ASHRAE Journal - January 2020 - 15

TECHNICAL FEATURE

Using EnergyPlus v 9.0.1, a real ofﬁce building located
in Toronto, Canada (ASHRAE Climate Zone 6A) was
modeled. A typical ﬂoor of the building was selected to
represent the building, as shown in Figure 1. The building has a ﬂoor area of 1,728 m2 (18,600 ft2) per story and
has been modeled with ﬁve thermal zones: four perimeter zones and one core zone, all representative of an
open-plan ofﬁces. The south façade has a gross windowto-wall ratio of 85%, while the east, west, and north
façades have a WWR of 41%, 41% and 12%, respectively.
The building is ventilated, cooled, and heated using a
rooftop package unit (RTU) with variable air volume
(VAV) terminal devices and hydronic radiant baseboard
heaters. Hot water for space heating is supplied by

20
15
10
5
0

6
4
2
Architect

Energy Modeler

Mechanical Engineer

Internal Gains Assumptions (Btu/h·ft2)

Methods

Internal Gains Assumptions (W/m2)

safety and egress.3 Mechanical engineers typically use
FIGURE 1 Case study model.
modeling to perform heating and cooling load calculation for the purpose of sizing HVAC equipment, while
energy modelers conduct modeling to demonstrate
that the proposed design meets the code speciﬁed
64.00 m
4.20 m
performance on an annual basis. Architects may use
energy modeling to guide selection of appropriate early
27.00 m
design strategies in relation to geometry and materials. Given the various modeling objectives, associated
risks and liabilities, the assumptions made by each
design discipline are typiTABLE 1 Occupant-related assumptions per stakeholder (total occupant heat gains is assumed to be 130 W4).
cally not the same. This disOCCUPANT
TOTAL OCCUPANT
LIGHTING EQUIPMENT POWER
WINDOW
MODELING
crepancy in assumptions can
OBJECTIVES DENSITY m 2/PERSON
HEAT GAINS W/m 2
POWER
DENSITY W/M 2 BLINDS USE
(ft 2 /PERSON)
(Btu/h·ft²) DENSITY W/m 2
(Btu/h·ft²)
lead to unsubstantiated and
(Btu/h·ft²)
oftentimes conﬂicting design
Architect
Form,
14
9.3
10
25
Always
decisions. This discrepancy
Orientation,
(150.7)
(2.6)
(3.2)
(7.9)
Open
WWR
can be due several factors
such as the lack of effective
Energy
Code
20
6.5
3.87
9.8
Always
Modeler
Compliance
(215.3)
(2.1)
( 1.2)
(3.1)
Open
communication amongst the
Mechanical Cooling Load
9.3
14
5.4
17.2
Always
design disciplines, the difEngineer
Calculation
(100.1)
(4.4)
(1.7)
(5.5)
Open
ference of tasks and duties
Heating Load
0
0
0
0
Always
among stakeholders, and the
Calculation
Open
degree of BPS literacy of the
design stakeholders. In other words, it is not expected
FIGURE 2 Light power density, equipment power density, and total occupant heat
that all design stakeholders will perform modeling at
gains assumptions per design team member.
same level of detail. However, they should have simi16
50
Equipment Power Density
lar assumptions to start with. This article discusses a
45
Light Power Density
14
simulation-based study that was conducted to demTotal Occupant Heat Gains
40
12
onstrate the impact that different occupant-related
35
10
assumptions have regarding the energy-savings poten30
25
8
tial of key design decisions.

condensing boilers with a thermal seasonal efﬁciency of
90%. The building also has an energy recovery ventilator
(ERV) with 85% sensible recovery serving the RTU.
Table 1 and Figure 2 show LPD, EPD, occupant density,
and equivalent internal gains assumed by the architect,
the energy modeler, and the mechanical engineer. These
values were obtained during interviews with the project
stakeholders. These values shown for each discipline
have different sources: designer experience and code of
practice (e.g., mechanical engineer assumptions), the
JAN UARY 2020

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ASHRAE Journal - January 2020

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