I only heated my pool from labor day last year till begining of Nov.
This is the HX I have, the SS650:
http://www.heatexchangersonline.com/poolheaters2.htm My pool is estimated @ 40k gal+ Once I fiddled with the flow going through the HX it worked really well. I had no controls on it because I knew in a short time I would be winterizing my pool. I just plugged my circ pump when I wanted heat, un plugged it when up to temp. I could fill my 6048 with wood, go to work, and come home to a boiler that needed wood but not empty. If I had to guess, I would say 30 to 40% of my pool pump discharge went through my HX then mixed back with the rest. I had approximately 100 degree water dumping into my pool. The thermostatic valve seems to be a "must". If I did not have this, my boiler would drop like a rock on temp. It works just like a car thermostat in the cooling system. If the water coming from the boiler is less than 150 it will return to the boiler and not flow to the HX. As the water temp rises, it allows it to flow. I know once my boiler dropped to 150 it would go no lower but never really recovered a whole lot either. But, it must have been heating because my water out of the HX was always hotter than water going in and my pool was always warm when I got home. Had it up to 85 degrees in Sept just to see how it would work.
This year, I am going to control the boiler water into the HX with an aquastat and 3way valve. I am also going to put a 24 hour clock timer on my control wire to the transformer for my valve so I can let the boiler "catch up" . I'm thinking maybe 3 hours on, 30 to 60 minutes off. When I get all hooked up this spring, I'll take some pics of the set up.