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RJ_4 Member Posts: 484
Can I get a couple of good explanations about when to have a dry return and when to have a wet return, also on a 2 pipe system is a vacuum pump recommended


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Dry returns run from the last radiator takeoff until the pipe drops down to floor level and heads back to the boiler, where it is know as a wet return.
    As air is the enemy, and must be pushed out of the way for the steam to enter the radiators, a vacuum system has some advantages. Do a search here for "sub atmospheric" or vacuum for many posts on the subject.--NBC
  • RJ_4
    RJ_4 Member Posts: 484
    thanks RJ
  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    A vacuum pump on a two pipe system can make for faster heat and slightly higher efficiency. However... they are also a pain in the neck to maintain. Really good venting will do almost as well!

    The choice of dry returns or wet returns or both is largely a design consideration. As @nicholas bonham-carter said, if you have dry returns, they must be vented -- usually right at the or very near the boiler, where they drop to let the condensate enter the boiler. In some systems, the steam mains are vented directly into the dry returns via crossover traps, in which case main vents on the steam mains are not needed (and, in some types of systems actually are harmful!). Many systems -- vapour particularly -- will have both wet returns, to handle the condensate, and dry returns, to handle some condensate but mostly air from the radiators.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Pumpguy
    Pumpguy Member Posts: 632
    Vacuum pumps are not always used on 2 pipe systems. As @Jamie Hall said, a vacuum pump can make for faster warm up time and improved efficiency.

    This is accomplished by increasing or changing the Delta P across the system, which allows running the boiler at a lower pressure for a given Delta.

    Keep in mind that for each 2" Hg. vacuum on the return line, you can reduce the boiler supply pressure by one PSI and still have the same Delta P you had with just atmospheric pressure.

    Something to keep in mind is, vacuum pumps, and vacuum performance in general is not compatible with high temperature condensate. Vacuum performance is best with cold condensate, but as a practical matter, I would say you're okay with 140 degrees F. or lower.
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963
    Website: www.nashjenningspumps.com

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • RJ_4
    RJ_4 Member Posts: 484
    Thanks everyone