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On some two pipe systems — Most "vapour" systems — there is or at least was once a gadget which kept the differential pressure between the steam main at the boiler and the dry return at the boiler from rising above a certain value — typically 8 ounces differential.

Several reasons for that, all of them good. If there are orifices or calibrated inlet valves, that's what they are set for. It prevents excess differential pressure on the traps and thus avoids damage. And you don't need any more than that.

However, that also means that any boiler pressure over 8 ounces gauge doesn't do anything additional to provide heat out in the radiation — all the extra fuel is doing is compressing the steam in the entire system. A complete waste of time.

So… if you have a vapour steam system, the vapourstat must cutout at or below 8 ounces gauge, There is no point, however, in setting the cutin too low — might just as well fire up again as soon as you can — so a differential of 4 ounces or so works just fine.

Now before someone starts commenting that the pressure control is a safety, not a control, I beg to differ — with an additional comment. The vapourstat set as above is very definitely a control, not a safety, and is used to modulate the effective firing rate of the boiler, since most steam boilers do not modulate otherwise (some coal fired boilers could, and did — but that's another topic). You also need a safety for overpressue, however, and that should be a pressuretrol — preferably a manual reset one, so someone will have to find out why the vapourstat didn't work — set at some reasonable pressure, such as around 2 to 3 psig.