DHW Load Exceeding DHW Capacity

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Does anyone know why eQuest allows my DHW assigned load to exceed my DHW
capacity in eQuest 3.65 (7175) DOE 2.3.

My project has 4 DHW boilers sequenced using parallel configuration. I'm
looking at an hourly report and there are hours of the day where the load
is 470 MBH and my first boiler is assigned a load of 470 MBH while its
operating capacity is 392 MBH. No other boilers are active for that hour.
Does some of that unmet load get carried over to the next hour?

Also, does anyone know how eQuest decides which DHW boiler (for multiple
boilers in parallel configuration) is active for any given hour?


Chris Hadlock's picture
Joined: 2012-01-23
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Hi Chris,

I?m a little rusty on how this might be different for DHW loops vs. HHW loops/boilers (it?s been some years since I did building energy simulations on the regular), but I?ve definitely modeled DHW systems on HHW loops if that?s pertinent.

What I recall is:

1. If needed, you can define and/or adjust the DHW loop?s (process) load schedule as-generated by the Wizards (which in kind will develop a single schedule for a project considering a weighted average of building occupancy types from your shell input screens).
2. As for sequencing, my intuition is that before you do anything additional (like define an EQUIP-CTRL command and/or set up a LOAD-MANAGEMENT), eQuest/doe2 will by default and generally
* Activate the minimum number of loop-connected machines to satisfy the load of the moment, and will evenly distribute that load between those machines.
* Assuming your primary equipment has different capacities, it?ll work out each interval what combination of equipment capacities ?best fit? the load on the loop and consider those machines active
* Assuming your primary equipment has equal capacities, it?ll engage the equipment in the order it?s defined, without trying to equalize runtime or similar.
* All of this may not be the most efficient sequence or otherwise reflect real-world design or installed conditions, but it?s typically a safe starting point assuming attached machines are of a similar type/vintage.
3. If you need/want to more explicitly define or revise default equipment staging behavior, then IIRC it?s good advice to try and accomplish that with an EQUIP-CTRL alone first, as combining with a LOAD-MANAGEMENT command can open up new options but can also grow unnecessarily complicated in practice, at least sometimes.

If you are observing loads in excess of an individual actual boiler?s capacity, without engaging a second unit, I would start with reviewing the boiler?s capacity input. Wizard-generated projects often allow primary hydronic equipment to auto-size to the load of the loop, in which case you?ll observe a blank field you need to input to specify rated capacity of the equipment. From there, assuming you have something input to cap or at least assert design capacity, both DW and HW boilers have inputs which allow for further restricting ratios of output beyond design and/or asserting minimum operating ratios, which could be fruitful to explore depending on context specific to your model and how it?s set up.

If your hard capacity limits for the equipment are not able to keep up with the loop load in a sustained/substantial fashion, I think you can expect to see something corroborating that effect in the detailed simulation report ATTN messages as a CAUTION and/or WARNING. IIRC there are also ?overload? hours that should be noted in one or more of the PS reports.

If everything seems to check out after reviewing capacity related inputs and additional units are just not engaging, I might next suspect and look into either loop inputs and/or user-generated sequence inputs resulting in making one or more primary equipment units unavailable in the simulation.


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Nick Caton, P.E. (US), BEMP
??? ????, P.E. (US), BEMP
Senior Energy Engineer
Energy Manager, Yokota Airbase
ESS - Energy & Sustainability Services
+81 . 070 . 3366 . 3317
+1 . 785 . 410 . 3317
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nicholas.caton at se.com
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Nicholas Caton2's picture
Joined: 2019-03-25
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