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Missing Hoffman Differential Loop

Recently installed 2 heat pumps for this customer but they would like to use their boiler also. They’ve owned the home only about a year. Last winter they had constant problems short cycling from water level bouncing below LWCO and then flooding the boiler. They had 3 or 4 people look at it. I don’t know all the “fixes” they did but some were disconnecting the electronic LWCO and adding a mechanical 1, adjusting the pressure (at 1 point up to 9 psi!) and adding boiler treatment once a week.
I’ve attached a couple of photos. All the traps are there including the ones in the basement between the supplies and returns. As post title says the differential loop is missing.
They want an estimate to fix it. Can it work with no loop? Obviously it needs a vaporstat and the near boiler piping repiped.


  • neilc
    neilc Member Posts: 2,671
    you mean the equalizer don't you?
    I don't see one, and you want one.
    that and all the rest of the pipe around that boiler is wrong also.
    known to beat dead horses
  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    Yes... you can make that work without the Differential Loop.


    Unless I'm blind -- which is quite possible -- the header and equalizer are catastrophes. What I think I'm seeing is two cute little risers coming from side outlets (and that style of boiler is prone to wet steam...) and going up, then enlarging to what might have been larger risers from an older boiler. Then at the top they join a header -- one at each end. With the steam main coming off in the middle. And the equalizer coming off one end at the elbow to that riser, but halfway up the elbow. Then the equalizer doesn't go down, but wanders over to the bull of a T, one side of which goes down and the other goes up and around and over to the bull of another T, one side of which goes up to what I take to be a dry return and the other side down to what I take to be a wet return out of the picture.

    Ah... no. I doubt that you can make it work properly -- Loop or no Loop -- without taking most of the near boiler piping apart, then redoing it. There are lots of ways to do that right. There are lots of ways to do it wrong. If it were mine, I'd probably enlarge the risers lower down, go up and over and down to a nice large dropped header, sloped down towards the equalizer so that the header can drain completely, with the order from the equalizer end being equalizer, steam main, riser one, and riser two. Get rid of the connection from the equalizer to the wet return -- with the Loop gone you don't need it or even want it. Put a nice big vent at the end of the dry return where it turns down (where are your main vents, by the way? That's where they should be).

    Then, if it really was a Hoffman Equipped system, it was intended to run on no more than 8 ounces of pressure -- so put in a vapourstat and set it for that (use a low pressure gauge to check calibration). That's ounces, not pounds!

    And put back the electrical low water cutoff. Removing it comes under the heading of "if you don't like the message, shoot the messenger" which never ends well. You can keep the mechanical one, too -- never hurts to have redundant safeties.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Gary_17
    Gary_17 Member Posts: 37
    The first thing I noticed was that there were no vents. I couldn’t figure out how it would work at all. I found Hoffman No. 8 valves on all the radiators and at the end s of the 3 mains. So did a little research and figured it must of had a differential loop at 1 point.
    I thought about eliminating the riser and equalizer on the right and supplying the other from a drop header and go from there. I also noticed the Hartford Loop was a little too high so basically all the near boiler piping needs replaced.

  • Danny Scully
    Danny Scully Member Posts: 1,416
    My guess is there wasn’t originally an equalizer, and that attempt at one now actually use to go to the differential loop. I would Repipe the near boiler piping (possibly replacing boiler) and Install a vaporstat. Also check your return heights and make sure you don’t need a false waterline as well.
  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    The reason it works at all at the moment is that there are vents on the radiators. The thing is working -- if that word applies -- as though it were a two pipe air vent system.

    So... what you want to do is get rid of the vents on the radiators and the stray "main" vents out in the system, and replace them with vents where the dry return drops to the wet return, at the boiler. Then go around the system and check all the traps, including -- perhaps especially -- the crossovers. I'm going to be that at least some of them aren't going to be working. Fortunately, it is easy -- and relatively inexpensive, as these things go -- to replace the guts.

    Also repipe the boiler, as @Danny Scully said. A fair amount of work -- but then, there is a fair amount of knuckleheading to undo there.

    Once you get it up and running right, if some rooms overheat you can partially close the inlet valves to those radiators -- that's one of the beauties of this type of system!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England