The Hoffman Differential Loop
If your a steam junkie, or just some lucky homeowner to have one of these devices, you'll hopefully think this is just the coolest thing ever devised. What the Hoffman Differential Loop does is to keep the water in the boiler. See, if you have to much pressure start to build up in the boiler, the water will have to stack up so high in the piping that it will stop returning to the boiler. On a coal boiler this could even have exposed the boiler to dry firing or explosion. The Hoffman Differential Loop solved this by making sure that under no circumstance could the pressure in the boiler exced 10 ounces higher than the return piping. It did this by bleeding pressure into the returns after 10 ounces was reached in the boiler. So if the boiler had 8 ounces pressure, the returns would be atmosheric. If the boiler had 9 ounces in it the returns would be atmosheric. If the boiler had ten ounces in it, the returns would be atmospheric, BUT if the boiler reached 11 ounces in it, then 1 ounce would be bled into the return pipes, and if the boiler reached 12 ounces, the returns would be 2 ounces. Beautiful huh! See at no time could the return pipes be more than 10 ounces different than the boiler. This means the water would always be able to return to the boiler. AND the steam would always flow since there was still a differential pressure between the supply and return pipes. Cool huh! This link here is to what I feel is the best description of a Hoffman Differential Loop I've yet come across. And this link is to the original patent for one of the loop designs.
By the way, this device has no moving parts. That means it will work just as well today with your gas and oil boiler as it did with the coal boiler. Please don't let anyone remove it!