DIY ventilation heat exchanger

[Piwoslaw]

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.

[Ryland]

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.

[Piwoslaw]

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:



And here is how it works:



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



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



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:



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.

[Heynow999]

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.

[Ryland]

Originally Posted by Heynow999

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.

[AC_Hacker]

Originally Posted by Piwoslaw

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.

Piwoslaw,

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

Make - Volume 18

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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

[Piwoslaw]

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...

[Daox]

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.

[AC_Hacker]

Originally Posted by Daox

Do you have any examples of the 180 degree flowing design?

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.

* * *

[Piwoslaw]

Originally Posted by AC_Hacker

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.

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).