Spoke to a B&G rep about the problem(s) and he didn’t feel the 2 1/2 reducers were the problem
He's wrong. Going from 2.5 > 3.0 will create a pressure drop, which you don't want, air bubbles.
but was focused more on the motor/pumps sizing. He speculated that the pump may be starving for water and causing the cavitation. Either because the pump is oversized (it’s not per the boiler GPM specs) or because of a restriction upstream.
That restriction would be the 2.5 mix valve acting as a choke, like I mentioned above.
Simple fix is put the 2.5 pump in (99.9% It's going to flow enough, being the intake on the boiler is 2.5, just need to check the specs), and that will solve the pressure drop from the 2.5/3.0 reducer by removing it.
4.0 Feed > 2.5 mix valve > 2.5 pump > 2.5 boiler intake.
Being this job was just "added possibly" to the system, "without a load analysis performed on the water supply" to ensure the two boilers running at the same time (or even just one) do not overload the supply, and create a low water pressure situation, they need to perform one to eliminate that as a cause, even with the change to the 2.5 pump. Nobody knows what systems are on that branch coming off the main at this point.
This "repair" is at the point where you need to start from the beginning (drawings), and cover all bases.
ETA: and yes, there should be test points to attach meters, or permanent installed, so you know what's going on.
You need those anyway to adjust the barometric, so you know what your changes / adjustments are doing.
