rise in boiler pressure?

I have a Weil McLain boiler and the piping was redone this summer as all the zone valves and piping was mounted on the boiler and now moved to the wall. A Spirovent was added. The question is the boiler pressure after a while rises to 20 psi after running at 16 psi for a while. The service man came over and set the water force regulator and the pressure tank. The unit ran for about 10 hours at 16 psi and then gradually rose to 20 psi. What can this be caused by.