Unless the exhaust pipe is several feet long, and made of copper, I doubt if there would be enough heat transfer to even get the water warm. Maybe if I could find a copper alloy capable of handling the output pressure of the power washer, and silver solder several wraps of it to the exhaust pipe, I might get enough heat. It would probably be simpler to coil up several feet of 1/2" copper tubing and blow the output of a kerosene or diesel powered turbo heater over it once the water leaves the pressure washer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!