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Stage and Rotate

John L
John L Member Posts: 118
Brad, thanks for the insight. Very informative as usual

John L.


  • John L
    John L Member Posts: 118
    Stage and Rotate

    Looking for insight on the staging and rotating of boilers and is it similar/different to the leading and lagging concept.
    When it is best to use either method.

    John L,
  • Brad White
    Brad White Member Posts: 2,398
    Staging and Rotation

    are similar and interrelated concepts, John.

    Rotation is more accurately called "lead-lag" but that too is not entirely accurate.

    In conventional/traditional "lead-lag" control, it presumes two or more units, boilers in this case. The common practice to have "first on, first off" with the second or subsequent boilers leading the charge for the next cycle. The idea of course is to have even wear and tear on the equipment.

    Nowadays, conventional controls have an "hours of use" meter for each boiler such that each 48 hours of operation shall force another boiler to take the lead next time. The hours of operation are adjustable, but 48 hours I have seen as a "pre-programmed default" in some controls. In our specifications, we state a default weekly swap-out, same day and time of day each week. Good a thing as any but it is all adjustable.

    Now, staging: This is more a function of capacity control than even wear and tear, especially where the plant consists of several sub-total boilers. In other words, each boiler capacity is a fraction of the total load. May be two boilers at 50% or 67%, three at 1/3, four at 1/4, etc. such that any one boiler out of service leaves you with at least 2/3 capacity if not 100%. (Some plants such as hospitals, prisons and mission-critical buildings "where you cannot send people home") have "N-plus-one" or more, such that any one or more boilers out of commission leave you with at least 100% capacity. Whew.

    In staging, (remember, capacity control), one boiler fires until it can no longer handle the load (drop in space temperature or target HWS temperature tells it so). The second boiler (or third, fourth, however needed) come on to meet demand.

    Here is how the rotation might then occur: First on, first off. Second on, second off.... thus each has a similar but not guaranteed number of run-hours. That last stage especially may have only a few compared to the first.

    This is why number of run-hours makes the most sense to me. Boilers run in order until their "numbers are up", assuming equal wear and tear is the goal.

    Does that make sense?

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Empire_2
    Empire_2 Member Posts: 2,343

    Just as a complement,...I love to hear your insight and thoughts as to your opinions. You are CLEARLEY an ASSET to this forum, just in case you haven't been properly thanked before.:-) Thank you!

    Mike T.
  • Brad White
    Brad White Member Posts: 2,398
    Thank you, Mike

    Wow, that was very nice of you to say. Thanks- it made my day!

    Still, I learn as much as I may help but it is appreciated!

    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Rich Kontny_5
    Rich Kontny_5 Member Posts: 116

    Sequencing is basically the same thing except you can change the sequence to assure equal use (or wear)

    Rich Kontny
  • hot_rod
    hot_rod Member Posts: 22,108
    many of the current mod cons

    have this feature built into the control. it even allows exercise programs, data logging, error logging and much more.

    The method for step firing mod cons differs from a traditional boiler sequence control. The control watches the output temperature and will actually ramp down the "first on" to keep it in the most efficient condensing mode and ramp in number two at low fire.

    tekmar probably explains it better than that ;)

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
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