Water level in boiler system
Holy cow.... STOP what you are doing.
A hot water heating system is "full" at all times. The difference is that there is a pressure reducing valve that limits the pressure to 12 or 16 pounds. If you pressureize your heating system to full city pressure, you'll have a royal mess on your hands. Your pressure relief valve on the furnace will start to puke water all over the basement, and your pipework (previously not used to those pressures) can declare it's leaks. Often, the bleeders will begin to leak too. I assume you used the "fast fill" lever on the pressure reducing valve when you did you test a bit ago.
Now... you might have a steam system too, with a "manual fill" valve, and you might not know you have steam and are confusing it with hot water. Does your furnace have a sight glass? (a glass tube about 1' long on the front where you can observe the water level). A steam boiler is only about 1/2 full of water.