How do you replace a expansion tank on a boiler system?

Turn off the makeup water and electrical service to the boiler. Isolate any and all valves in the system. Thread the old tank off, thread tape and/ or thread sealant on male ends of new tank and thread it in. Bleed air from system as needed using makeup water. Dependent on the piping arangement, if the tank screws in the bottom of an air scoop, an air eliminator atop the scoop may be removed and then replaced after all air is purged. Note: in using makeup water valve/ bypass, do not go over 30# of pressure. Most residential boilers will have a safety relief valve that is set at 30# that will blow off at this pressure and make a mess. The makeup water pressure reducing valves are generaly set to maintain 12# of pressure. Most pressure tanks are precharged at the same 12# of pressure on the air side. Verify these pressures and that all air has been purged from the boiler and pumps, before refiring the boiler. lc