Two Pipe steam hoffman differential loop

Do you have a copy of The Lost Art? There's an excellent writeup in there on the Hoffman Equipped System.

To start off here, though...

First, the system is intended to operate on no more than 8 ounces of steam pressure. Anything more and it it operates at all, it will operate rather poorly. So start there.

Second, the system is rather particular about venting. You must have all your venting at the elbow where the dry return(s) enter the top of the Hoffman Differential Loop. There must be no vents, repeat: NO VENTS, anywhere else on the system. Misguided additional venting somewhere out there -- whether on the mains or on the dry returns -- will raise all kinds of havoc when the Differential Loop operates (which it will, at about 8 ounces, by design). So remove the vent on the end of the steam main.

Instead, what is required -- and if someone put a vent at the end of the steam main they may have also messed this up -- is that at the end(s) of any steam main(s) there must be a crossover trap -- which is just an ordinary usually 3/4 inch radiator trap -- piped as follows: a nipple vertically up from a T on the main; 90 degree over and a nipple to the inlet of the trap, then a nipple down to a T on the corresponding dry return from the outlet of the trap. This is not to pass condensate -- but acts as a king size vent for the main.

Note that if the main pitches away from the boiler, there must also be a drip at the end of the main to a wet return -- and, most likely, a drip from the end of the dry return in the same area. Make very sure that the wet return in that area is below the water line in the boiler.

There is no need for a check valve on the return. To place one there will cause problems.

Have fun! They are great systems, but they are a bit sensitive to being mistreated.

I don't think that that's going to be a happy boiler...