Hot water not heating up enough on Hawk 2 boiler?

I have a Hawk 2 boiler. When I turn on the hot water, the boiler fires up immediately and keeps running but the water only reaches warm. Last night, I found the pressure on the guage on front of the boiler was done to zero, so I brought it up to 2 bar (as I had been told by the installer) and that persuaded the heating to work. Any suggestions?