Buzzworth,
I've had a geothermal Water Furnace since 1994. It's excellent (supposed to be the most efficient of all heating and cooling methods) although I'm sure my heating and cooling bill would be lower if I didn't have a cathedral ceiling!
My technician says I have tops a couple of years until I need the unit replaced. So it appears about 16 to 18 years is the life of these units? Or perhaps it's just the compressor, I'm not sure.
At some point I will go to a closed loop system that is horizontally or vertically placed vs. my present open loop system if there is no effective way to ream out the water line. Take a look at the photo below and you will see why. This is what the inside of my water line looks like that discharges into the pond. (Open loop)
Here's the same discharge end with the pond drained down for maintenance.
I assume it's this way the entire length of the line due to iron and carbonate deposits. If someone can assure me I'm wrong I'd be glad to hear it. At some point I fear it will clog and effect my Geothermal unit. My water is extremely hard at 400 to 500 mg/l with about 2 1/2 mg/l or iron, and if one's water is not, it may not be an issue.
I think putting your water line in the bottom of the pond would be O.K. as long as you don't ever plan on seining the pond! That's why I have to forego that option as I seine my ponds annually -- although I could possible use one of the smaller ponds -- and not seine it. It could come to that due to cost issues.
I do wonder if the loop would be more efficient in the ground vs. in the bottom of a pond that gets down to 39 F. -- in northern temperate climates -- vs. a little warmer 6 or 7 feet below the ground. Not sure what the temps gets down to down there. Perhaps it's not much warmer?
Here's something some folks neglect on heating that can really make a difference: Passive heating of your house via windows facing south. I have lots of windows on my log home facing south and we keep our geothermal unit off from October through November. Actually we shut it off in September too to as their is not a great need for cooling. Sometimes it gets too warm up into the 80's in the house in October due to the windows and we have to close the blinds!
Here's some info I gleamed from my geothermal technician as I was interested in using geothermal water for my fish raising:
My open loop which is a 2 1/2 ton system drops my well water source discharge temp ~ 10 to 17 F. in the heating mode to about 40 F.
In the cooling mode it increases the well water temp ~ 16 F. to about 70 to 75 F.
The lower the flow the higher the temperature change.
If anyone considers an open loop because it discharges into their pond and thinks it will nullify evaporation, I have my doubts. My discharge is 20 gpm but from appearances I don't think it adds that much water to my .62 acre pond.