Page 294 - Mechanical Engineers' Handbook (Volume 4)

2 Performing an Energy Audit—Analyzing Energy Use 283

made to calculate HVAC energy consumption from engineering principles and data. First,
the building’s heating and cooling loads are calculated. These depend on the operating sched-
ule and thermostat settings, the climate, heat gains and losses from radiation and conduction,
the rate of air exchange, and heat gains from internal sources. Then energy use is calculated
by taking account of the efficiency with which the HVAC systems meet these loads. The
efficiency of the HVAC systems depends on the efficiency of equipment such as boilers and
chillers and losses in distribution through pipes and ducts; equipment efficiency and distri-
bution losses are usually dependent on load. In all but the simplest buildings, the calculation
of HVAC energy consumption is sufficiently complex to require the use of computer pro-
grams; a number of such programs are available (see, for example, Ref. 2). The auditor will
usually make some investigation of all of the factors necessary to calculate HVAC energy
consumption. However, the effort involved in obtaining data that are sufficiently accurate
and preparing them in suitable form for input to a computer program is quite considerable.
For this reason, the deterministic approach is not recommended for energy auditing unless
the calculation of savings from energy conservation measures requires detailed information
on building heating and cooling loads.
Statistical approaches to the calculation of HVAC energy consumption involve the anal-
ysis of records of past energy consumption. In one common statistical method, energy con-
sumption is analyzed as a function of climate. Regression analysis with energy consumption
as the dependent variable and some function of outdoor temperature as the independent
variable is used to separate ‘‘climate-dependent’’ energy consumption from ‘‘base’’ con-
sumption. The climate-dependent fraction is considered to be the energy consumption for
heating and cooling, and the remainder is assumed to be due to other uses. This method can
work well in residences and in some small commercial buildings where heating and cooling
loads are due primarily to the climate. It does not work as well in large commercial buildings
because much of the cooling load in these buildings is due to internal heat gains and because
a significant part of the heating load may be for reheat (i.e., air that is precooled to the
temperature required for the warmest space in the building may have to be reheated in other
spaces). The easiest statistical method to apply, and the one that should probably be attempted
first, is to calculate the energy consumption for all other end uses (lighting, domestic hot
water, office equipment, etc.) and subtract this from the total consumption; the remainder
will be HVAC energy consumption. If different fuel types are used for heating and cooling,
it will be easy to separate consumption for these uses; if not, some further analysis of the
climate dependence of consumption will be required. Energy consumption for ventilation
can be calculated easily if the operating hours and power requirements for the supply and
exhaust fans are known.
Whatever approach is to be taken in determining the fraction of energy consumption
that is used for HVAC systems, the auditor should begin his or her work on these systems
by determining their operating hours and control settings. These can often be changed to
save energy with no adverse effects on a building’s occupants. Next, maintenance practices
should be examined. This examination will usually be initiated by determining whether or
not a preventive maintenance (PM) program is being conducted. If there is a PM program,
much can be learned about the adequacy of maintenance practices by examining the PM
records. Often only a few spot checks of the HVAC systems will be required to verify that
the records are consistent with actual practice. If there is no PM program, the auditor will
usually find that the HVAC systems are in poor condition and should be prepared to make
extensive checks for energy-wasting maintenance problems. Establishment of a PM program
as part of the energy management program is a frequent recommendation from an energy
audit.