Why does my boiler keeps losing water from pressure relieve valve?

Hi: Good question with many possible answers and one likely one. The valve is corroded at its seat face and leaking. Most likely needing replacement or re-machined seat face and matching seal. Hot water tends to do this a good bit. Be careful : let the thing completely cool and depressurise before you touch it. Never guess, heat energy stored in water is massive and will scald you faster than that 500 lb fish through your best hook. jimiwane >>>>>>>>>>>> Perhaps a problem with the expansion tank such as a bad bladder or an air leak if its not a bladder type of tank.