There is a process called 'flashing', where you lightly pass a propane torch over
the surface to burn away the coating, without (maybe) burning the plastic..
IIRC, that was a bit difficult for me. :o
RC Airplane/Coroplast - Wikibooks, collection of open-content textbooks

When I have painted Coroplas, I used paint made just for plastic, PVC etc.
Krylon Fusion, supposedly bonds to the molecules of plastic surface being painted.
But it didn't stick to Coroplas very well. I could easily remove it using a fingernail.. :(

Easy stuff to cut, but, not very easy to glue.. For making an exchanger 'X' stack,
I would try a very small shot of spray glue between sheet layers. Just a mist of Duro All-Purpose Spray Adhesive.

I don't know about polystyrene and polycarbonate, but I remember someone telling me that polypropylene doesn't react with anything, so it's almost impossible to paint or glue.

Here's some info from EcoModder:
Coroplast Kammback: joining or bending?

Now that I think about it, the sheets do not need to be firmly glued. They can be bound together, maybe also pressed, so only a layer of thermal paste is needed.

corocopper
 

On the company website. it does say that the physical properties of the material can be modified prior to the extrusion process, implying that the company is interested in working with customers in that manner,

I sent an email to the company inquiring about thermal enhancing additives, but haven't heard back yet.

I will post if & when I hear something.

-AC_Hacker

I wonder how well a very small coro-core would work, as a test bed?
Maybe 6" x 6" ??

I think this would be the perfect place to start...

-AC_Hacker