[Richard and Rhonda]

Steve,

The rust inhibitor in coolant can also weaken with time. After working on another owners Aquahot and seeing first hand that the boiler had rusted out from within, I added a pint of coolant rust inhibitor to my system just for grins.

It's kind of a mystery to me why all the engine manufacturers recommend changing coolant anywhere from 3 to 10 years. As a ChemE I know ethylene glycol does not go bad or deteriorate with time, so has to be one of two things. Either the additive package which contains rust inhibitors and pH buffers is used up, or the system is contaminated with wear products from the coolant system. Perhaps both. Just wondering out loud.