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Hoffman Differential Loop

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Phil_26
Phil_26 Member Posts: 1
I'm working on a two pipe steam system. I have no historical data on the system. It has F&T traps at the end of the steam mains discharging into the receiver of the condensate pump. There are also thermostatic traps on top of the condensate mains w/inlet connected on top of steam mains. These are also at the end of the mains. We have cast iron radiators at ceiling of basement level and cast iron radiators on main floor, All above the level of the mains. Current project (per architect spec) was to repair all traps, replace condensate pump (Hoffman Watchman), and install thermostatic radiator valves on all radiators.On start-up, after all the radiator valves were satisfied, the vent on the receiver tank blows a full pipe of steam. Instant sauna. This happens at 3 psi ( lowest setting on pressuretrol). There is a Hoffman differential loop above the boiler (see figure 9, pg 234, Lost Art of Steam Heating) that appears to be the culprit. All traps show a temperature drop indicating that they are holding, but the loop shows the same temp across it indicating it to be the source of the steam. My reading seems to indicate that this might be a leftover from a time of gravity return. Can I remove it? If so, remove all piping? Would I still need the vent at the high point of the condensate mains? How about those thermostatic traps at the lower end of the mains? Another topic: any suggestions on how to control the boiler now that we have all those thermostatic radiator valves? The boiler currently is on a wall stat, but it seems to me that the radiator valves can stop steam flow and leave the boiler running against the limit. Sounds like poor fuel economy. Help, please!

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  • Unknown
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    Wow...

    At that steam pressure, you'll need to remove it. You won't need a vent control, as the condensate tank is the vent, now. Cap the live steam line to the Hoffman loop. Drain the returns into the tank such that the air can vent through the tank without needing to blow through any trapped water. The tank needs to be at the same location as the old main vent was, so flow directions of air and water remain the same. The only check valves you need are the ones between the boiler and the feed pump.

    Control the boiler on outdoor air temp. Use a Heat-Timer brand controller for most efficiency, or a simple air temp stat to control the boiler.

    Noel
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    Pressure is wayyyy too high

    That's a Hoffman Controlled Heat Vapor system. It was designed to run at roughly 8 ounces. The Differential Loop blows over at 10 ounces on the smaller models and 14 ounces on larger ones. That's what produces the readings you're getting.

    Far from removing the Loop, the solution here is to install the proper pressure control. You want a Vaporstat- the new mercury-free model is L408J-1009. Set it to stop the burner at 8 ounces.

    At these low pressures, you can also get rid of that return tank and pump, and the extra F&T traps at the ends of the mains.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
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    Pressure is wayyyy too high

    That's a Hoffman Controlled Heat Vapor system. It was designed to run at roughly 8 ounces. The Differential Loop blows over at 10 ounces on the smaller models and 14 ounces on larger ones. That's what produces the steam leak you're getting.

    Far from removing the Loop, the solution here is to install the proper pressure control. You want a Vaporstat- the new mercury-free model is L408J-1009. Set it to stop the burner at 8 ounces.

    The piping at the ends of the steam mains and dry returns should look like that on page 23 of this Library .pdf file:

    http://www.heatinghelp.com/pdfs/310.pdf

    The radiator trap piped between the steam main and the dry return is an air vent for the steam main. The air from the mains and the radiators goes thru the dry return and out the vent on the Differential Loop.

    Once that system is de-knuckleheaded, it will work quickly, quietly and efficiently.

    At these low pressures, you can also get rid of that return tank and pump, and the extra F&T traps at the ends of the mains.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • BRIANJ
    BRIANJ Member Posts: 118
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    Hoffman Valves

    If you installed TRV's in place of Hoffman Valves would you be interested in selling them. Give me a call.

    Brian 973-237-0431
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