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Unread 02-02-2010, 08:00 AM   #1
bobkoure
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Default What (else) to do with heat exchanger knowledge

I've been looking at heat recovery ventilators lately.
These are basically an air-to-air heat exchanger with inside-house air flowing out and outside air flowing in. Because some heat is transferred from the outgoing air to the incoming air, you're not throwing away all the energy you used in heating (or cooling) inside air.
But you still get some ventilation. Turns out, most modern homes need more ventilation than they've got. What with "cash for caulkers", it's going to end up being a lot more homes than that.

Why mention it at PC? Well, firstly, there's a good deal of knowledge about airflow here. Secondly, we're a bunch of hobbyists who are already thinking about heat exchange and building things out of sheet metal. (BTW, it looks like HRVs are pretty simple to build from sheet stock). The current HRVs out there don't seem to be all that efficient (hard to get numbers, which is often a sign that they're not spectacular) so there's room for a hobbyist to make a breakthrouh - sort of like PCs and cooling, say, ten years ago.
Not as sexy as having the fastest (or the quietest) PC on the planet, but possibly interesting to some folks here.
Did I mention that there's a market for these things? If you figure out how to build one that's cheaper or smaller or more efficient, there may be a business in it for you.

IMO the limit to a2a exchanger efficiency is the airflow boundary layer. Maybe a decent place to start tinkering...
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Unread 02-03-2010, 03:38 AM   #2
billbartuska
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Default Re: What (else) to do with heat exchanger knowledge

While you can calculate the efficiency and cost of running a A/A heat exchanger, the cost will always be higher than not adding outside air. As soon as you start adding outside air, there is a cost for doing it. You can lower that cost, but it will always be a cost.

What do you get for that cost? Well, it's health, and you can't asign a cost to that.
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Unread 02-03-2010, 07:24 AM   #3
bobkoure
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Default Re: What (else) to do with heat exchanger knowledge

Or you figure you need a certain minimum flow of fresh air (there are standards - which my house failed, BTW). Take that as a given (open window, ventilator without heat recovery, whatever). At that point, whatever heat / cooling you recover counts as free energy.
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Unread 02-04-2010, 03:43 AM   #4
billbartuska
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Default Re: What (else) to do with heat exchanger knowledge

Quote:
Originally Posted by bobkoure View Post
Or you figure you need a certain minimum flow of fresh air .... Take that as a given... At that point, whatever heat / cooling you recover counts as free energy.
My point exactly. Yes then there is money to be saved.

Another problem would be condensation. If you take to much heat out of the exiting air you could get condensation. Perhaps that's why what's on the market is inefficient.
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Unread 02-04-2010, 09:47 AM   #5
bobkoure
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Default Re: What (else) to do with heat exchanger knowledge

There's definitely condensation. And, yeah, state change represents quite a bit of energy. But which way is that energy flowing? Water requires energy to vaporize - and gives it up on condensation. So, the warm stream is giving up even more energy on the way past. I'm pretty sure that'd help efficiency...
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