[Trainer]

Boy, just to share how challenging it is to diagnose long distance, I drove down from Baytown to Sweeny to do an annual service on a heater this morning. First thing I did was remove the radiator cap and check the coolant level (now why didn't I ask Bill to do that?). It was down 1/2 gallon but the overflow tank was full. So either the cap or hose was leaking.

Sure enough after finishing the service, when I fired it up and the tank started to heat up, the tube started to leak. Cut off 1.5 inches, reconnected and fired back up and all was well.

So I second the recommendation to observe the coolant level in the overflow tank rise as the heater warms up and fall as it cools down to ensure the tank stays full all the time.