Best Of

You aren't likely to ever see any pressure difference between the boiler and the end of the main, not in a house anyway.

@DanHolohan 's "A one pipe system will often add humidity to your home whether you want it or not" is a joke. He is saying it will add appreciable humidity if it is leaking somewhere. If it is working properly it should only lose a small amount of water, like a gallon to a couple gallons a month, not enough that you have to feed it every day or even every week. If you have to add water more than once a month you have some sort of leak somewhere.

The water should be boiled in to steam in the boiler, be condensed back in to water in the radiators(and a little bit in the piping) and that liquid water should return to the boiler through whatever return system that particular system has, either dedicated returns or through the main to a return at the boiler or some combination of the 2.

It is essentially a closed system except it fills with air when it is off and that air is vented out when the system is running to make space for the steam. A little bit of water evaporates in to that air that is vented out, that is why you lose a little bit of water.

If you are adding water every day either you have large steam leaks and will likely have condensation on walls and windows and such, it is leaking out of a buried return where you can't see it, it is leaking out of a hole in the boiler and up the vent, or it is coming out of a vent or a relief valve as liquid water.

Note that on one pipe steam a partially closed radiator valve will frequently cause the radiator to fill up with water, valves must be all the way open or closed on one pipe steam.

Then it is in the wrong spot. Up top it will tend to close prematurely before the radiator is properly filled with steam. This effectively makes the radiator smaller. What vents do you have on those small radiators? When you say they are making the hissing "when the heat first on" do you mean when first on during a morning recovery?

At those pressures I wouldn't expect any hissing, and even less so at the start of a call for heat when the radiators are still getting heated by the steam. Closing off 1/3 of your radiators, though, will make your boiler even more oversized than it already likely is and that's not advisable.

Try opening them and report what happens.