The pressure won't destroy the boiler if the pressure relief valve works, but it adds fresh water to the boiler which will kill it from rust. So, you want to stop that. It sounds like you have a ceiling hung expansion tank, or multiples. In many cases, opening the drain valve of the tank will not drain it. It becomes vacuum locked because there is only one exit. In stubborn cases, a small hose is inserted through the valve or hole and up to the top of the tank to break the vacuum. The water in the tank must be cold and it makes a mess. The tank must be completely emptied. If it's a continuing problem, I would install an Extrol tank, sized properly. Many times a ceiling hung tank will never give trouble. You don't seem to have a fill valve problem or a pinhole in your domestic coil, if you have one. Since it responds to the heat/cool cycle, I would completely drain the expansion tank or replace with Extrol(s).

i think you will find that the code would say that a boiler has to have a auto water feed open to feed boiler when ever needed.. I guess this could change from state to state. paulbm

Thanks for info..when I look at my autofill I see two holes at the bottom..one is where the "bob" sits and one is, I'm guessing, where the water flows from the pool for the bob to sense..my actual overflow is separate. It sounds like then that the auto fill usually only acts to fill..any drainage is a result of the overflow. Correct?

Pictures? When I think of a zone valve I think of this so am not following your description. Here is a link that might be useful: Honeywell Zone Valve