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Air Block in Old Gravity Fed Hot Water Heating System

Barnzi Member Posts: 1
Last plumber, who really knew this system, told me it was one of the last Gravity Fed systems in the county. Then he moved to Vermont. Most of the newer Heating folks are kind of young - like sixty. The real young ones just look, eyes glaze over and they then say the same thing: get a new system. PROBLEM: AIR BLOCK in HOT WATER LINE? Solution:"go on line and see if some OLD types can help." Actually, anyone who can share an idea as to where to start would be really welcome. TY Real Old Man would appreciate any help.


  • mattmia2
    mattmia2 Member Posts: 9,747
    Where is the air? Does it have an automatic water feeder? What kind of heat emitters does it have? Is it cast iron radiators or cast iron baseboard or something else? Does it have the original boiler or does it have a more modern boiler?

    Bleeding it when it is set up as gravity isn't much different than bleeding a gravity system that has been converted to forced circulation except that you will have to get it all out through the radiator vents, there is little circulation to pick up the air and move it to an air eliminator by the boiler so you might have to bleed the radiators a couple times and if some piping is trapping air such that it can't get to a radiator you might need to open some valves to force flow in that section.
  • Jamie Hall
    Jamie Hall Member Posts: 23,438
    Be sure while you are purging or bleeding one section or radiator that you keep enough water in the system!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ch4man
    ch4man Member Posts: 296
    old guy here..... 1st boilers dont bleed, service guys do.
    boiler systems are either vented or purged. all gravity boilers i've seen utilize CI radiators and were piped by the deadmen. purposely done so whether firing or not any air will naturally rise to the tops of the radiators. these are to be vented.
    if pipes have sagged that may trap air.
    do you have a closed compression tank or an open x-tank of the top floor in the house? if so be carefull pressurizing that system.
    are you sure there is enough water in it? old boiler, old altitude gauge (pressure gauge) may not be accurate
  • jumper
    jumper Member Posts: 2,289
    Here's how a rich guy does gravity. Replace the open tank in attic with a closed steel tank strong enough to hold vacuum. At that time install an overflow pipe beneath tank going back down to boiler room. Put a valve or valves on end of that overflow. Open valve(s); add water until it overflows. Now apply vacuum to that overflow with a device that won't be damaged by water like venturi. Close valve(s) on overflow pipe. When the heat goes on air will end up in that expansion tank up there. You may have to evacuate tank once more but hopefully never again after that. And no more bleeding.

    Gravity does not need pressure. Just enough water to fill every radiator. I've seen a three storey mansion work this way.