My anti siphon valve is not by the heat exchanger, but I guess the concept is identical. I have a loop of copper (40mm, and rising well above the heeled water line) into which is sweated a spigot (5mm) Onto the end of this I have some nylex tubing attached to a $2 one way valve. A good alternative can be to replace the one way valve with a longer piece of tubing and feed this into the cockpit above a cockpit drain.
This avoids you having to periodically peep over the side to see if there is water in the exhaust, as the capilliary will continue to provide a fine stream of water down into your cockpit drain whenever the engine is operating (and the water pump is working), yet when the engine stops, the siphon is broken.
"if at first you don't succeed....Redefine success"!