The pressure in the system should be between 12 and 18 PSI. The pressure might be a little higher depending on the height of the building. The pressure needs to be positive for a several reasons. The benefit of never allowing the piping to fall into negative pressure is avoiding the entrance or air pockets into the piping which would cause noise.

If the pressure is lower than the recommended pressure, you will need to introduce water through the pressure reducing valve. When you add water to the boiler sytem, you are raising the pressure. Ensure the water is turned off so thatthe system does NOT continue to fill by itself. If the system has a leak - even a small leak - the worse thing that could be done is to leave the cold water line open to system. One reason is, the system will be really inefficent, because it is having to heat up cold water all the time.  A more a important reason, is that each you introduce new water to a boiler system, you have the potential to bring in more impurities (iron).  These can collect in the boiler in the from of scale. Over time this can create hot spots and cause the boiler"s heat exchanger to over heat. This can cause a premature leak.  It can also cause water hammering. If your boiler keeps losing water pressure, it is highly recommended that the leak be found and repaired.