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Derate oversized boiler?

budalb
budalb Member Posts: 20
in THE MAIN WALL
I have an oversized boiler that hit the target supply temperature very quick, causing short cycling. It is Navien NCB-240. I read the manual and found that I can set the max firing rate to be between 50-100%, which essentially derates the boiler, solving the oversize issue. Any downsides? Thanks

Comments

  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
    Use the SlantFin app to determine the heat loss of the building as a guide to how oversized you are, and report back here for more info.—NBC
  • budalb
    budalb Member Posts: 20
    I did a rough heat loss calculation. It is about 40,000 btu/hr. The NCB-240 is rated as 112,000 btu/hr output. The contractor used the same boiler capacity as the old oil fired cast iron boiler. With the higher efficiency, the boiler is even more over-sized. If I just derate the NCB-240 to 50% or 60% of its full capacity, does setting the partial firing rate permanently hurt the boiler life? I guess it won't, because the boiler is designed to be modulating. Am I right?
  • motoguy128
    motoguy128 Member Posts: 393
    Yes, you can lower max firing rate. That will sometimes help scenarios where the boiler tends to ramp up too fast then short cycle.

    The other adjustment you might make is to widen the range of temp where it shuts off and when it cycles back on.

    Next you need to make sure it’s not over pumping. Downfiring max rate is usually needed when a boiler is overpumped. IF flow is too fast, (internal pump flow rate is around 10GPM on high speed and I think 6 on low speed if pipied primary/secondary as required). There is no need for the primary pump to go faster than the secondary.


    What’s the application? Temperatures? Minimum fire rate is around 15k on that size. It’s 11 on the smaller model, so not a big difference, but most homes In northern climates need the extra DHW capacity.

    I‘ve installed about 6 of these Navien NCB’s in the last year. Don’t have any call backs once sent up correctly if the radiant load is designed right.
  • budalb
    budalb Member Posts: 20
    Thanks. I am in CT. I guess the firing rate for DHW is a separate setting.
  • motoguy128
    motoguy128 Member Posts: 393
    Yes.  Separate setting. 
  • budalb
    budalb Member Posts: 20
    This boiler is installed residential home in northeast region.

    I am trying to set burner on/off temperatures. After reading the manual another time, I have the following questions.

    "Setting the Burner-Off Temperature" (0-54F): The manual says "During space heating, the boiler turns off the burner when the space heating supply temperature meets or exceeds the burner-off temperature." I guess the burner will turn off if >= (SWT target - ) . For example, Set SWT = 180F, burner-off = 4F. The burner turns off when SWT hits 184F. Correct?

    "Setting the Burner-On Temperature" (5-54F): similar interpretation. For example, Set SWT = 180F, burner-on = 10F. The burner turns on when SWT drops to 170F. But it appears to me that the burner turns on when SWT drops to ~140-145F. Any comments?

    Thanks
  • GrundfosHVACOEM
    GrundfosHVACOEM Member Posts: 2
    Does the Boiler have a o-10vdc out put or PWM to control the boiler pump? The best and may help is to have the Boiler and Boiler pump to be in sync The Boiler ramps up so does the Grundfos pump the Boiler slows down so does the Boiler pump
  • budalb
    budalb Member Posts: 20
    I don't know. This question is too hard for me.
    GrundfosHVACOEM said:

    Does the Boiler have a o-10vdc out put or PWM to control the boiler pump? The best and may help is to have the Boiler and Boiler pump to be in sync The Boiler ramps up so does the Grundfos pump the Boiler slows down so does the Boiler pump

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