one of our radiators wasn't bleeding, and the boiler pressure was 9 so i called the technician. he showed me how to let more water into the system . after that the radiator bled. and he set the pressure for 15. now it has gone back down to 9. can you give me any idea of what the problem could be? i called a help line and they said i need a plumber to drain the water tank.
i had a new "back flow " thing put on last year. Sorry i think that's what it's called. so i don't think that's it.

thanks.