What is the pressure of air in the domestic expansion tank? Since you have already replaced the relief valve we know it isnt that. The expansion tank must be one specifically for domestic water, not one used for boilers. What is the model of tank? The air pressure in the tank as compared to the water pressure in the house is important. It will tell you what it must be in the instructions. If you dont know the water pressure in the house you dont have enough info to go any further. It will help answer questions.
