Boilers sized by eQuest

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Hi -
My model let eQuest to size the boiler capacity based on 'secondery' demand to
see how large the size of the boiler will be. There are 2 equally sized
boilers. I made the capacity and capacity ratio of each boiler default (blank).
The output report shows the rated capacity for each boiler is 4.037 MBtu/h, and
boiler A works at all spectrum of part load for most hours when heating is
needed, while the boiler B only work several hours at very low part load
percent. The peak load for boiler A is 4.125 MBtu/h, the peak load for boiler B
is only 0.058 MBtu/h.

Since boiler B works less efficiently, I try to downsize the boiler capacity to
reduce equipment cost and heating energy use. If I input a capacity ratio of
<0.5 for each boiler, eQuest will give out a warning that the hot water loop
heating capacity is less than the secondary demand. However, the percent of
hours outside throttling range was not affected.

Is the warning a concern? Or any alternative way to reduce the size of the
boiler and make them ?work? more efficiently in eQuest?

Many thanks in advance.


yizhao1 at's picture
Joined: 2011-09-30
Reputation: 0


Have a look at the PS-H report for your boilers. It's likely that eQuest
oversized the boilers dramatically. You could look at the percentage of
time the main boiler spends above the 50% of full load bin and if the number
of hours is not significant, limit the boiler capacity accordingly.

You can also control the boiler staging with an equipment control
(EQUIP-CTRL keyword). Have a look at the DOE2 help files for instructions
on how to set one up. An excerpt: "This command allows you to override the
default load allocation routines (in which the program automatically selects
the mix of equipment with a combined capacity most closely matching the load
but preventing an overload condition)."

Hope this helps,

Steven Savich

Steven Savich's picture
Joined: 2011-09-30
Reputation: 0

Try running the simulation with one boiler to get the auto-sizing. Then enter your own sizing for each when you add the second boiler, whatever that percentage would be.

David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

I tried put the capacity ratio of 0.5 for each boiler, the results remained the
same. eQuest said if no CAPACITY-RATIO is specified, the capacity defaults to
the maximum design-day heating loop capacity, divided by the number of boilers
attached to the loop, and multiplied by the loop?s SIZING-RATIO.

The heating energy use drop a lot and boiler B will work a little bit better if
a ratio of <0.5 is used for each boiler.

Quoting Xiang Liu :

yizhao1 at's picture
Joined: 2011-09-30
Reputation: 0

Sizing in eQUEST is always a problem. I ran an office model with a HW loop.
PV-A report calculates the heating capacity of the HW loop to be 0.834
MBtu/h and size each boiler to be 0.417 MBtu/h. However PS-D indicates the
peak load of the loop is 0.4117 MBtu/h, less than the capacity of a single
boiler. This is why one boiler is enough to meet the load.

If we want both boilers to carry similar load, the size of each boiler shall
be 0.206 MBth/h, half of 0.4117 MBtu/h. The sizing ratio of each boiler
ought to be 0.206/0.834 (HW loop capacity from PV-A report) =0.247.

This change brought similar loads to both boilers and improved their overall

Xiang Liu's picture
Joined: 2011-10-02
Reputation: 0