assuming that the pressure vessel is ok
you are going from one to two bar because of water expansion when heated.
I would only expect 1 to 1.5 bar
it could be you have a lot of radiators or the vessel needs replacing or re-charging.
At 3 bar the pressure relief valve will operate and excess water will be shot out through the wall. This is not dangerous but not desirable.
dave broadbent, June 2007
