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Easiest way to tell if my radiators are piped in series or parallel?

SENorthEast1895er Member Posts: 83
Newbie homeowner here. I’ve got a 1895 home with cast iron radiators and i’d like to install TRVs. What are the easiest ways to tell if they’re piped in series or in parallel?

Easiest i can think is to close some radiator valves that would be early in the series (2nd floor) and see what happens on the third floor. Next easiest would be to use a tiny camera (3mm thing on a flexible wire) and try to look in the floor cavity to see what is going on.

System might’ve been gravity fed at some point; there are pumps now. Main feed in the basement is 4” cast iron, give or take.

Anything i’m missing here?


  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,241
    If they were installed correctly back then they would NOT be in series.  Can you post some pictures of Radiators boiler and any exposed piping?   Mad Dog
  • Mad Dog_2
    Mad Dog_2 Member Posts: 7,241
    But, yes...If in series ANY valve you shut on a radiator would stop flow to all.  You generally only see series on Slant Fin typ baseboard.  This is not to say you will never see it, but aside from a small zone that has two radiators in series, it will mostly just be piped that way by mistake.  Mad Dog
  • PC7060
    PC7060 Member Posts: 1,239
    edited April 2023
    Typically older systems are piped series parallel with large (and often long) supply and return lines.   Each radiator is piped with dedicated lines off the supply and return as illustrated below. 

    Can you post photos as @Mad Dog_2 requested?

  • ethicalpaul
    ethicalpaul Member Posts: 5,792
    When the system is running, take the temperature of the incoming water at each radiator and see if some are a lot lower than others. Maybe that would work.

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