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Open gravity system

I looked at a system in a building recently that is all old cast-iron radiators. There’s a cast-iron boiler in the basement originally it would’ve been a coal fired one. The building is over 100 years old. It is a gravity feed system but I was told that it is an open non-pressurize system. Apparently there’s a pipe sticking out on the roof that vents to the atmosphere. So when the fill valve is opened to manually add water to the system The excess water runs out onto the roof and drains into the gutters. Was this a common practice? And how complicated would it be to convert it to a high-efficiency mod-con boiler?


  • Jamie Hall
    Jamie Hall Member Posts: 18,966
    Very common. And very very simple. And works marvelously well. No moving parts!

    It is, however, quite possible to convert it to an all singing, all dancing modern system. Not to say that it's simple. But it's quite possible. There are several ways to do it, but the common thread in all of them is balancing: pumped systems circulate in a completely different way, and getting the balance right again can be a very interesting (and frustrating) exercise.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 15,349
    Look more closely- you'll find that roof pipe comes from an expansion tank either in the attic or a second-floor closet. You'll want to remove this tank and use a closed tank in the basement, along with an air separator.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service