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Will standing water (ie. not drained) in condensate pipes cause rust?

elfie Member Posts: 266
seems that if the condensate pipes are not fully draining, the leftover water may cause rust

is this an issue?


  • Jamie Hall
    Jamie Hall Member Posts: 22,972
    You need two things...

    to cause rust: damp or wet conditions and, more important, dissolved oxygen.  So this is sort of a yes but no situation.  In general, if you aren't adding much water and the venting is working as it should, there will be very little dissolved oxygen in the condensate; hence, very little rust.  Yes it will rust, but the rate is very slow.  Now if the pipe were open to the atmosphere and damp, then it may rust rather rapidly.  But not if it's full of condensate.  So I wouldn't say it's a problem...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,849
    Like he said...

    Very slowly. When my wet return started rotting out and leaking, I convinced myself that the whole problem was that it had sat, full of water, year round, for godknowshowmany years, and I decided to replace it with a dry return and put a drain valve at the point where it returns to the boiler so I could drain it every spring.

    Looking back, the drain valve was a god idea--I can drain the sludge out every week or two so it doesn't gunk up the boiler--but the dry return was an overreaction. As I was cutting up what was left of the old pipe, I noticed that, in some places, there was more corrosion on the outside of the pipe, especially where it had been painted. It seems the paint starts to peel as the pipe starts to rust, but it doesn't flake off, so it forms a loose layer around the pipe that traps the condensation that forms when the basement gets warm and damp.

    So, indirectly the water inside is part of the problem, because it allows condensation to form when the water is cooler than the surrounding air, but you can avoid this by insulating the pipes and not painting them.

    If you decide to drain the return, make sure it all drains out. You don't want to leave a horizontal pipe half full of water, because that will expose a lot of surface area to the air. In a typical wet return, air is only in contact with water at the top of the water column in the hartford loop and each drip line. If you have two drip lines and the pipes are all 1" NPS, that's a grand total of about 2.5 square inches.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • elfie
    elfie Member Posts: 266
    will standing undrained water in steam mains cause rust?

    what about a steam main that is not fully draining condensate.  this would be in a situation where the main is lower than the condensate return pipe and when operating the steam pressure lifts the condensate to the condensate pipe.

    however, when the steam pressure is absent (eg, a modulating system) it results in an undrained water from the steam main.

    is keeping standing water in the main likely to cause corrosion over time, and also allow standing water to absorb oxygen

    the comment about condensation that can accumulate on painted pipes in areas where paint chipping/separation occurs.
  • gerry gill
    gerry gill Member Posts: 3,078
    yes it could lead

    to premature corrosion. We just replaced a section of 2'' steam main that held water an sprung a leak where the water sat.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

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