I have a Raypak 168-t (circa 1982) with with an Armstrong s-15 pump. When the burner is off, the pressure is stable at 12 psi. However, when the burner fires, the pressure goes up to 32 psi and the pressure relief valve (rated at 30 psi) begins to drip. The pressure remains at 32 psi until the burner shuts off, and then returns to 12 psi. I had a technician look at the boiler today. He said that the pump needs to be replaced. It is only drawing 0.1 amp and is rated at 0.7-1.1 amp. My thoughts were that the pump either works or it doesn't, regardless of the amperage that it draws. Hot water is getting to all of my rads. Does this sound like a reasonable explanation for the high pressure during firing? I did have the fresh water line from the street replaced by the city last November, which has increased my water flow from 1.75 gpm to 6.5 gpm. I thought that might be relevant, but there is a pressure regulator in the fresh water intake line to the boiler. Also, I have a rad in the attic (fed by 3/4" copper from the boiler) which was added by the previous owner at some point. Although this rad has had presure since I bought the house 5 years ago, it never got hot. Now, since the city water line replacement, it gots hot along with the other rads. Any opinions would be welcome.
It sounds like your expansion tank is waterlogged. Depending on the type, it may need to be drained, or if it's a bladder type, replaced. Once you resolve that, you may want to replace the pressure relief valve. Once they get opened regularly, they don't always reseal.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013