We have exhausted most of the obvious to try and correct this high pressure problem however it continues. We have an older weil-mclain oil fired furnace which would cause the Pressure Release Valve to pop and release water. After replacing the valve twice the problem continued. We decided to replace the old expansion tank with a new diaphragm style tank, replace the pressure release valve as well as the main water pressure regulator. After this the running average pressure is even higher than before 23-25psi vs 17-19psi. The pressure spikes when the furnace is called to come on (as would be expected). Air also seems to keep getting into the radiators which we bleed. They seem ok for a few days/week and the air returns. I assume this might be excess steam getting back into the system. The furnace temperature is set to 140-160F which I donâ€™t think to be too high. Our plumber and us are at a lose for what the potential problem might be. Is there something we are overlooking? Is the temperature set to high? Any help would be greatly appreciated.