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Bleeding HWB zones while boiler is hot

1075NH
1075NH Member Posts: 34
in THE MAIN WALL
If this question has been asked and answered in another thread my apologies. I couldn't find it. My question is in regards to bleeding the air from zones on a residential cast iron Hot Water boiler. I have heard a lot of opinions on the "dangers" of bleeding zones while the boiler is hot. Namely cracking the boiler by introducing cold water. Especially when bleeding multiple zones for any length of time. Some people say let the boiler cool down to room temperature. In a perfect world, sure. But i have never heard of any boiler service tech going out on a call and saying they need to come back hours later when the boiler is at room temperature to bleed it. Is this an "Old wives tale"? Do i really need to leave the house with no heat or hot water for hours while i let the boiler cool before bleeding zones? Is it better the leave the boiler turned on while bleeding zones to heat the incoming water? I have asked a lot of plumbers and techs this question and the majority say it's fine. But a few insist you will cause cracks in the boiler that will fail sooner than later.

If you don't let it cool down to room temp what is a safe temp to allow bleeding zones?

Thoughts?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 24,115
    edited November 2020
    How much water are you bleeding out? And are you circulating at the same time? If it's only a gallon or two -- and I can't see it being more than that -- particularly if you are circulating at least the boiler, I can't see it being a problem. If you are taking a pretty dry system which is hot -- maybe shut down on low water or something evil like that -- yeah, that's a problem. Don't do it.

    And I presume that the makeup water is going into the system piping, not directly into the boiler.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • 1075NH
    1075NH Member Posts: 34
    Jamie Hall said:

    How much water are you bleeding out? And are you circulating at the same time? If it's only a gallon or two -- and I can't see it being more than that -- particularly if you are circulating at least the boiler, I can't see it being a problem. If you are taking a pretty dry system which is hot -- maybe shut down on low water or something evil like that -- yeah, that's a problem. Don't do it.

    And I presume that the makeup water is going into the system piping, not directly into the boiler.

    I'm talking about bleeding/purging air from the system using the standard procedure. So a hose connected at the purge station, return valves closed, PRV set to Fast Fill. So in some cases you are talking about flowing a lot of water for several minutes per zone. And the makeup water is feeding into the return side of the boiler (which is common).

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