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The first question depends on what type of boiler we are talking about, #2, fall on a drain line I always ran drain line with as much fall as I could get into them, I've worked on jobs where the steam condensate line froze solid when it had little slope in it & I'm talking about a 8" lines. Before I go into a boiler the gas lines are disconnected & blanked off, electric power to the boiler is locked out & tried out, water drained from the boiler jacket & having it cleaned out is nice but never seems to happen & any steam lines hooked to the boiler are also blanked as well. So your rules above are not good enough.