what is wrong with my boiler?

i'm going to say the expansion vessel has lost its charge , thats why your pressure is going right up , the pressure relief valve might be letting out water , due to this , showing up as very low pressure when cooling down . check for water dribbling out of pipe that goes to outside . if so replace expansion vess and press relief valve

aaron
December 2010

Sounds as though you have air in the system that expands with heat. turn off the heating and then go around all the radiators and vent them until water flows from the vent (use an old piece of cloth or old tea towel to collect any vented water.) Also make sure the automatic air release valve in the boiler has the finger nut open two turns so that any air is released from the boiler

plumbob1
December 2010