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Steam heating with hot water loop banging

PACMAN Member Posts: 3
Hello, I put in a Weil McLain steam boiler with a hot water loop. The heat works great.  however when the steam and the hot water loop thermostats both turn on there is loud banging in the hot water system. What am I doing wrong? 


  • EzzyT
    EzzyT Member Posts: 1,295
    Your missing the bypass piping from the supply to the return before the pump. Plus the pump should be a bronze pump not a cast iron. 
    E-Travis Mechanical LLC
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    Also -- how high is the hot water system above the boiler? If it is above the water line, it may lose prime between runs, and when the pump starts you could well get some banging in it as it fills.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    PACMAN Member Posts: 3
    Thank you all for your help. The bypass has been installed. The reason the pump is so big is because I put back the exact same layout and pump as he had with the existing boiler. The hot water loop is not new.  The steam heats the first floor. The hot water loop is on the second floor. The hot water loop piping is about 220’ of 3/4” copper piping including copper baseboard. The boiler is not draining while the boiler is pumping. If I put a taco box and make the hot water loop priority, do you think this will help the banging?
    EBEBRATT-Ed Member Posts: 15,544
    I am thinking your trying to pmp 212 degree water and the pump is cavitating. You need to pipe it with a bypass and set it so the HW loop stays below 180. You said when it is not steaming the hw loop is ok. You also may be pumping too much water. If you have a valve on the pump discharge try throtteling that. Don't close a valve on the pump suction