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 psi just like it's supposed to but once it starts heating up it gets up to 30 at which point it starts dripping out through the pressure relief valve again.
Since it's happening while heating and the PRV seems to work properly (turned off the valve right after the prv to isolate) I looked at the expansion tank next.
The expansion tank is the old style - just a steel tank mounted horizontally above the boiler. The inlet to the expansion tank comes from the bottom of the air scoop. It's getting very little water in it, much less than a gallon and not hot water when I've checked it recently. Don't know how to proceed here. Water will come in to the expansion tank but apparently not enough.
where does your domestic hot water come from?
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 ago. Boiler still overpressurized.
I notice that the pressure goes up when the hot water's being pumped and heated, but the expansion tank have very little water in it, only a few quarts, and cold water at that.
The tank must be totally drained, there is more then a gallon of water in it. The tank will not drain with out letting air into it. It's like holding your finger over a straw.
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 the riser of the baseboard hot water to the house and open the drain just above it. The lateral after that goes into the air scoop and the exp. tank inlet comes from the bottom of the air scoop. So that should allow air to come into the tank from the inlet to replace the water coming out.
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.
Come spring, consider replacing the expansion tank with a bladder type (i.e. Extrol) tank. These are a lot less hassle in the long run.