Creating Efficient HVAC Systems
Heating and Cooling Fundamentals for
Commercial Buildings
Heating, ventilating, and air conditioning (HVAC) systems account for nearly half of the energy
used in a typical commercial building. There are a number of ways to better manage your
energy budget by focusing on this important area. This whitepaper describes the types of HVAC
equipment, key terminology including efficiency ratings, and the impact of the building envelope
on heating and cooling energy use.
Equipment
The HVAC industry has its own language to identify different
equipment types. “Unitary” refers to equipment that contains
all of the components necessary to heat, cool, dehumidify,
filter and move air in one or more factory-made assemblies.
Unitary equipment is available in packaged or split system
designs.
The most common type of commercial system is the packaged system design, which provides both heating and cooling to about 70 percent of the commercial building floor
space built over the last 30 years. Packaged units are typically sized from five to 30 tons in cooling capacity. They are
generally mounted on the rooftop but can also be installed
at ground level. The evaporator and condenser are kept together in one package, delivering conditioned air directly
into a room or ductwork.
A split system is a combination of an indoor air handling unit
and an outdoor condensing unit. The indoor air handling unit
contains a supply air fan and an evaporator (or cooling) coil.
The outdoor condensing unit consists of a compressor and
a condenser coil. Split systems are typically found in smaller
commercial buildings.
create heat, but merely move — or pump — it from one
place to another. These work best in moderate climates.
Cooling
Cooling is produced through the clever use of refrigerants,
and HVAC professionals use different terms such as mechanical compression, vapor compression or direct expansion to describe the basic cycle. A conventional mechanical
vapor compression system consists of four major mechanical elements: a compressor, a condenser, a metering device
and an evaporator. Essentially, indoor heat is absorbed by
the liquid refrigerant in the evaporator coil. This causes refrigerant boiling (evaporation) and transforms the refrigerant
from a cold liquid to a warm gas. The vapor continues on to
the compressor, where it is mechanically raised in temperature and pressure. It then circulates to the condenser coil,
where heat is rejected and the vapor condenses to a liquid.
The liquid refrigerant is metered back into the evaporator
via a thermal expansion (TX) valve, which regulates the flow
of liquid refrigerant using spring pressure in the valve body.
The amount of liquid refrigerant released must be restricted to keep the evaporator pressure low enough to support
evaporation. The refrigerant enters the evaporator again as
a liquid-rich vapor/liquid mixture and the evaporation portion of the cycle is repeated.
Heating
The heating side of an HVAC system can include electric
resistance, natural gas or heat pumps. In the case of electric
resistance, the heating elements used are similar in concept to those used in a toaster — only quite a bit larger.
Most manufacturers use heating elements in the 10 to 15 kW
range, and then combine multiple elements to achieve the
desired heating capacity. The heating outside design temperature (coldest expected temperature) varies by climate
zone, and is used in conjunction with heat loss calculations
to size the heating portion of the HVAC system.
Natural gas heating is also used in unitary HVAC systems.
However, there are some differences compared to a conventional gas furnace for a home due to the common placement
of these commercial HVAC units on building rooftops. The
piping to deliver the natural gas to the rooftop or other locations should be considered in the system installation costs.
A heat pump works much like a conventional air conditioner,
except it can also run in reverse during the winter to provide
heat. Heat pumps are more efficient because they do not
Compressor designs may be reciprocating, screw, scroll
or centrifugal types. Reciprocating types are like the pistons in a car engine, displacing an increment of vapor with
each stroke. This type is costly, noisy, bulky and limited in
total cooling capacity. Screw compressors also displace
increments of vapor but do so with counter-rotating screw
shafts that mesh together. This results in a higher flow rate
with small exterior dimensions. Scroll compressors function similarly to screw compressors except only one helical
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coil rotates; the other is stationary. Scroll compressors are
cheap and efficient, but limited in cooling capacity. Centrifugal compressors are like jet engines. Vapor is continuously drawn into the center of a rotating impeller with radial
blades, and then thrown out toward the periphery of the impeller by centrifugal forces. The velocity of the gas is transformed into pressure when it is forced to decelerate within
the impeller structure. They can achieve very high cooling
capacities more efficiently.
Cooling “load,” usually expressed in refrigerant tons (RT) or
tons, is a big driver in HVAC equipment selection. One ton of
cooling is the amount of heat energy (12,000 Btu) required
to melt 2,000 pounds (1 ton) of ice in one hour. Commercial facilities and manufacturing plants with larger cooling
loads often use chillers, which produce chilled water for air
conditioning or process cooling applications. Chillers typically use mechanical vapor compression (centrifugal, screw,
scroll or reciprocating) to achieve the refrigerant phase
change that cools. The waste heat is then discharged to
the outside air, either through an air-cooled condenser or
cooling tower. Chillers using reciprocating compressors are
generally found in the 15 to 200 ton range, while centrifugal
chillers range from 200 to 1,500 tons. Chillers with screw and
scroll compressors serve the mid-size capacities.
Local climate, fuel cost, fuel availability, building type and
capacity requirements are the most important factors in the
HVAC system selection process. When deciding which type
of HVAC system best fits your situation, operating, installation and maintenance costs should also be considered.
Building Envelope
Your building envelope — doors, windows, walls and roof —
protects your indoor environment from the elements. The
condition of your building envelope strongly impacts HVAC
system energy consumption and occupant comfort. As a
general rule, roughly 35 percent of a typical building’s energy
loss is through the roof. Twenty-five percent is lost through
the walls, 20 percent through windows, and the balance is
lost via fenestration through spaces like cracks and doors.
A detailed assessment of the building envelope is required
to make a heat loss calculation. Similar-sized buildings with
variations in the number of windows or the degree of insulation will have heat loss calculations that are quite different.
One of the most cost-effective steps is to weatherize windows
and doors by filling cracks and placing weather-stripping
around gaps where air can escape. Also, properly insulated
walls and roofs can substantially reduce heating and cooling costs. Tightening up your building envelope is a great
first step in an HVAC upgrade, since it can provide a quick
payback toward lowering your energy costs. You may even
be able to reduce the size and cost of your new equipment!
Local building codes can provide specific recommendations for insulation levels and best practices. You can also
reference the American Society of Heating, Refrigerating
and Air-Conditioning Engineers (ASHRAE) standards, the International Energy Conservation Code (IECC), or the Building Owners and Managers Association (BOMA) standards
for more information.
Efficiency Ratings
Determining the efficiency of your existing HVAC equipment
is the first step in deciding whether to upgrade to a more efficient unit. Since the efficiency ratings for HVAC equipment
can be confusing, a brief description of applicable HVAC
terms follows:
A British thermal unit (Btu) is a commonly used unit of measure for energy use in heating and cooling equipment. A Btu
is the amount of heat required to raise the temperature of
one pound of water by one degree Fahrenheit. One kilowatthour (kWh) of electricity contains 3,412 Btu.
Energy efficiency ratio (EER) is a measure of how efficiently
a cooling system operates when the outdoor temperature is
at a specified level (95 F). A higher EER means the system is
more efficient and will use less energy for the same cooling
output. Typically, EER is used in rating commercial cooling
systems that have a cooling capacity greater than five tons,
with ASHRAE specifying values from 9 to 12 EER depending
on size.
Seasonal energy efficiency ratio (SEER) is a measure of efficiency over an entire cooling season. Residential units are
rated in SEER, with a higher SEER indicating higher efficiency. Simply put, SEER is the total amount of cooling the air
conditioner provides over the entire cooling season divided
by the total number of watt-hours it consumes. Manufacturers must now meet a minimum of 13 SEER, though efficiencies over 20 SEER are available.
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Coefficient of performance (COP) is typically found on chiller systems, gas cooling equipment and heat pumps. COP
measures how efficiently a system operates at a single outdoor temperature — typically 47 F for heating or 95 F for
cooling. A higher COP means higher efficiency. The COP
is equal to the Btu output divided by the Btu equivalent of
electricity to produce that energy. COP ratings can vary from
less than one to greater than five.
Heating Seasonal Performance Factor (HSPF) is a ratio used
to rate heat pumps, determined by dividing the seasonal
heating output in Btu by the seasonal power consumption in
watts. Like SEER ratings for air conditioners, HSPF is a seasonal measure. HSPF ratings of seven to nine are common
for air source heat pumps, while geothermal (water source)
heat pumps typically have HSPF ratings greater than 10.
Annual fuel utilization efficiency (AFUE) measures the efficiency of heating equipment on an annual basis. AFUE is
calculated by dividing the total heat delivered to a conditioned space for a heating season by the total fuel used (in
Btu). Higher AFUE ratings indicate higher efficiency. Ratings
of 80 to 96 AFUE are typical for furnaces.
Upgrading Your Equipment
If you have an HVAC system that is more than 15 years old, you
may want to consider an upgrade. Newer systems are 20 to
40 percent more efficient than older technologies, which can
result in a good return on your investment. When deciding to
replace, however, you should also consider the maintenance
costs and condition of your current system. Full-load Value
(FLV) is the most useful efficiency rating for electric HVAC
equipment. FLV is measured in kilowatts per ton capacity
(kW/ton) and a lower FLV is preferred. COP and EER can easily be converted to FLV using these conversion factors.
• FLV = 3.516/COP
• FLV = 12/EER
Knowing FLV, the capacity in tons, and cooling equivalent
full load hours, you can make a rough estimate of energy
consumption:
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The best available chiller efficiency is in the range
of 0.5 kW/ton
The best available packaged rooftop units operate
at 0.8 kW/ton
The best available packaged terminal air
conditioners (PTACs) operate at 1.0 kW/ton
When purchasing a new system, size equipment to the current load requirements rather than attempting to match the
old design. Oversizing can result in higher installation and
operating costs, shortened equipment life and poor humidity
control.
Oversizing is frequently caused by reliance on old nameplate data or rule-of-thumb formulas for general estimating.
When sizing your new system, take into account insulation
or lighting system upgrades that may have reduced space
conditioning needs. Newer systems also provide a higher
level of performance than older models. Consult with an
HVAC engineer or trusted equipment supplier about properly sizing your system.
HVAC Accessories
An economizer is an HVAC accessory that incorporates
bypass vents in the air handler to allow cool (30 F to 55
F) outside air to be brought directly indoors without being
cooled by the evaporator. It is often called a “free cooling”
air system since no cooling energy is used. Economizers are
subject to requirements spelled out in ASHRAE 90.1, a standard that provides minimum requirements for energy efficient designs for all buildings except low-rise buildings. This
standard states that economizers are required for systems
larger than 5 tons in cool/dry climates (West, Southwest) and
for systems larger than 11 tons in cool/moist climates (Midwest), but not in warm/moist climates (Southeast).
Heat recovery ventilators (HRVs) are a heat exchange system between incoming make-up air and exiting exhaust air.
In the summer, the temperature of outside air is reduced by
cooler air from indoors that is being exhausted. In the winter,
it works in reverse. HRVs can recover about 60 to 70 percent of heat in warm exiting air as cold outside air passes
through the heat exchanger on its way into the building.
Cooling energy consumption (kWh) =
Capacity (tons) x FLV (kW/ton) x Cooling Load Hours
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Energy/enthalpy/desiccant wheels are similar to HRVs, except that the heat exchanger is coated with a moisture-absorbing desiccant material. Desiccant wheels can reduce
about 70 to 80 percent of the thermal energy in incoming
humid summer air, because they remove moisture as well as
heat. HRVs and desiccant wheels are best used in climates
with extreme temperatures.
Thermal energy storage systems use HVAC equipment
to make ice or chilled water during the night. When air is
passed over this cold media during the day, you can reduce or eliminate the HVAC contribution to peak demand.
Nighttime operation of the HVAC system may also qualify for
lower time-of-use rates.
Conclusion
HVAC systems are complex, but understanding the language
of HVAC and principles of operation will help you achieve
optimum indoor air quality while minimizing energy use. A
well-executed preventive-maintenance program, combined
with building upkeep and equipment upgrades, can provide
substantial energy-cost savings.
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