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  1. #1
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    Running both main boiler and stand-by boiler at the same time

    I was performing an energy audit at a fire station on Kennedy Space Center in Florida when I came across this situation. There are two Fulton 270-MBH gas-fired boilers in the mechanical room for reheat/humidity control of the HVAC system. During the site visits it was found that both boilers were set at 160 °F and both were running at the same time. The design drawing Sequence of Operations states that of the boilers one shall be a stand-by boiler. Note 1 states that the stand-by boiler should be set at 70 °F.

    When I contacted the hot water shop supervisor he told me that they have always ran the system that way because the shop only works first shift and no over time is authorized so if the main boiler went down that the fire station (24/7 operation) would be without reheat for up to 16 hours or up to 2 days if it was the weekend. He opined that running 2 boilers wouldn't use any more energy because the water would just heat up twice as fast, same total energy in.

    I'm looking for a second opinion. The controls are allegedly +/- 10 degrees on these boilers. One that was set at 160 degrees F was reading 168 degrees.

  2. #2
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    Well, if it was engineered as a standby and therefore a single boiler has enough capacity to cover the full load, running both could cause both boilers to short-cycle. However if they need unattended failover capability, there will need to be some kind of control system that can detect a failure and stage on the standby automatically. It sounds like the original design assumed that there would be someone onsite all the time that could switch on the standby.

  3. #3
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    Thread Starter
    Thank you for your well thought out reply. What does it mean "short cycle" of a boiler and what are the ill effects?

  4. #4
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    Quote Originally Posted by markk1pe View Post
    Thank you for your well thought out reply. What does it mean "short cycle" of a boiler and what are the ill effects?
    Short cycling means that the boilers run in short bursts. This can cause excessive wear and tear for a variety of different reasons. Boilers like to have long run times, which isn't possible if they are oversized.

  5. #5
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    Quote Originally Posted by rjk_cmh View Post
    Short cycling means that the boilers run in short bursts. This can cause excessive wear and tear for a variety of different reasons. Boilers like to have long run times, which isn't possible if they are oversized.
    If they are Fulton Pulse boilers they should condense and short cycling wont hurt them as much.
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  6. #6
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    Quote Originally Posted by HVAC_Marc View Post
    If they are Fulton Pulse boilers they should condense and short cycling wont hurt them as much.
    Good point, although the model of boiler wasn't specified by OP. There's a few variables possible in this situation and we don't have much info to go off of.

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  8. #7
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    Agreed.
    This forum requires that you wait 20 seconds between posts. Please try again in 50 seconds.

    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Pecmsg - "NEST should only be used with 3-M filters to guarantee total destruction of the equipment!"

    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  9. #8
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    I agree. We would need more specific boiler info to answer your question OP. Some boilers can modulate to meet load. Others cannot handle cold return water temps, etc....

  10. #9
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    Thread Starter
    From the original post: "two Fulton 270-MBH gas-fired boilers"

    Here is a link to the literature:
    https://www.fulton.com/fultonDotCom/...?ptc=hw&uid=12
    Last edited by markk1pe; 04-11-2018 at 05:58 PM. Reason: add information and link

  11. #10
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    Quote Originally Posted by markk1pe View Post
    From the original post: "two Fulton 270-MBH gas-fired boilers"

    Here is a link to the literature:
    https://www.fulton.com/fultonDotCom/...?ptc=hw&uid=12
    Fulton does more than Pulse/condensing boilers, so the size alone doesnt tell us anything.

    The short run cycles shouldnt hurt them much. But, they wont reach steady state either for maximum efficiency.

    The water temperature should rise at the same rate whether or not you have 1 boiler or 10 running on it. BTUs are BTUs. (dependent upon piping configuration).
    This forum requires that you wait 20 seconds between posts. Please try again in 50 seconds.

    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Pecmsg - "NEST should only be used with 3-M filters to guarantee total destruction of the equipment!"

    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  12. #11
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    if you would increased throttling range on standby it should stay off till water temp dropped.

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