Expansion vessel full of hot water?

The full expansion vessel can't be the cause, but it IS a symptom of another problem.
There is something blocking the water flow after the expansion vessel, the water level within will obviously go up as it have nowhere else to go.

Turn the furnace OFF. Turn the pump off if it don't turn itself off. As the water cool down and contract, the water level will go back down.
Call a plumber to check the pipes now.

Electro
April 2016

\try this

https://www.youtube.com/watch?v=4meZgMAUecw&app=desktop

Roger
April 2016