engine coolant change
there should be drains on the engine block and the heat exchanger (petcock type value on exchange and a screw neddle type value on block) They are usual easy to find as they should have a lenght of clear tubing hanging down to see. there are also raw water drains so be sure you get the right ones.It helps to leave the coolant cap off when you drain the system,if I remember correctly it holds about 1 gallon. (Plus a little more depending on the hot water heater set up)