Check out a slim jim type plate heat exchanger for a "right" way to do this. Those quotes would drive me to DIY in a heartbeat as well.

If you are more adventuresome, consider using metal pipe in the pond. I don't know where you obtained the 600' per ton design rule of thumb, but I gotta figure that metal pipe immersed in water that is free to circulate would transfer heat much better than plastic buried in damp / wet soil. It is true plastic pipe has much lower conduction than metal but plastic is preferred underground owing to its durability, lower cost, and the fact that, underground, pipe wall conductivity is not the limiting factor for heat transfer - surrounding soil is.

Immersed in water free to circulate via natural convection, plastic pipe's wall conductivity likely DOES become the limiting factor, so metal, I'm thinking copper, would perform much better.

Heck, for not a lot of money you could design a heat exchanger empirically:

Getcha 50-100' of 7/8" OD refrigerant tubing. (start with 50') It'll come coiled up in a flat box. Remove from flat box. Spread the tubing into a slinky shape maybe 10' long. Sweat garden hose fittings to the copper slinky, run hoses to it, sink it in the pond.

Using a cheap fountain/ statuary / utility pump run 5 or so GPM through that apparatus. Source it with a couple hundred gallons of water warmed by some temporary means - maybe a kiddie pool painted black on the interior or run a hose into a pool from your house's water heater. Whatever. Run the pump and measure flow (time to fill 5 gallon pail) and heat transfer (Delta-T supply vs return water)

If heat transfer is OK but you want more, yank up the slinky and double the copper portion.

Once you get this all ironed out, tell (in writing on the contract you sign) the Waterfurnace installer that you take responsibility for the waterside flow and temperature conditions. All you want him / her to do is place the unit, connect the ducts and thermostats / zone controller if applicable. Since you are taking on all the risk and responsibility for the waterside, their price should drop to cover the unit, install labor, and reasonable markups. $9k sounds like plenty...

I like the WF Envision (what you mean by 'gold-plated'?) for its 2 speed and efficiency as well as lower water side heat exchanger head losses.

Disclaimer - the foregoing is free-and-worth-every-penny-paid advice via the internet for an adventuresome DIYer willing to risk electrocution, drowning, cranky family members, etc.

PS: No more fishing in the pond...

I would be very hesitant about using galvanized anything in a pond.  In fact, careful ph measurements should be done before using copper.  I would not worry as much about corrosion with stainless steel or plastic.

Alton

Alton
    Thank You for reply.  Years since I cked my ponds PH, don't remember the reading, but it can't be far off, the fish haven't complained. [;)]  Not worried about leaks, running stright H²O without antifreeze in loop. May have to add a little corosion inhibitor for the pumps benefit.

Climate Master recommends 300 of PE pipe per ton.


You don't have to reinvent the wheel here,  there are plenty of pond loop systems out there working with PE pipe.

I bet that 50 ' of copper will cost more than 300' of PE pipe.

Resident time in a big tank is kind of meaningless since most of the water in the tank will not be close to the edge of the tank.


Below is from the Climate Master Manual:


Pond/Lake Applications
Pond loops are one of the most cost effective applications
of geothermal systems. Typically 1 coil of 300 ft of PE pipe
per ton [26 meters per kW -- one 92 meter coil per 3.5 kW
of capacity] is sunk in a pond and headered back to the
structure. Minimum pond sizing is 1/2 acre [0.2 hectares]
and minimum 8 to 10 feet [2.4 to 3 meters] deep for an
average residential home. Actual area can be 1500-3000
sq. ft. per ton [39.6 to 79.2 sq. meters per kW] of cooling.
In the north, an ice cover is required during the heating
season to allow the pond to reach an average 39°F [3.9°C]
just below the ice cap. Winter aeration or excessive wave
action can lower the pond temperature preventing ice caps
from forming and freezing, adversely affecting operation
of the geothermal loop. Direct use of pond, lake, or river
water is discouraged because of the potential problems
of heat exchanger fouling and pump suction lift. Heat
exchanger may be constructed of either multiple 300 ft.
[92 meter] coils of pipe or slinky style loops as shown
in Figure 18. In northern applications the slinky or matt
style is recommended due to its superior performance in
heating. Due to pipe and antifreeze buoyancy, pond heat
exchangers will need weight added to the piping to prevent
fl oating. 300 foot [92 meter] coils require two 4” x 8” x 16”
[102 x 203 x 406 mm] blocks (19 lbs. [8.6 kg] each) or 8-10
bricks (4.5 lbs [2.1 kg] each) and every 20 ft [6 meters]
of 1-1/4” supply/return piping requires 1 three-hole block.
Pond Coils should be supported off of the bottom by the
concrete blocks. The supply/return trenching should begin
at the structure and work toward the pond. Near the pond
the trench should be halted and back fi lled most of the way.
A new trench should be started from the pond back toward
the partially backfi lled fi rst trench to prevent pond from
flooding back to the structure.