If you have a boiler rather than a furnace, it will have an expansion tank separate from one (you might have) for your potable water tank.
In a boiler, the pressure should not get to much more than (typically) 20# or so, and the expansion tank is normally set to the normal pressure of the system. The T&P valve for a boiler will trip if the pressure gets much above the normal pressure. This is a different valve than one designed for a WH, each has its own settings for both pressure and temperature where they start to vent.
If you install a T&P meant for a boiler on the WH, it will dump water. If you install one made for a WH on a boiler, you may blow seals before it trips.
If you get 100# in the boiler, you likely have either a leaking boiler make-up valve or the new bladder tank is shot, or the pressure is too low, or it is not the right size, and is filling up with water.
Maybe a picture of what you have would shed some light.