DIY well drilling / trench digging for GSHP

[AC_Hacker]

Originally Posted by Blauhung

...95 therms in January (2010) works out to be a bit over 12,000BTU/hr.
...70 therms in January (2011) works out to be a bit over 8,800BTU/hr.

This is really useful information for your project.

Originally Posted by Blauhung

...200' of pipe under 10' of depth (where ground temp doesn't vary as much). 5X40' holes will give me exactly that...

For my project, I did a similar calculation. I now think I was being overly optimistic.

Where loop fields are concerned, size really does matter... bigger and deeper is better.

For one, I under-estimated the effect that temperature changes close to the surface would have. I also learned there is also a temperature change time-lag (more depth = more time lag) factor that I now appreciate more than I formerly did.

Last winter, I ran a test, using my little heat pump which I had previously measured to output around 4500 BTU per hour. I hooked it up to a car radiator and had a box fan blowing through it. I set it up in my basement which is not insulated and I let the thing run initially without any thermostat, for 24 hours a day, 7 days a week, for about two months. I had never used the radiator and fan setup before, but I just wanted to see how everything worked out on a sustained basis. All I logged was outside temperature, basement temperature, and loop water temperature.

Since I only went down 17 feet, which is really shallow, my whole loop field is greatly subject to the transient temperature swings, but since the heat travels slowly, the effect was somewhat muted because the cold pulses (and warm pulses) took a while to work their way down.

But still, whenever there was a period of unusual chill, I could see it clearly in the loop water temperature change about three days later... same for an unusually warm period. If there was a sustained 10 to 15 degree downturn... three days later, I would see my loop temp drop by two or three tenths of a degree.

There was also a warming effect caused by significant rainfall... with a similar time lag as above.

This diagram from the CLGS Installation Guide (#21020) book I mentioned previously helped me greatly in understanding what was going on...

Long Term Loop Temperature Decline

I also saw a steady, sustained decline in loop temp over a two month period, which was due to heat removal from the ground. During this time, on a couple of occasions, I turned the heat pump off for a day or two, to make changes in my heat pump setup, and I could see that the loop temp had 'sprung back' somewhat, but subsequent running of the heat pump showed that pretty quickly, within a matter of hours, not days, the loop temp worked itself back down to near what it had been before the equipment shut-off... which indicated to me that the full heat reserves take a considerably longer time to completely restore than I would have guessed.

My conclusions from all of this...

Where loop fields are concerned, size does make a difference... bigger and deeper really is better

My initial idea of a shallow borehole loop field does work, but my optimism as to the total available heat is now somewhat tempered. In other words, I learned something from this experience.

I would expect that there would be similar behavior from a loop field that was deeper and larger (larger in terms of loop filed size to heat load). I would suspect that a deeper field would reflect changes more slowly, and to a lesser degree. I would think that a bigger loop field would show similar changes but again, to a lesser degree.

Assumption Error

My calculation regarding boreholes, that...

200 = 200
200 = 2 x 100
200 = 4 x 50
200 = 8 x 25
200 = 16 x 12.5

... is not correct, and that the shallower the boreholes are, the greater my error was. So a loop field composed of shallower boreholes will require more boreholes do deliver the required heat. I do think that a correction factor can be developed and that the above diagram holds the key to the formulation of this correction factor.

Having said all that, I am getting useful heat from my loop field, and possibly enough useful heat to meet my original design target... because serious insulation is also part of the project.

When I originally laid out my loop filed, I left enough space to add an additional 7 or 8 boreholes, which I can definitely use. Due to my arm injury, I may or may not be able to do it this summer, time will tell.

Hybrid System

When I started this project, I bought a small 3/4 Ton mini-split ASHP to see me through this project. The mini-split has worked out far better than I had anticipated, delivering reliable cheap heat, even on very cold (12 F) days, when it was obviously struggling to stay alive (low COP during that low temp, too). The climate in which I live has a pretty moderate heating degree day level of 4,000 to 4,500. Also, being as how Portland's weather is marginally maritime, heating requirements are fairly modest, compared to the central northern states & Alaska. In fact, there were a significant number of days this winter when the air temperature was higher than the loop field temperature. Those days are golden days for an ASHP.

So, my current thinking is to combine a too-small mini-split ASHP with a too-small GSHP heat pump and leverage the best qualities of each. On the days when the air temp is higher, the heat loss from the house is not so high, and the ASHP is working at it's highest COP, so the ASHP alone is capable of doing the job... On days when the air temp takes a huge dive, the ground loop temp is looking pretty darn good, especially if I haven't been drawing it down all winter, then the GSHP comes into it's own.

I still think that the best way to do the hybrid setup would feature a hydronic radiant floor. The GSHP is already a good fit, but my current ASHP is not. An ASHP hacked to be air-in and water-out would be ideal.

So it is with tremendous interest that I am watching developments on the ASHP Hacks thread.

Best Regards,

-AC_Hacker