I would use a real heat exchanger. I've installed a couple stainless steel heat exchangers in the last couple months. The ones I've used would allow for a 1" connection to be plumbed with your steam or hot-water source and 1.5" connections for your pool piping. My cost on these units was only $425 and with 180 degree supply-water they provide 70,000 BTU's. I'll go have a look at the brand names for you tomorrow. Regular PVC pipe is rated up to 150 degrees; if you think the water in your pipes will be warmer than that, use CPVC, which is rated to about 210. If you have more questions, let me know.

Brad
www.waterworkspools.com