I bought a brick sized stainless steel 20 plate heat exchanger from ebay, commonly used for bio-fuel set ups. It was about a third the price of the copper coil ones, and I figured the flat plates would allow for bettter heat exchange, plus I hoped that the s/s might allow food grade water warming, which the copper ones advise against. It gets the water to over 40.C on the first pass, so has proved quite effective. I recently upgraded to a 17lpm pressure switched water pump, and even with the faster flow rate, it's still hot.

