I noticed the pressure on our Utica MGB125hd boiler was up around 40 psi. I was concerned because I read that the range for a gas boiler should be between 12 and 30 psi.

I bled one of the radiators and there wasn't any air but I noticed the pressure went down a bit so I let a little more water out from the radiator at the bleeder valve and the pressure went down some more. I probably let out about 10 ounces of water from the system.

It is now running at 27 psi when hot. What would cause the pressure to go up like this?

The boiler is about 20 years old. I asked the manufacturer how long they are supposed to last and they told me 18 years. I am thinking maybe a faulty relief valve somewhere. I would appreciate any thoughts on why this might have happened.