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Excess Boiler Capacity Issue

elfie Member Posts: 264
in this situation, 2 large 14 section cast iron commercial boilers were purchased about 10 yrs ago (steam and hot water).  hot water boiler used for one section and steam boiler used for another area.

combined capacity is nearly 6 million btu per hour (daily btu capacity is 144 million  -  24 hr x 6 million).  On the coldest day of the yr. the total NG use equated to about 275 therms, which converts to 27.5 million btu's created.  On warmer days, this amount is closer to 175.

I have estimated that the excess daily capacity is about 115 million btu's (144m less 27.5) and this was on the coldest day of season. On the coldest  day, capacity used was about 20%. Since it was the coldest day of yr, the boilers were likely operating fairly constantly throughout day to satisfy demand.

Ideally, it would have been alot better to install 2 smaller hot water boilers and 2 smaller steam boilers (so one could be shut off). presently, no backup systems exist.

Is there agreement that the excessive capacity is on the excessive side and are my assumptions reasonable?  and in light of the rather severe excess capacity, what can be done to mitigate the problem short of replacing boilers which tragically is just not feasible.

thanks for any thoughts on situation.


  • JStar
    JStar Member Posts: 2,752

    What are the exact sizes of each boiler?
  • elfie
    elfie Member Posts: 264
    capacity for each boiler

    each boiler capacity is 3 million btuh.

    additionally, for the steam boiler, the available capacity from radiators us about 800,000 btuh; and for this space it only uses half the capacity (space can easily be heated with half of these rads being turned on).

    so it seems odd to use a 3 million btuh boiler.


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,564
    A shocking suggestion (forgive me)

    Could the steam side be re-piped as hot water, and connected to the new tag-team of 2 hot water boilers, whose controls would determine which one would fire? Of course it would be dependent on the suitability of the existing steam radiators to be hot-water compatible.

    This must be a government building, to have such excessive capacity.--NBC
  • JStar
    JStar Member Posts: 2,752
    Hot water

    You can lower the hot water boiler's supply temperature to match the connected radiation's ability to overcome the heat loss. If you know the exact BTUH rating of the radiation, we can reference the supply temperature you will need. Add on a proper outdoor reset, and you could really cut a lot of that uneeded energy usage.

    The steam boiler...well, maybe you can downfire it a little. Probably not enough. Is there anything in the building that could utilize a steam to hot water heat exchanger, such as the domestic hot water, maybe a swimming pool? Then you could be using that extra steam BTUH for something useful.
  • elfie
    elfie Member Posts: 264
    chronic short cycling concerns

    in light of the substantial excess capacity and sharp restrictions on the available rads to deliver the full potential of the boiler, might this be a problem over time to the boiler.

    it never has a chance to go full out when in use.  does this lack of using it to its full potential create a long term health problem (short cycling cant be healthy for the cast iron boiler and the burner machinery).
  • JStar
    JStar Member Posts: 2,752

    Short cycling is never good. You could produce excess condensation in the sections or chimney.

    Maybe, if it's cost effective, you can remove some of the boiler sections.
  • JStar
    JStar Member Posts: 2,752
    I hate to reply to myslef, but...

    If you removed a few sections you could keep them on site for future use. Then, hopefully, you can derate the power burner, or worst case, change it out.
  • Zman
    Zman Member Posts: 7,330
    Differential and buffer
    1. On the hot water boiler the best (economical) way to reduce short cycling is to increase the on/off differential.There are many controllers that will do this automatically. Warm weather shut down and outdoor reset would also help. Check out tekmar controls.
    2. A buffer tank would also help with the cycle lengths although it may not be needed given the volume and mass of your cast iron boilers.
    3. Carl

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
This discussion has been closed.