I have this boiler with a coil for domestic hot water. Once the pressure relieve valve started to leak I replaced it right away since it was at least 15 years old. That did not resolve the problem, so I checked the expansion tank, model 110, and there was no air at all in there. I pumped 12 PSI of air as it comes from factory, the tank holds the pressure fine, but the valve still leaks. For kicks, I pressurized the tank to 30 psi and the valve still leaks!!! That is with the boiler off, so that is just from the water pressure of a well which has very little pressure... The expansion tank is about 15 years old. I think there is a 3rd thing I could look into on this situation but I cannot remember... Any suggestions would be greatly appreciated.