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Old 04-13-09, 01:08 PM   #18
AC_Hacker
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Default AC_Hacker does a heat transfer test...

Most of the sources I respect advise doing a heat transfer test to determine the rate at which heat will be transferred into the earth (for cooling) or out of the earth (for heating). This transfer rate will be the same in each direction, and will determine how much loop-field (trench or borehole) will be required to heat or cool your house. This is important because the loopfield is the most expensive part of the GSHP installation process if you are hiring it out, and is a lot of work if you are doing it yourself. So it's a good idea to get it right. Too little loop-field won't be able to supply the heating or cooling required. Too much, while not such a bad idea, means greater expense and greater work.

So I went on a google-frenzy to try to locate testing and evaluating proceedures, and after many, many hours, finally turned this up:

http://www.geokiss.com/tech-notes/TCTestingSum.pdf

So, I consider this test to be my first attempt at testing and likely to be a candidate for improvement, but this is what I did...

I had used a hand auger, meant for fence posts, and had dug to a depth of twelve feet in about 3 or 4 hours. I was going to fill the hole back in, but I decided to try to get some use out of the hole along the way. I buried a double loop of CPVC pipe I had laying about in the garage and attached some garden hose to the input an output of the loop. (see photo)

Then I filled the hole back up with the dirt that I had laboriously augered out and used water on the dirt as I went, so it would settle well.

I hadn't actually found the testing method document that I'm linking to above at the time I started testing, so I was just sort of making it up as I went along. I reasoned that if I introduced a known amount of heat-energy into the CPVC-loop, the ground would absorb the heat-energy at a rate that I suspected would decline. I further reasoned that if I monitored the temperature of the water, it would tell me something about the rate of absorption of heat by the ground. If the ground absorbed the heat at a high rate, the water temperature would be low, if it absorbed the heat energy at a lower rate, the water temperature would be higher.

So here's a sketch of my setup and a few photos:




this shows the hole I'm doing the tests in. I used CPCV pipe (not recommended) with a double-loop (not much advantage over a single loop)...



I thought it would help to eliminate error if I used good pipe insulation on the hose. Bottom photo shows digital thermometer and kill-a-watt. A good analog thermometer would work just fine, but the digital is much easier to use. The kill-a-watt meter has a 'watt' function. It was set up to measure both the pump & the heater together, since they both give off heat. I found that the watts vary over time, so they need to be recorded at every chosen interval. It also has an elapsed-time function (called 'clock'), very handy.


this shows the set-up in the cooler box. I initially thought that it would be a good idea to put the coffee cup warmer inside of something so it wouldn't melt the foam box. I nixed this idea, and suspended the heater from the hose with a bread wrapper twist-tie.

If you read the proceedure I am using for my guide, you'll notice that the proceedure call for measuring the input and the output temperatures from the ground loop that is under test and arithmetically averaging them. I made the simplifying assumption that the water swirling around in the cooler box would physically average the temperature.

(NEXT POST: thermal transfer test data & analysis)

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Last edited by AC_Hacker; 05-27-09 at 02:07 PM.. Reason: in-line images...
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