Hot water loop demand and boiler sizing

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Energy Modelers,
I am getting the following warning message when I type in the actual boiler capacity:

"Hot Water Loop: the heating capacity is smaller than the secondary demand. Primary = -2000000 and Secondary = -2992234."

If I let eQuest size the boilers, everything is I know it has to do with me putting in the actual boiler capacity for the proposed design. However, the issue here is there is no way the max heating demand is almost -3.0 MBh as it states in the warning message. I can not figure out where eQuest is getting this number; especially since the SS-D report (which should tell me the absolute max load demand on the building HVAC system - including outside air) says the maximum heating load is -1035.026 MBh. Can anyone please help me determine why I am getting this error, where the -2992234 number is coming from, and if the SS-D report really is the actual maximum HVAC load demand? Thanks everyone for their help. Your time is very much appreciated.

Josh Doherty

Josh Doherty's picture
Joined: 2011-09-30
Reputation: 0

I constantly struggle with the same issue (loop sizing) and whether or
not it is actually having an impact on energy consumption.

In a recent project I did, I had the same results as you. However, I
went to the hour-by-hour report page for the plant, and using your
example below: I would get something like -2992234 buth at 8am,
-1000000 at 9am, -2900000 buth at 10am, etc.. The heating load was
bouncing high and low, meaning the loop had to be sized for -2992234 but
the actual maximum heating load that your system would see in actual
operation was closer to the average of these two "bouncing" numbers (ie
-2000000 in your case).

Never figured out why, and posted a question here and received no
answers. Stepped utility block was 15 mins, didn't have any weird
temperature setpoints, etc.

Ultimately I don't think these warning messages are too concerning, but
I also know NEVER to use eQuest to do my load calcs for maximum
heating/cooling equipment sizing.

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

I had a similar error recently and adjusted LOOP-SIZE-RATIO on the Basic
Specifications tab for the loop

The safety factor for loop sizing. (Note that safety factors could have
already been specified within the secondary HVAC systems attached to this
loop). This keyword is used only when SIZING-OPTION = SECONDARY-EQUIP; it is
ignored for the PRIMARY-EQUIP sizing method. When primary equipment is to be
auto-sized by the program, this keyword modifies the default capacity
assigned. Note that this keyword affects only the auto-sizing of the primary
equipment based on secondary loop loads. It has no effect on the design loop
flow; the loop flow is based on the sum of the coil flows or other loads
attached to the loop. However, the design pump flow can be modified via
keywords in the PUMP command.

Rich Tesoriero's picture
Joined: 2011-09-30
Reputation: 0