I actually did this. I have had a OWB for 2 years and year number one i was sitting outside phila sweating my proverbial you know what off and thought, hmmm i wonder if it would work. So i took out my calculator and figured that using cast iron radiator off of craigs list for free. I needed for my 3900 square foot house I would need 2000 inches of 1/2" pipe for the water entering at 100 degrees F to be reduced to 58dg F ( I have a fresh water spring that moves 3 gallons per minute. it is about 18inches wide and 6 inches deep.
So I dug a little pit in the brook, then I went and got rubber in a can spray (as seen on tv). Then used swimming pool paint. (that didn't work) So then i went with roofing tar from home depot. I put it everywhere then put the spray rubber on top of that to contain anything from messing with the water quality of my spring. So far It works.
Since I installed my pump close to my Forced air exchanger. I use that set up but I installed inside the house several just in case ball valves in case i wanted to add some extra hot water radiator baseboard heat in my basement. So, it cuts off the water from the stove and connects to my 3/4" pex which runs to my stream for the stove. I just placed some black 3/4"inch next to it, hooked it up with the extra valve that I had installed originally and there is a shut off valve in front of the extra valves.
OKAY got that... now here is the tricky part. IT FREAKIN WORKED. however, a small problem arose. It does not get rid of the humidity. Living outside philadelphia/wilmington, DE area it gets sticky here. so, i used my calculator to figure out with my size house that you need to run a 150 qt per 24 hour dehumidifier all the time. Since to buy one of those was about 3500 bucks i said no thanks. however i used 2-72 qt dehumidifiers in the house. Well, i got it to work by august 30th of this past year and bam... cold sept no real heat wave... so it worked and my humidity was around 48 percent for the house which acutally keep the house cool without running the central Air on days more than 80 but never really had bad humidity after that. So, all i know is the first test i did was that I let the OWB run at 90-95 dg F for a week and my house was 75 degrees with 82 percent humidity... That's what spurred on my next set of tests that i reveled here. So, I hope the calculations help you guys. oh copper was better than cast iron you needed less that 2000 inches. But i was looking for a cheap test, so i went craigs list FREE way and used cast iron. So i would show a picture but thing is buried in my stream. it is 2ft tall and 9 loops stacked on top of one another x 2 radiators. So imagine 2 radiators against a wall that are connected by a small nipple then dropped "laying down" into a 3 foot hole in a stream. The water flows on average 3-4 GPM across the fins showing. Above the radiator it is 58 degrees (water temp) and 50 ft down stream the temp is 70. So it does work