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There are air bleeds near the top of each radiator that need to cracked open slightly to let the air out, close the bleed valve as soon as tiny squirt of water comes out... do all of the radiators, one at at time.
Next go the boiler and lift the handle on the pressure relief valve, very briefly STAND CLEAR OF THE OUTLET OF THE RELIEF VALVE .
Next look for any other air bleeds, those are all at high points in the system.
Now go back and bleed the radiators again.
Beyond that, the boiler temperature control needs to be set at least 10 degreesF lower than its current setting.
This is step one of what could be a much longer trouble shooting process.
We can go from here.
Make sure that boiler water pressure stays between 15 pisg and 25 psig during this bleeding process.