Ashrae Baseline Model & Unmet loads

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I have several VAV units with reheat serving several zones in my Ashrae
baseline model. It is autosizing all of the capacities, so why would I
have unmet loads? I have cooling and heating set to size at 1.15 and
1.25 as Ashrae states, and I have cooling control set to "warmest." I'm
sure it is a control issue, but I can't figure out what exactly it is. I
attached my model, any help would be very greatly appreciated!

Amanda E. Martz, EIT, LEED AP

Martz, Amanda's picture
Joined: 2011-09-30
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I have reverse action set, a range of 6 degrees, night cycling set to
"cycle on any," and a 20 degree delta T. You're right, those usually do
the trick, that's why I'm so stumped.

Martz, Amanda's picture
Joined: 2011-09-30
Reputation: 0

sometimes unmet load hours are logged during the morning start-up warm-up
periods. What's happening is that for every zone (at every hour) the
program is trying to meet the loads with in a 60 minute (1 hour) approach.
If you have a building that has intense internal loads occuring right at the
hour of start-up or warm-up operations it will take more than 60 minutes to
get that space fully conditioned, so now you have 1 hour (per zone) logged
as "hours unmet."

I don't have the chance now to look at your sim file, but maybe set your
start-up schedules to give a 2 or 3 hour building warm-up cycle and see if
that helps your unmet loads.

that's my quick thought/2-cents.


Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400


The main problem is the mechanical penthouse where you?re supplying at 70 F to maintain 90 F. The indoor sizing temp and hourly T-stat setting were the same, which I admit sounds fine in theory. However, in terms of eQUEST auto-sizing, the resulting airflow will just barely be enough ? it will not be sufficient during peak months as indicated by the high number of unmet load hours (nearly all of which were occurring in July and August). The 1.25 cooling sizing ratio is for coil capacity, but that does not help in terms of airflow, and insufficient airflow is nearly always the cause of unmet loads (I have discussed the opposing nature of sections G3.1.2.2 and G3.1.2.8 here: You?re right that it?s usually a control issue, but this case is a little different because your system is on 24/7 to maintain a constant temperature. This is why night cycling did not make a difference.

Before going further, it may be beneficial to take a look at G3.1.1 exception b. I think the mechanical floor may qualify. However, the entire baseline system does not appear to follow Table G3.1.1A; the inclusion of CHW and steam meters implies a district thermal system. I?ll accept what I see at face value without a deeper understanding of the design. However, as noted in the bullets below, I believe you?re trying to model System 7.

In the attached project I made the following changes to AHU-504, the one serving the mechanical floor. Note that only the first two modifications were necessary to eliminate all unmet cooling hours, the rest are simply to incorporate different sections of Appx G:

? Set indoor cooling design temp to 80F with 60F supply temperature, but hourly T-stat setting was left at 90 F. You?ll note that this increased the supply air CFM by about 13%. If 80F/60F is too far off from the proposed design, try gradually increasing the design temp back up toward 90F with the same delta T until the unmet hours start appearing again. If you?re trying to maintain 90F, I would assume the proposed system was at least designed with a little buffer, maybe 85F or something in that range.

? Set indoor heating design temp to 65 F and a Zone Entering Max Supply Temp to 85F for the same reason as described above ? you want at least a few degrees buffer between the design temp and hourly T-stat temps. This introduces 65F unmet heating hours to EL3 South Perim Zn (G.S12), but I don?t think this is a real zone unless the architect is into 87 SQFT triangular rooms. It appears to be a byproduct of trying to achieve the 3rd floor?s custom footprint. If you cleaned up the building geometry, these hours would go away. To illustrate, I added two Air Walls to this zone with the proper NEXT-TO spaces defined, and the unmet heating hours were eliminated.

? Changed the CHW and HW valves from three-way to two-way (the building is small enough that the CHW & HW pumps should be riding their curves per G. and G3.1.3.10, and the pumps were already set to one-speed).

? Set Max Cooling Reset Temp to 65F and Min Cooling Reset Temp to 60F (for a design supply of 60F) per G3.1.3.12. Originally it was set to a Zone Entering Min Supply Temp of 70F and a Max Cooling Reset Temp of 50F, which don?t agree with each other.

? Ensured all conditioned zones under AHU-504 have a heating and cooling schedule assigned, the same design temps, the same thermostat type, and the same 6F throttling range. There was a mix of different conditions, which I assume was a result of trying to address unmet hours.

? Restored default under the Heating/Unitary Power/Furnace section (there is no furnace). However, I was confused by the inclusion of HW coils in the AHU and electric zone reheat. Is this supposed to be System 7 or System 8? The standard VAV terminal units and water-side configuration imply System 7, so I set the Heat Source to ?Not Installed? and the Zone Heat Source to ?Hot Water Loop?.

? Set the Hot Water Loop operation to ?Demand? (there?s nothing in Appx G that says it must be on Demand, but Standby is not a realistic secondary loop operation and a good LEED reviewer will call you on it)

? Reversed the high/low temps in schedule ?Reset temp HWday? ? they were backwards from what G3.1.3.4 indicates.

Try following the above on the other systems and see what it does for your unmet hours. Let me know if you have any questions or concerns about what I did.



Dakota Kelley's picture
Joined: 2011-09-30
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