Modeling Boilers

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Equest World,

I am trying to fix the boilers in the model and I have a question (both
conceptual and modeling) related to what capacity to model. This is a new
construction project where I am following 90.1 appendix G.

This is what is going on with the heating boilers (I'll use simplified

- There are 7 hot water boilers that are used for the HHW (total
capacity of 2500 MBTUH)

- There are a electric fan forced heaters in bathrooms and
stairwells that (70 MBTUH)

- There is a community room in the basement that has a separate
unit that provides both heating and cooling (heating capacity of 70 MBTUH)

- The heat from the boilers is sent mostly through baseboard
radiators and a few hydronic systems (total heating capacity of (1200 MBTUH)

- From the above info, the capacity of the boilers is 2500 while
the total capacity for the heating units that need the hot water is 1200.

o Is this common?

o Did the designer screw up?

o How would you model it in both the baseline and proposed model? (ie
which capacity am I using, or should I just have eQuest autosize the boilers
off of the cooling design?)

I hope this makes sense. Thanks again for everyone's continued help.
Sooner or later I will have enough skills where I can start answering calls
for help like mine.


Kevin Kelly, LEED AP

Kevin Kelly's picture
Joined: 2011-09-30
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In my (limited) experience, it is not at all uncommon for the boilers'
capacity to exceed that of the heating water loop. Often times this is
done intentionally to prepare for building expansion in the future.
Double seems a bit odd, but perhaps the engineer was instructed by the
building owner to do so for redundancy or perhaps the building owner was
planning on a major expansion in the future. Maybe he just wanted to
CYA, too.

To answer your modeling question, I personally never allow eQuest to
auto size plant equipment. (Regardless of Baseline or Proposed) The
reason for this is because different efficiencies exist for the
equipment depending on how loaded the equipment is. In order to properly
simulate the amount of energy that your plant must consume to provide
for the correct heating (or cooling) load, eQuest has to know the
part-load performance of the equipment and how often that equipment
operates at those partial loads. It is also imperative that the correct
boiler curves are input to calculate this part-load performance as well.
Most of the time eQuest's curves suffice, but I have made my own before
in some instances where I felt I could better match the particular
equipment that was going to be used on site.

Hope this helps,

Gary Schrader

Gary.Schrader at's picture
Joined: 2011-09-30
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