Using measured equipment load profiles to 'right-size' HVACsystems and reduce energy use in laboratory buildings (Pt. 2) Page: 7 of 14
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Some designers suggest that equipment load is not relevant for rightsizing, because HVAC
equipment sizes are driven by ventilation requirements, not equipment loads. While this may be
true in some instances (e.g. laboratories with low equipment loads and high minimum
ventilation rates), it cannot be generalized. It should also be noted that ventilation requirements
themselves are often set at an unnecessarily high level. Some organizations have recently
revisited their minimum ventilation requirements and revised them downward. The chiller
tonnage analysis above, which assumed 1cfm/sf minimum ventilation requirement, clearly
shows that equipment loads affect chiller tonnage in a wide range of climatic conditions. It is
recommended that designers at least do a sensitivity analysis during design, to assess the impact
of equipment loads on HVAC sizes.
The most common argument against rightsizing is the risk of under-sizing and who carries that
risk. As many design engineers have observed, the legal and contractual basis for design services
rarely rewards rightsizing, and almost certainly will penalize under-sizing. Right-sizing requires
that owners and designers come to agreement on the basis for right-sizing and the associated risk
management. This was the case with the design for the central plant of the new University of
California campus as Merced. In order to avoid the problem of over-sizing the central plant, the
owner took the initiative to right-size the plant based on benchmark data from other campuses
[Brown 2002]. Instead of just using design values that assume a worst-case estimate, a "most
likely maximum" (MLM) load was also used and the difference between the MLM and the design
loads were value-engineered to reach a reasonable margin of safety.
The Labs21 Environmental Performance Criteria [Labs21 2005, Mathew et al. 2002] recommends
the following approach for right-sizing HVAC systems based on measured equipment loads:
"...For each comparable laboratory space, obtain one week (7 days) of continuous power
metering at a distribution panel level of all laboratory equipment, including plug loads and
hard-wired equipment....Metering data should be obtained while the spaces are fully
occupied. Continuous metering data should be time averaged over 15 minute time periods.
Design heat load criteria for each typical laboratory space in the facility should then be based
on the maximum load indicated over the metering period..."
It should be noted that this approach represents a minimum requirement and longer or more
detailed measurements may be required for specialized situations.
In addition to using measured loads for right-sizing, probability-based analysis (PBA) can also be
used, especially if there are no comparable laboratories available in which to measure loads. A
more detailed description of PBA is beyond the scope of this paper, and is provided by Martin
. PBA is essentially a "bottoms-up" approach to derive diversity factors for space loads
based on aggregating probability of use for each heat source in the space. The two approaches -
PBA and load measurements - can be used in conjunction with each other, providing two
reference points for right-sizing.
3 Simultaneous Heating and Cooling
3.1 Measured Load Variation
As noted in the introduction, over-sizing is only one of the problems resulting from incorrect
estimation of equipment loads. The other major problem is the under-estimation of load variation
across different laboratory spaces, which in turn exacerbates the problem of simultaneous heating
and cooling, particularly for systems that use zone reheat for temperature control. Figure 5 shows
the range of 15-min interval power for various laboratory spaces in the UC Davis laboratory
building referenced earlier. The maximum for most spaces is under 6 W/sf. A few are between 6
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Mathew, Paul; Greenberg, Steve; Frenze, David; Morehead, Michael; Sartor, Dale & Starr, William. Using measured equipment load profiles to 'right-size' HVACsystems and reduce energy use in laboratory buildings (Pt. 2), article, June 29, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc901308/m1/7/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.