How can I increase boiler system pressure so bleeding 3rd floor radiators is easier?
To get the minimum fill pressure of you system you can determine this in one of 2 ways:
#1, measure from the top of the highest point in the system to the pressure gauge on the boiler and divide that distance in feet by 2.3. For example if that dimension is 30 feet (30 divided by 2.3 = 13.0 PSI. This is the minimum pressure in the boiler for a cold fill and with nothing running. Also you should add 2-3 lbs pressure as a safety factor. of the system.
#2, Begin filling the boiler and have a 2nd person be at the top radiator with the bleed valve open and a container to catch any water that is discharged. When the water flows from that highest radiator, the pressure on the boiler is the minimum setting, and again add 2-3 lbs for safety factor. That is it.
Adding any more pressure will do very little to speed up the bleeding process. You can change the relief valve to any pressure as long as it does not exceed the pressure rating of the boiler and any device that is in the system. Theoretically, you could install a 50 psi relief valve but why would you. A higher than necessary pressure adds nothing to the operation of the system.