first a bunch on questions since I am not at all sure that what you plan is such a good idea.
How will you circulate the water
through this pipe? I assume you have a hot water storage
tank, since a "one shot2 heating
requires a completely different setup. Do you have a pump?
Second, and my greater worry: How will you stop it from heating
? Stopping the pump won`t suffice unless you drain the whole coil - which again is virtually impossible unless you have an exhaust going straight up. If you don't stop heating, at some point your system will partially go to steam, which means your safety
valve (you do have one, correct?) will open and you will dump your hot fresh water.
Remember - there is a reason why everybody else goes the other way around and uses the engine cooling
loop for heating: First, you've got the pump already, second, the temperature under normal conditions will never get to a critical level, and third, it is already freeze protected - so no worries in Winter. If you haven't progressed too far, that's where I would recommend you go...