District Energy / Fourth Quarter 2012 21© 2012 International District Energy Association. ALL RIGHTS RESERVED.

Understanding Building Energy Models:Speaking your customer’s languageTim Griffin, PE, LEED AP, Principal, RMF Engineering Inc.;Dave Crutchfield, PE, LEED AP, Principal, RMF Engineering Inc.

Feature Story

system has on our customers’ sustainabil-

ity goals. To achieve a certain LEED rating

level, all the rating system’s prerequisites

plus a certain number of points must be

achieved for meeting standards in sev-

eral categories. By far the most points

are available for demonstrating that a

potential project building’s annual energy

use will be significantly less than a build-

ing that meets minimum code-regulated

efficiency. To demonstrate compliance,

designers develop computer-generated

energy models.

Project teams pursuing a LEED rat-

ing for proposed or existing buildings

that are or will be connected to district

energy must account for that district

energy in these models. Since a signifi-

cant portion of a building’s energy use

is often directly related to the genera-

tion of heating and cooling, the impact

of district energy on the outcome of

these models can be significant. This

has often caused heartache and confu-

sion within the district energy commu-

nity – most often from a lack of under-

standing of how these energy models

are developed and function.

Therefore, to best respond to our

customers’ needs, we must first have

a basic understanding of how these

models are developed and utilized. The

intention of this article is to provide

this basic understanding and to high-

light the key issues related to modeling

and district energy, including how a

model is developed throughout each

phase of a project.

To best respond to our customers’

needs, we must understand

how building energy models are

developed and utilized.

Building Energy Models: The Basics Building energy modeling is a pow-

erful tool that predicts building energy

use. Different types of energy models

are developed at various stages of the

design and construction period to pro-

vide data that can verify or disprove

suggested energy efficiency measures.

To understand how to assist modelers,

district energy system operators should

understand what the model is intended

to do during each stage of the design

and construction. To complete a model,

District energy system customers

from college and university

campuses, federal facilities

and municipalities are often interested

in achieving a U.S. Green Building

Council (USGBC) LEED® (Leadership in

Energy and Environmental Design) rat-

ing on their facilities. Why? Because

their customers are demanding it, and

in many cases the law requires it for

publicly funded buildings. In the spring

of this year, USA Today reported on

a Princeton Review survey that found

68 percent of more than 7,000 college

applicants said a campus’s commitment

to the environment would play a role

in their decision to apply to or attend

that school (complete survey at www.

princetonreview.com/greenguide). For-

tune 500 company shareholders and

customers also want their organizations

to demonstrate environmental steward-

ship, and commercial tenants will often

pay higher rents for environmentally

friendly office spaces. The USGBC’s

LEED rating system provides a brand

that allows building owners to satisfy

these concerns with their stakeholders.

Therefore, it is imperative that we, as

district energy system owners, designers

and operators understand the impact our

22 District Energy / Fourth Quarter 2012 © 2012 International District Energy Association. ALL RIGHTS RESERVED.

the engineers developing the energy

models need information from archi-

tects, other engineers and even contrac-

tors. In cases involving district energy,

modelers also need input information

from system operators.

Before the existence of the USGBC,

building energy modeling had been uti-

lized by mechanical engineers to deter-

mine building and space heating and

cooling loads, and to compare system

options in terms of both energy effi-

ciency and lifecycle cost. This tool gave

engineers the data needed to assist

owners in making the best overall sys-

tem selections. Over time, the goal of

building modeling evolved to be finding

the balance between many competing

factors, such as occupant views, increas-

ing the amount of fresh air to the build-

ing, appropriate lighting levels, control-

lability of the system, first costs and

energy costs.

Today, many construction projects

are required to meet sustainability goals

by local law through code requirements

or by direction of the building owner to

pursue voluntary rating systems such

as LEED or Green Globes. One of the

main sustainability goals consistent with

these programs is to optimize building

energy efficiency. The reduction of the

building’s energy usage relates directly

to an overall reduction in lifetime

owning/operating costs. Studies have

shown that often less than 2 percent of

a project’s construction cost is spent in

the form of design fees that determine

greater than 90 percent of the overall

lifetime cost in the form of energy usage,

maintenance, etc. With this much at

stake, energy modeling can become a

useful tool for owners as they are asked

to make decisions on the project. In

addition, energy modeling can verify

to authorities having jurisdiction that

minimum energy efficiency goals have

been incorporated into the design.

Building energy modeling is a com-

mon tool that allows engineers to math-

ematically model the building’s perfor-

mance over a period of time to gain an

understanding of the potential building

energy usage. There are three popular

energy modeling periods: during design,

preconstruction and post-construction.

Since each energy model is done when

information is either not available,

becoming available or completed, there

are specific requirements and concerns

with each energy model created.

Design Phase Modeling Energy modeling during design is

typically used to compare options pre-

sented by the design team. The goal of

modeling at this stage is generally to

develop a set of guidelines and base-

lines for the building design. The energy

model at this phase often includes

different HVAC system types, utility

rate structures and energy efficiency

measures. Energy efficiency measures

include building feature alternatives.

While modeling during the design

phase is useful, the energy modeler is

typically using many assumptions to

complete the model. Having multiple

assumptions would present a problem

with a later energy model, but during the

design phase, as long as the assumptions

are reasonable and applied equally to the

proposed building and baseline building

models, they provide a reasonable level

of comparison between input options. At

this project stage, using the model to

create comparisons that allow the verifi-

cation of design assumptions has more

value than a single detailed model.

Modeling many different energy

efficiency measures is critical to ensur-

ing that the building will ultimately be

Mechanical engineer Matt Mumpower develops a building energy model.

Co

urt

esy

RM

F En

gin

eeri

ng

Inc.

Figure 1. Preconstruction Energy Model and Finished Building. (Note the architectural element added

after the design was completed.)

Source: RMF Engineering Inc.

District Energy / Fourth Quarter 2012 23© 2012 International District Energy Association. ALL RIGHTS RESERVED.

constructed to be as energy-efficient

as possible. The most common options

being modeled are changes to the build-

ing envelope, building orientation,

fenestration and HVAC system types.

Architectural changes, such as shown

in figure 1, can have a profound effect

on building energy use, but they often

come with concerns about how they

affect building aesthetics. Being able

to discuss the energy savings of each

architectural change helps lead the

design team to develop truly energy-

efficient design concepts and not sim-

ply focus on HVAC system changes.

The solar load on the building is

a major component of the mechanical

system size. Modeling different building

orientations allows the owner and design

team to understand how shifts to the

building’s direction can affect lifetime

energy usage. The correlation of build-

ing orientation to window location leads

to the ability to optimize lighting levels

in the building. Harvesting daylight and

reducing lighting loads can have a pro-

found impact on lifetime building energy

use, so incorporating that into an early

energy model is valuable.

Options for the HVAC system type

are frequently modeled to understand

which systems may be more effective

for the building. It is common to com-

pare typical direct expansion systems to

water-cooled systems or even ground-

coupled systems. This allows the owner

to understand not only the potential

energy savings among the options but

also to investigate first-cost differences

and lifecycle cost differences with the

benefit that educated system selections

are made representing total lifecycle

cost savings.

At the end of this stage, the fun-

damental building choices are usually

made and a design direction clearly

established. It is also at this stage

where the proposed building’s ability

to meet LEED energy efficiency require-

ments is first determined as well as its

potential to achieve points for enhanced

energy efficiency.

Included in this analysis is the

potential impact that connecting to

district energy may have on both these

questions. This is often where the deci-

sion whether to connect to district ener-

gy or provide standalone thermal sys-

tems is made. Although more detailed

analysis occurs during preconstruction

phase modeling, designers often must

know during this phase of the project

whether or not physical space and infra-

structure are required to accommodate

thermal generation equipment in order

to move the design forward.

Preconstruction Phase Modeling After the initial energy models are

completed during the design phase and

a design direction has been defined, a

preconstruction energy model, often

described as the “proposed building”

energy model, is developed, which

serves a new and different purpose.

At this stage, the proposed design

energy model is used to compare to

a separate energy model representing

code-minimum efficiencies. The inputs

for the proposed design energy model

are taken from the latest design docu-

ments so the proposed building energy

model accurately represents the actual

envelope components, fenestration,

orientation, HVAC system type, etc. The

proposed building model should begin

to remove as many of the assumptions

as possible that might have been built

into the earlier design models. The

model will include building shading,

self-shading, improved insulation, high-

performance glass, etc.

The energy model at this stage

should not be used to predict

energy consumption or set

energy budgets.

With both the code-minimum and

proposed design building models devel-

oped, a comparison between them is

used to express the energy usage of

each model. This stage of the design is

generally the earliest appropriate time

to discuss building energy savings, as

many of the input variables are solidi-

fied in the design drawings. It is also

the point where it is possible to deter-

mine the approximate number of LEED

points related to energy efficiency for

various options and, as a result, confirm

the impact that connecting to district

energy may have on achieving a desired

LEED rating level.

It is important to note that the

energy model at this stage should not

be used to predict energy consump-

tion or set energy budgets. While the

number of assumptions built into the

model at this stage is less than the

number built into the model earlier,

there are still unknowns. Assumptions

such as occupancy numbers, schedules,

control schemes and final weather

data are still in progress at this stage.

ASHRAE Standard 90.1 provides the

following disclaimer: “Neither the pro-

posed building performance nor the

baseline building performance are pre-

dictions of actual energy consumption

or costs for the proposed design after

construction. Actual experience will

differ from these calculations due to

variations.”

The next energy model, post-con-

struction, is more appropriate for com-

parisons to energy bills. The goal of the

model at this stage is to compare the

baseline model to the proposed building

model to see potential energy savings.

Post-Construction Phase Modeling Post-construction energy modeling

is the phase in the project when the

design and construction are to the point

where the majority of the assumptions

used in previous models can be validated.

The energy model at this point can be

made quite specific to the project.

The starting point for this model

is the as-built documents. All envelope

components, fenestration, HVAC system

type, occupancy type and schedule, con-

trols schemes, lighting levels and gener-

al power requirements can be accurately

modeled using the design documents

and shop drawings for the project’s

actual construction components.

At this stage, the energy model is

often used to compare to the actual

energy bills. It is common for the ener-

gy bills to be different from the com-

24 District Energy / Fourth Quarter 2012 © 2012 International District Energy Association. ALL RIGHTS RESERVED.

Figure 2. Variable Air Volume System Screenshot, eQUEST® Software Version 3.64.

Source: Seth Spangler, RMF Engineering Inc.

pleted final energy model. This is usu-

ally due to differences in typical meteo-

rological year weather data used in a

model versus the actual weather profile

for a particular year or actual building

usage varying from modeled building

use schedules. An energy model that is

within 10 percent of the actual energy

bill is generally considered one that pro-

vides good accuracy. Often the energy

model is manipulated to match trended

data from the building control system

to allow for precise schedule matching

and electrical usage.

The goal of the model at this stage

is to verify that the building is perform-

ing as expected and, if it is not, to help

isolate underperforming systems or

components.

The Guts of the Model The output of an energy model will

estimate the total annual energy use of a

proposed building. This includes energy

used for comfort heating, cooling, domes-

tic water heating, lighting, and general

power use for computers, non-HVAC

equipment, etc. General power require-

ments are referred to as “plug loads”

and are set in a model, per ASHRAE 90.1

guidelines, at 25 percent of the total

annual energy use unless a modeler can

provide evidence they are lower. Domes-

tic water energy use is determined by

occupancy loads and schedules. Light-

ing energy use is determined by lighting

types and use schedules.

The bulk of the modeling exercise,

however, goes into determining the

annual energy use required for heating

and cooling. To do this, the model first

calculates the space heating and cooling

requirements for all 8,760 hours of a

typical year based on regional weather

data; building envelope information

(wall and roof insulation, fenestration,

building orientation); exterior shading;

sensible and latent heat from building

occupants and outside air; and sensible

heat from equipment.

Of the five major component inputs

that go into creation of the model, the

HVAC system type tends to be the one

that requires the most understanding

and agreement between owner and engi-

neer. There is a multitude of HVAC sys-

tems that can be provided to condition

a space and therefore inputted into the

model, and with each system there are

inevitable tradeoffs between first cost

and operating cost, differing mainte-

nance requirements, etc. It is typical for

a large amount of time to be invested in

determining the optimum HVAC system

during the modeling period so that the

owners will understand the modeled

system and agree that the system is

appropriate for their long-term opera-

tion and ownership.

The model then determines the ener-

gy required, based on the efficiency of

proposed heating and cooling equipment,

to achieve desired indoor air tempera-

tures and humidity levels. Schedules are

input for control features, such as night

setback and expected changes in daytime

occupancy load. The sum total of all of

these factors over the year estimates the

building’s total annual energy use.

(For sample screenshots from

energy modeling software, see figures

2 and 3.)

Meeting the Standard Knowing the amount of energy, in

some metric such as dollars or kilo-

watts per hour, that a building is esti-

mated to use annually, however, is not

enough to indicate whether the build-

ing meets or exceeds energy efficiency

goals. To do this, there must be a way

to determine how much annual energy

the same building (in terms of size

and geographical location) would con-

sume if it were designed and built

based on minimum energy efficiency

standards. In LEED parlance, this code-

minimum model is referred to as the

Figure 3. Energy Use Summary Screenshot, eQuest® Software Version 3.64.

Source: Seth Spangler, RMF Engineering Inc.

District Energy / Fourth Quarter 2012 25© 2012 International District Energy Association. ALL RIGHTS RESERVED.

“baseline building.” To determine this,

first a computer model of the same

building must be generated. However,

this model assumes the insulation fac-

tors, fenestration, equipment types and

efficiencies, and control strategies are all

set to code-minimum levels as defined

by the currently adopted version of

ASHRAE Standard 90.1. The result pro-

vides the anticipated annual energy use

for the code-minimum building if it were

designed to meet the minimum require-

ments of ASHRAE’s standard.

It would seem that all that is needed

at this point to demonstrate building effi-

ciency is to compare the results from the

proposed building to that of the baseline

building; yet, it is not that simple. Since

energy comes in many forms and units,

it is difficult to achieve an apples-to-

apples comparison. Just converting all

energy forms into Btus or kilowatts per

hour does not achieve this goal. For

example, a Btu of natural gas and a Btu

of steam are not the same in terms of

usable heat, so a surrogate is needed to

negate the differences in energy forms.

The surrogate chosen by the USGBC and

ASHRAE is dollars. All energy forms in

the model are converted to their cost

using the actual utility rate structure of

the proposed building. Are dollars the

perfect surrogate? No, yet using dollars

does seem to achieve the best balance

between accuracy and simplicity.

To comply with LEED guidelines,

you are comparing the total

estimated annual energy cost of

the proposed building against

the baseline building.

So, to comply with LEED guidelines,

you are comparing the total estimated

annual energy cost of the proposed

building against the baseline building.

As an example, if your proposed build-

ing estimated an annual energy use of

$700,000 and your baseline building

estimated $1 million, your calculated

energy savings from the modeling is

30 percent.

Within this discussion, there are

two principals that must be understood.

The first is “energy neutrality,” which

expresses the USGBC’s goal to prevent

the cost advantages of one fuel over the

other from affecting the comparison.

For instance, if your proposed energy

model uses natural gas as the heating

fuel source, your baseline energy model

cannot use electric heat, which would

generate bigger dollar savings for the

proposed building. Instead, the baseline

energy model must also use natural gas.

An interesting example of this occurred

with a company in Fairbanks, Alaska. Its

proposed facility received its heat from

a coal-fired district steam plant. For the

baseline building, the company assumed

fuel oil for heating, which would have

been the only realistic choice based on

the location. As a result, the low cost of

coal in Alaska compared to the relatively

high cost of fuel oil generated significant

cost savings. However, the USGBC reject-

ed the company’s model and instructed

it that the baseline building model must

use coal-fired unit heaters, with minimum

efficiencies set by the U.S. Environmen-

tal Protection Agency, in order for the

comparison to be accurate. (For more on

this story, and an in-depth explanation

of energy neutrality, see “LEED 2012: Do

dollars matter to the USGBC?” in Second

Quarter 2012 District Energy.)

Second is the concept of a “virtual

plant.” When a proposed building receives

district energy service, the USGBC does

not want any advantages in utility rate

structure differences for the district ener-

gy system versus the building to impact

the model. For example, if the system is

able to purchase electricity at a better

rate due to its scale than the building can,

these dollar savings cannot be passed on

to the building in the model. Instead, the

modeler must assume the district energy

system is a virtual plant on the same site

as the proposed building and, as such,

must use the same rate for electricity as

the building.

Now Let’s Throw in District Energy Energy models designed to dem-

onstrate energy efficiency improve-

ments over a baseline can be complex.

Factoring district energy into the mix

only adds to that complexity. Relative

to achieving a LEED rating, the mod-

eler must address two issues. Under

the current 2009 rating system, the

proposed building must meet the mini-

mum energy requirements outlined

under LEED’s Energy and Atmosphere

Prerequisite 2 – Minimum Energy Per-

formance. This prerequisite requires

modelers to demonstrate an estimated

10 percent improvement over the

baseline model for new buildings and

a 5 percent improvement for existing

buildings going through a major reno-

vation. Failure to meet this prerequisite

will prevent a project building from

achieving any LEED rating level.

Energy models demonstrating

building energy efficiency can

be complex; factoring district

energy into the mix adds to

that complexity.

The second issue a modeler faces

falls under Energy and Atmosphere

Credit 1 – Optimize Energy Performance.

Under this credit, a new building has

the chance to achieve up to 19 points

– by far the most points available in

any LEED credit category. Therefore, it

is imperative project teams pay close

attention to what can be achieved under

this credit, especially if they are pursu-

ing a high LEED rating level such as

Gold or Platinum. Modelers achieve

points by demonstrating the proposed

building has much higher anticipated

energy efficiency than the baseline

building. For example, a new building

model showing 12 percent energy sav-

ings would achieve one point with

an additional point awarded for each

2 percent increase in energy savings up

to the maximum of 19 points.

When district energy is involved,

the plot thickens. In the USGBC’s dis-

trict energy guidance document, titled

“Treatment of District or Campus

Thermal Energy in LEED V2 and LEED

2009 – Design & Construction,” dated

26 District Energy / Fourth Quarter 2012 © 2012 International District Energy Association. ALL RIGHTS RESERVED.© 2012 International District Energy Association. ALL RIGHTS RESERVED.

Aug. 13, 2010, the modeler is given

two options of approaches for verify-

ing compliance with Prerequisite 2

and determining the number of points

available under Credit 1. Although

this makes the district energy analysis

more complicated, it is an improve-

ment over the first version of this doc-

ument released in 2008, which required

two separate proposed models in all

cases. Now one model will suffice.

However, two are still often needed to

determine which path will generate the

most points.

Option 1 is the simpler of the two.

To complete an option 1 model, the

modeler does not need any information

from the district energy system opera-

tors. This was designed so that if a pro-

posed building receives district energy

and the system operators do not have

or refuse to provide necessary informa-

tion, the modeler can still meet LEED

requirements. However, there are limits

in the number of points that can be

achieved primarily due to these factors.

In option 1, the district energy sys-

tem energy efficiency is not a factor.

Modelers run their proposed and base-

line model to determine the amount of

chilled water, steam and/or heating hot

water required to meet the annual load.

Then, these numbers are run through

published formulas designed to predict

the cost of electricity or fuel at the plant

that would be consumed to produce the

thermal energy required to meet the

building’s load. Therefore, the only way a

building can achieve the required energy

savings under option 1 is by designing a

building that requires less thermal ener-

gy. So, if plug loads account for 25 per-

cent of a building’s annual energy use,

and the generation of thermal energy is

35 percent of a building’s annual energy

use, that only leaves 40 percent of the

building’s annual energy from which the

modeler and project team must carve

out energy savings. In this scenario, to

meet the prerequisite minimum of 10

percent energy savings, the total avail-

able energy that can be reduced must be

reduced by 25 percent. This is a signifi-

cant challenge!

Option 2 includes the energy effi-

ciency of the district energy system. In

this option, the system operator must

provide system energy efficiency data

to the modeler for inclusion in the

model. These data must reflect total

system efficiency. As an example, if a

system supplies chilled water to a cus-

tomer, to complete an energy model,

the modeler must know the energy

loss, as a percent, between the point

electricity enters the chiller plant to

the point the chilled water enters the

proposed building. This will allow the

modeler to calculate approximately

how much electricity was purchased to

both generate and deliver the chilled

water the facility will use annually. The

customer can then convert this electric

use into a cost based on the building’s

rate structure.

The proposed energy model

includes the actual district energy

system and is compared to a baseline

energy model that does not. Instead,

the baseline energy model includes

on-site, code-minimum equipment

that uses the same type of fuel as the

proposed building (remember energy

neutrality). For example, if the district

energy system would supply electric-

generated chilled water to a proposed

building, the baseline energy model

may use on-site air-cooled equipment,

direct expansion equipment, etc. The

type of equipment used in the baseline

energy model is specified in ASHRAE

90.1 based on building type and size.

However, often the district energy

system in the proposed model is also

being compared to a proposed model

with standalone building equipment,

such as small magnetic bearing chillers

and packaged condensing boilers. The

manufacturer’s published efficiency of

this type of equipment is impressive.

The likelihood that these efficiencies

will not be achieved originally or main-

tained as the building ages due to poor

building maintenance and complex

controls equipment is not considered

in the initial analysis.

Conclusion Cost has always been and always

will be a key component when poten-

tial district energy customers consider

the benefits of connecting to the sys-

tem versus building, operating and

maintaining their own building ther-

mal systems. The reliability of district

energy as well as the ability to free up

valuable building real estate for other

uses have also been powerful argu-

ments for becoming customers. Today,

the overall efficiency of district energy

compared to other options is a new

and growing factor influencing cus-

tomers’ choices as they pursue LEED

ratings or work to meet local energy

ordinances.

District energy systems often

employ strategies – such as combined

heat and power production, thermal

energy storage and renewable energy

use – that offer sustainable benefits.

These benefits are primarily recognized

through energy models. Therefore, it

is essential that district energy execu-

tives, marketers and operators under-

stand the impact of their system on

these models and make decisions on

future investment while considering this

impact as a factor.

Tim Griffin, PE, LEED AP is IDEA’s liaison with the U.S. Green Building Council and serves on IDEA’s board of directors. He is a principal and branch manager with RMF Engineering Inc., a firm specializing in district energy

system planning, design and commissioning. A registered engineer and a LEED Accredited Professional, Griffin has a Bachelor of Science degree in mechanical engineering from North Carolina State University and a Master of Business Administration degree from Colorado State University. He authored the book Winning With Millennials: How to Attract, Retain, and Empower Today’s Generation of Design Professionals. He may be reached at tgriffin@rmf.com.

Dave Crutchfield PE, LEED AP, is a mechanical engineer and princi-pal with RMF Engineering in charge of the firm’s Southeast Region, which includes offices in South Carolina, North Carolina and Georgia. Based in Charleston, S.C.,

Crutchfield leads production and growth of that division, and he oversees building system design and energy modeling for facilities in the southeastern U.S. A seasoned engineer with particular expertise in the design of research laboratories and higher educa-tion facilities, he joined the firm in 1993 as a project engineer in the Buildings Division. He can be con-tacted at dave.crutchfield@rmf.com.