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Vapor Differential Loop slows the flow or speeds the flow ?

mstrbill Member Posts: 37
edited September 2019 in Strictly Steam
Hello steam pros................ Does anyone have 1st hand knowledge of weather a differential loop slows the flow of air and then condensate returning in a dry return when the pressure is above 1 psi with a modern boiler and standard pressure control , and running long on a cold day . I understand the loop works to put the condensate back with pressures above 10 OZ , however , from what i have read about the loop (Hoffman) to be exact , it injects steam to keep the differential going and in the process , closes the air vent . Does this effectively slow down the heating of the home when used with a pressuretrol on a modern boiler due to the air valve being closed - opened - closed - opened etc. during the higher than 10 OZ events , which can be many when its cold ? OR , will the system work faster if kept as designed as a vapor system and i add a vaporstat in place . Seems as though when above 10 OZ . the cycling of the diff. control will take more time to release air and create condensate flow . Its difficult to decide about the vaporstat because the new ones do not compare to the accuracy of the old merc. ones and who wants to return to the customers home often to change the expensive vaporstat . All input is welcome !


  • Steamhead
    Steamhead Member Posts: 16,885
    By the time the pressure gets that high, all the rads should be full of steam. If not, check traps and venting.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
    As the OP noted, the differential loop acts to maintain the differential pressure between the steam main and the dry returns. It does this by allowing steam from the boiler to enter the dry return, closing the main -- and only -- vent (note here: if there are other vents elsewhere on the system, things will proceed to work very poorly, if at all. Take them off!). Once enough steam has entered the dry return, the loop will reseal from returning condensate, and the system will operate normally, although at a higher than normal pressure relative to the atmosphere.

    There will be some loss of heating while the loop resets, which may take a few minutes -- so you really don't want to have it happen.

    It can be avoided by keeping the pressure where it belongs -- below 8 ounces (some loops trip at slightly lower pressures) and the way to do that with modern often oversized systems is with a vapourstat as a control (you need a pressuretrol, as well, as a safety). It is true, as the OP notes, that the microswitch type vapourstats may come out of the box off calibration. They are easily calibrated using a low pressure (0 to 3 psi) gauge, and the calibration should be checked on all installs as a matter of course.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mstrbill
    mstrbill Member Posts: 37
    Thank you for your comments . Yes there are other vents , 2 others , and i will remove them and add the vapor stat . I will get the number off of the loop device but i believe its the larger one (03) .. I will also run it at 8 oz . the loop is much higher than the water line so i will measure it exact and label etc . i am aware of one loop seal at the other end where the return drips to the wet , and i will measure that to and adjust if necessary . Keep up the good replys .