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Hoffman Differential system

I have had the pleasure of being introduced to a Hoffman differential loop system. The first firing of the new boiler, water level steady. Vaporstat set at 12-14 oz. Boiler ran well for hours with all radiators hot.
Next day system is not heating all radiators, and cycling on vaporstat. While the boiler was dropping pressure, the water line disappeared from the glass trips my 1st and 2nd low water cut offs. System is in a vacuum and I am no longer making steam. Once the water level returns to the glass, low water safetys reset, 90 second prepurge, We start making steam again. burner only fires for about 90 seconds before making the vaporstat. Very interesting stuff. Could it be a problem with the Hoffman Loop? I have flushed the wet and dry returns, replaced all cages in the traps. Anyone have experience with this system?


  • Abracadabra
    Abracadabra Member Posts: 1,948
    Has boiler been properly skimmed several times?
  • Bug512
    Bug512 Member Posts: 52
    Oh I had the same issue. Dirty / oily water will cause the level in the boiler to drop. Skim, skim and skim some more. You have to flush all the oils and contaminants from the boiler.

    This is what mine looked like. Link Here

    Gene in Northern NJ
    NJ HVACR License 19HC00537600
  • Fred
    Fred Member Posts: 8,542
    As has been said, skimming is the solution. This kind of activity in the sight glass is classic. It ran fine the first day because the water was relavively clean. Oils were on the sides of the boiler block and on the walls of any new pipe. Heat and steam washes it into the water and ultimately, because oil is lighter than water, it lays on the surface of the water. Draining the boiler won't remove it, it will just cling to the sides as the water level drops. Skimming, and probably several times will resolve your problem.
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    Does the differential loop trip? It is very sensitive to pressure and i would almost expect it to.

    However, it isn't the problem.

    Building that much pressure in 90 seconds means that air isn't getting out of the system for some reason...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ScottLichon
    ScottLichon Member Posts: 2
    I skimmed the boiler 5 times to date. The water is sparkling.
    It seems the hoffman may not be tripping.....
    My thought is that the inner tube is possibly plugged....
    At 10 oz. Itshould be passing steam.
    The loop is luke warm at best.