Random hot water?

Before the boiler kicks in for what? Hot water or heating?
I will assume hot water. What is the flow of water like, same as before? If not check the isolating valves on the boiler, May not of been turned on fully with recent work, Also what was fault, no warranty with work done? IF ALL valves are ok, but flow is less than it used to be, Could be the part was not installed correctly, Changing a heat exchanger would not normally affect water flow.
IF water flow is about same as before, Could be a problem with pressure sensor. Opening a second tap, would decease the pressure in the system a bit more.

Sheepdog
March 2011