You nailed the problem of being under the drain field.   It is not recommended to put the loop under a drain field for the reason you stated.  What size heat pump is going in your house?    Slim Jim heat exchangers   http://www.awebgeo.com/AWEB.asp?file=Contact.asp

work great for pond loops.

We haven't got to the sizing part yet, we're still in the initial stages of planning. The more I know going into negotiations and selecting the players the better off I'll be. What we are looking at now is about 6,000sf with sips and precast foundation, geo thermal with outdoor wood furnace back up which will act as primary heat for the carriage house and parking area.

I'm confused....  Is the geothermal heating the 6000 sf  or the carriage house and parking area?   By parking area are thinking of a snow melt radaint system under the parking?

The ground source is heating the 6,000sf but we plan on using a wood furnace as back up. Just so the wood furnace isn't doing anything for the years between breakdowns we thought we would use the wood furnace for snow melting (actually ice melting around here) and heat the carriage house.

The slim jim solves a problem I saw with fish hooks and plastic pipes.

Your system will probably be between 6 and 10 tons. I would think that your pond would support that, but a design professional needs to do the calculations. I think the Slim Jim company can do the design for you.

Somewhere in my wanderings around the 'net looking for info on ground loops systems I came across a discussion of using the drain field for a heat source. The short response was, "NO." The reason given was the drain field is a biologically active part of the entire waste disposal process, and that process needs a warm environment to work. If you suck the heat out of the drain field you stand a chance of killing the biological activity and causing the drain field to fail.

Do a search to see what you can find. I had no need to pursue the subject since I will be on a city sewer system and so have no links to give you.

Very god point. Between loss of drainfield in case of repairs and killing the bacteria I think I have my answer.

Looks like I'll be doing a pond closed loop unless I can't get the water supply. I looked at the Slim Jim, it seems a useful product an 8 ton pannel is between $5 - 5,500. They suggested at min. a pond 12 ft deep and a surface area at least as large as the heated portion of the house.

I take it you have a spring to supply the pond. You will need a reliable number for that before you proceed very far. Taking your pond dimensions at 120 x 30 x 10 that's 36000 cu ft of water. 1 Btu = 1F temp change per pound of water. If you can drop the pond by 10F you can pull out 22,464,000 Btus of heat. If you have a 6 ton heat pump running at a COP of 3 you'll suck out 48,000 Btu per hour of run time. If your heat pump runs 12 hr/day you'll get about 40 days of heating out of the pond.

If you take 48,000 Btu/hr for 12 hours per day and suck that out of the water flowing from the spring, and drop the water temp 10F, you will need over 100 GPM spring flow all day long. That is one huge spring, I think!

This should give you some idea that you are looking at some big numbers for your system! Of course, 6000 sf is a big house! Big house, big heat load, just that simple!

There are plenty of pond loops out there working on a lot less volume of water than the previous poster calculated. Some of them are static ponds as well. A pond loop is a great option for geothermal. Good luck with yours.

Before you get to far into it be sure to check into whether the soils in your area will support a 30 ft wide pond being 15 ft deep.

dmaceld,
You're forgetting that the pond is not an isolated environment but will gain heat from the ground and sun. Just at what rate I'm not sure. But I don't think no matter how hard I try, I cannot turn a pond in to an ice cube with 10 tons of heating although I wonder if I could not add to the thickness of ice for ice skating.

Geodean,
You're right I'm finding general numbers centering around 10 ft and 300-400sf surface area per ton so 6,000 should be fine, but bigger is better. I just want to see a real calculation instead of rules of thumb. The original house design is 4,000 sf but with changing the roofline, installing stairs to access the attic to reclaim a bunch of space in unusable attic and excavating under the attached garage we're squeezing out another 2,000+ sf. It still needs to be heated and cooled though.

Gregj,
I'm assuming that I don't have clay soil for now and expect to install a liner. More expensive but then the ground isn't mushy. Fortunately I only have to close one end of a draw to make the pond so I only have to worry about reinforcing about 30 ft of berm.

My point with those numbers was just to show that sometimes what looks like a minor issue at first, may in fact be much bigger. You're right, he needs to have someone who is familiar with this type of project do the design. I can see several factors to consider, the size of the pond being only one. The total heat draw over the heating season is the main one, and the drawdown rate during the heart of the heating season. Then there's the issue of air temperatures, colder temps will draw heat out of the surface of the pond more so than warmer temps. But warmer temps will increase evaporation. On the other hand, high humidity will supress it. Solar heating is a factor. If his area has a lot of sunshine during the heating season that will replenish the heat in the pond. Then there's ground water level. If he's in a high ground water area, and the pond is deep enough, he may have a steady supply of ground heated water flowing through the bottom reaches of the pond. And then of course, a small heat pump will draw heat out of a large pond a lot longer than a large pump will from a small pond.

If I was in a position to consider a pond as a source for heat, I'd jump on it in heartbeat!

The earth's heat supply is endless. The transport mechanism is what's limited. It's important to know what those limits are. And if those limits are greater than the need, bingo, you've got a solution!

Actually, that would work in your favor! The COP of your heat pump isn't as good at 32F as it is at 45F, but there's a lot of heat to be sucked out of the water when it freezes. At freeze point you can get more than 143 Btus/lb compared to 1 Btu/lb dropping from 33F to 32F.

Keep in mind too, that when a shallow pond is freezing the warmest the water can be is 39F at the bottom. At 39F water is the most dense so any water warmer or colder will rise above it. Obviously, the water at the top can't be warmer than 39F or it wouldn't be freezing. So if your proposed pond will freeze in your climate, your heat pump will be drawing on 39F, or colder, water during that time.