"Pump and dump" systems are a lot less to install. The issues are water quantity and water quality. There would be a problem if you used pump and dump and had times where there was insufficient water quantity in gpm. The qualtity of the water will determine maintenance cost - pumps and stuff get dirty/coated with rust, etc. Can lead to lots of filter changing, breakdowns, etc.
Closed loops are quite a bit more to put in, but eliminate quality and quantity issues. Where we are in NE Ohio all we install at my company are closed loops. But we potentially have quantity and quality issues.
Pond loops will operate more efficiently if they don't get buried in the mud. Heat transfer is maximized by contact between the surface area of the pipe (polyethylene) and the water. The water being a fluid will convect (cooler water is denser and falls, etc.) increasing heat transfer as compared to contact between the pipe and the mud.
When we install pond loops we put spacers between the pipe coils so they don't touch and allow water to convect more efficiently. 1/2" spacers holding 3/4" coils of pipe apart. And we use concrete blocks to weigh them down and to hold them up off the bottom. Great structure!
The pond systems seem to perform very similar to the ground loop systems we bury horizontally at 5' depth, and the vertical systems (150' deep). The "efficiency" of the heat transfer has a lot to do with the amount of pipe vs. the size of the machine. If the installing contractor puts too little pipe in the ground for the size of the machine (in tons of capacity) it will hurt efficiency.
We have seen pond closed loop systems not work properly due to no spacers. The contractor just took 500' loops of 3/4 polyethelyne pipe, headerred them together and put them in the pond. No convection. Very poor performance. Contact with the mud would be a lot better than having the pipes contacting themselves.
As to the pond temps yes it makes a difference but not as much as you think. Ground loops acutually get warmer all summer as the ground is used as a dumping ground for heat - peaking at about 70 or 80 degrees in Sept. here in NE Ohio. Conversely they ground loops build up ice - as much as 6" - as heat is extracted and peak at coldest in February or March. The ice thaws as the machine "hits the loop" less - the heat migrating from the ground to warm the loop is a slow gradual process. The ice is probably still there in May when cooling season begins (very efficient at that point). So the loop changes "incoming" temperatures over the year, yet the system is still giving you 4 times the energy consumed - a very efficient way to heat and cool. Point being the efficiency does not change that much when the source changes temperature over the year.
A bigger concern would be the size of the pond. If you add or take away heat from a smaller pond the temperature change would be significant and a big problem for both the pond and the performance of the system. We usually start with requiring 1/2 acre pond with at least 12' of depth. There are variables of course but this just to give you an idea.
WaterFurnace has a new unit now that has a COP (efficiency rating) of 5. This translates into 5 units of energy for the home for each unit (kW) consumed. Awesome.
