Environmental Housekeeping Handbook APPENDIX C HVAC background information ___________________________________________________________
Unitary vs. centralized HVAC systems
Commercial buildings smaller than 20,000 square feet typically use
factory-built, air-cooled “unitary” HVAC equipment. Buildings larger
than 100,000 square feet and multi building campuses generally use siteassembled or engineered “centralized” HVAC systems. Buildings with a
square footage between 20,000 and 100,000 square feet may employ a
combination of multiple large packaged units (for example, one unit per
wing of an office building) or small built-up systems. Performance
comparisons between unitary and engineered systems, or among systems
of either type, should consider the performance of the entire system,
rather than just the chiller or the condensing unit.
The principal advantages of central HVAC systems are higher energy
efficiency, greater load-management potential, fewer and higher-quality
components that require less (but more skilled) maintenance, and
architectural and structural simplicity. The main advantages of unitary
systems are lower initial costs, independent zone control, lower failure
risk, and less floor space occupied by a mechanical room, ducts, and
pipes.
Performance measurements
There are a number of metrics that can be used to compare the efficiency
performance of various HVAC systems. The Air-Conditioning &
Refrigeration Institute (ARI) defines standardized test procedures to
determine the efficiency metrics for a limited scope of HVAC systems.
The tests used to evaluate the performance vary based on the HVAC
equipment being tested. The following list summarizes major HVAC
performance metrics:
Cooling capacity is rated as the amount of heat energy a cooling unit can
remove from a space per hour, expressed in BTU per hour.
Energy Efficiency Ratio (EER) is the ratio of the cooling capacity
(BTU/h) to the power input value (watts) at any given set of rating
conditions expressed in BTU/watt-hour. The current standard is an
energy efficiency ratio (EER) of 8.9 for systems with a capacity of 65 to
135 thousand BTU per hour (kBTU/hr).
Coefficient of Performance (COP) is defined differently depending on
function. For cooling, COP describes the ratio of the rate of heat removal
109 to the rate of energy input in consistent units, for a complete cooling
system as tested under a nationally recognized standard. For heating, it is
the ratio of the rate of heat delivered to the rate of energy input in
consistent units, for a complete heat pump system as tested under
designated operation conditions. Conversion: COP = EER * 3.4.
Note that both COP and EER are calculated under controlled laboratory
conditions and usually do not reflect the efficiency of performance under
actual use. The seasonal energy efficiency ratio (SEER) and the heating
season performance factor (HSPF) address the need to reflect actual use
by measuring efficiency in field situations.
Seasonal Energy Efficiency Ratio (SEER) is the total heat removed
from the conditioned space during the annual cooling season, expressed
in BTU, divided by the total electrical energy consumed by the air
conditioner or heat pump during the same season, expressed in watthours. U.S. federal appliance efficiency standards currently require
minimum SEER ratings of 13. The highest efficiency models available
can have SEER ratings up to 23 for central air units.
Heating Season Performance Factor (HSPF) is the total heat added to
the conditioned space during the annual heating season (expressed in
BTU), divided by the total electrical energy consumed by the air
conditioner or heat pump during the same season (expressed in watthours).
Integrated Part-Load Value (IPLV) is a seasonal efficiency rating
method for representative loads from 65,000 BTU per hour and up. This
rating applies to units that have stated partial capacities, such as units
with staged compressors. Units are tested at full capacity and at each
stated partial capacity, and those values are then used to calculate IPLV.
Additional information
For additional information on HVAC design, see:
Nontechnical introduction to HVAC: NCDEnr. 2003. “Energy Efficiency
in
industrial
HVAC
systems.”
accessible
at:
http://www.p2pays.org/ref/26/25985.pdf
Technical discussion of HVAC systems: Benjamin, Reynolds, Grondzic,
and Kwok. Mechanical and Electrical Equipment for Buildings, 10th
Edition. John Wiley & sons, inc. new York.
Notes
1
American Council for an Energy Efficient Economy (ACEEE). 2004.
Online guide to Energy-Efficient Equipment. High-performing HVAC
systems. accessible at: http://www.aceee.org/ogeece/ch3_index.htm.
110 Environmental Housekeeping Handbook 2
ERPI Office Complexes Guidebook, Innovative Electric Solutions.
Chapter 6—Heating, Ventilating, and Air-Conditioning (HVAC).
December 1997. tr-109450, p. 195.
3
Air-Conditioning and Refrigeration Institute. (2006). 2006 Standard for
Performance Rating of Unitary Air-Conditioning Equipment and Air
Source Heat Equipment.
111