I think you are talking about your auto-fill valve. This would normally add water into the heat loop to maintain the preset water pressure (most are adjustable, I think, but come preset to about 15psi). Many boilers have a low-pressure cutout, and won't even fire if the pressure is at 5psi, mine won't run until it gets over 12, and the autofill is set to 15. If the pressure is too low, you can get steam in the boiler's heat exchanger (think the pressure cap on your car's radiator). To keep from potentially polluting your potable water, there's also a backflow preventor device before the autofill device. If you don't have an autofill device, you still would have the backflow preventor, but then you'd have a valve to manually fill the boiler.