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Orifice Plates

FWDixon Member Posts: 78
So I've been reading up on old overhead gravity hot water heating systemes and came across something called an "Orifice Plate" that the old-timers installed in the upper story hot water rads to ensure flow to the lower floors. The article also said that when converting to force flow, the plates needed to be moved to the first floor rads.

I understand having the plates on the top floor in a gravity system, but why move them for force flow? Also, how did the old timers know how big to make the orifice, trial and error or something more scientific?


  • Ironman
    Ironman Member Posts: 7,119
    Move or Remove

    I would ReMove them. We encounter these in old homes in the old town part of our city. We always ReMove them.

    They were necessary to balance the gravity flow because the hotter water would naturally seek the highest level.

    Sometimes it becomes necessary to place some downstairs when converting to forced flow due to the greater volume of water that has to be heated for upstairs, but not often. If the radiator valves function, adjusting them is usually sufficient.

    Every job is different and results may vary.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • jumper
    jumper Member Posts: 2,050
    just make sure

    Just make sure those valves are up to throttling duty. That means no water gets past packing.
  • FWDixon
    FWDixon Member Posts: 78
    Re: Orifice Plates

    I do not know if I have them in my system, just curious how the size of the opening was determined and why it may be necessary to put them in the first floor rads when converting to force flow.
  • JStar
    JStar Member Posts: 2,752

    In a gravity system, the water will favor the highest level because of the hot water's buoyancy. The orifice plates were added to add resistance to the upper level radiators, and force water into the lower levels.

    In a pumped system, the water will favor the lower levels because they have less resistance (less pipe and fittings).

    If you just blindly add a circulator to a gravity system with orifice plates, you'll end up overheating the lower levels, and losing most/all flow in the upper levels.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    The ones I've seen...

    Were all the same size regardless of their location. You can't miss them though. THey'd be in the union receiver of the radiator valves serving the radiator. The ones I have seen were all roughly 1/4" opening.

    The ULTIMATE balancing device would be to use non electric thermostatic radiator valves. They automatically adjust themselves to the real time load, closing off those radiators that are warm, forcing flow to the ones that are open.

    In addition to orifi', the dead guys also use to NOT ream the pipes on the upper floors, and DID ream the pipes on the lower floors. Crafty bunch of fitters those guys were.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • FWDixon
    FWDixon Member Posts: 78
    Re: Orifice Plates

    Interesting, thanks for the info.
  • jumper
    jumper Member Posts: 2,050
    just a quibble

    I always figured upper floor got more flow because the column of cooled water in return weighs more? Oddly enough, in old three storey homes the middle floor was coldest.