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Drained the expansion tank last night and now it's got perfect pressure, between 12 and 16 just like it's supposed to. Thanks for the help.

John if I read you right I only think I'm draining the expansion tank when really it just stops putting out water because no air is coming in when opening the purge.

Then I'll close the valve on...

House hot water is supplied by a coil running through the boiler. I should've mentioned that too, eliminated that possibility by turning off the cold water feed into that coil over night few days...

My boiler is over-pressurizing consistently for the past week when heating. If I lower the pressure by letting some water out from the relief valve in back of the boiler, the water fills to about 12...