I have a residential boiler that keeps running over 30 PSI causing the
pressure relief valve to drain on the floor.

When I first examined the system, I noticed the expansion tank (with
diaphram) was completely filled with water. I replaced the expansion
tank with a new one and turned the system on. The pressure still
exceeded 30 PSI.

How do I find the source of the excessive pressure and fix it?

There is a reducing valve that says 12 PSI on the factory label. Going

into the reducing valve is a cold water supply line with a shut off
valve. It looks as if a previous repairman has backed off the adjusting

screw a few turns from the factor setting since there is the absence of

red paint on some of the threads.

I have not adjusted the reducing valve nor cleaned the strainer.

I can drain the system, get the pressure down, fill the system to about

12 PSI and then run the system:

- If I run the system at this point with the cold water supply valve
closed the system appears to stay around 18 PSI. My gut tells me I
need to keep this supply valve open when operating the system to safely

supply new water to the system if needed.

- If I run the system at this point with the supply valve open, the
pressure will go up to over 30 PSI. If I then let the system cool, it
appears that the pressure will go down to only about 18 PSI. Why
doesn't the cooled down system go back to the 12 PSI condition as when
I started it?

Should the water supply line going into the reducing valve be kept open

when normally operating the system?

Should I try to adjust the reducing valve?

Should I try to clean the strainer on the reducing valve?

Should I adjust the 12 PSI pressure on the new expansion tank?

The water temperature appears to reach about 155 degrees after an
extended period. Should I reduce the water temperature to reduce the
pressure? What's an idea residential boilder water temperature?

FYI - the house is 3 stories above ground so I need to circulate water
35 feet from the basement. Some threads lead me to calculate that I
need about 20 PSI to circulate water that high. If that's accurate,
how do I keep the pressure between 20 and 30 PSI.

Sorry for all the details and asking too many questions here but
thought they might help give one of you experts a hint as to what the
problem may be.

Any advice on solving this boiler pressure situation would be
appreciated.