ACHARYA NAGARJUNA UNIVERSITY
College of Architecture & Planning
th Semester, Bachelor of Architecture
HVAC (heating, ventilation, and air conditioning) is the technology
of indoor and vehicular environmental comfort.
HVAC system design is a sub discipline of mechanical engineering,
based on the principles of Thermodynamics, Fluid Mechanics,
and Heat transfer.
HVAC systems use ventilation air ducts installed throughout a
building to supply conditioned air to a room through outlet vents,
called diffusers; and ducts to remove air through return-air grilles.
HVAC is important in the design of medium to large industrial and
office buildings such as skyscrapers and in marine environments
such as aquariums, where safe and healthy building conditions are
regulated with respect to temperature and humidity, using fresh air
The three central functions of heating, ventilating, and airconditioning are interrelated, especially with the need to
provide thermal comfort and acceptable indoor air quality within
reasonable installation, operation, and maintenance costs.
HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure
relationships between spaces.
The means of air delivery and removal from spaces is known as room air distribution.
The starting point in carrying out an estimate both for cooling and heating depends
on the exterior climate and interior specified conditions.
In modern buildings the design, installation, and control systems of these functions
are integrated into one or more HVAC systems.
For very small buildings, contractors normally capacity engineer and select HVAC
systems and equipment.
For larger buildings, building services designers and engineers, such as
mechanical, architectural, or building services engineers analyze, design, and
specify the HVAC systems.
Basing HVAC on a larger network helps provide an economy of scale that is often
not possible for individual buildings, for utilizing renewable energy sources such as
Thermodynamics is a branch of natural science concerned with heat and its relation
to energy and work. It defines macroscopic variables (such as temperature, internal
energy, entropy, and pressure) that characterize materials and radiation, and
explains how they are related and by what laws they change with time.
Fluid Mechanics : Fluid mechanics is the branch of physics that
studies fluids (liquids, gases, and plasmas) and the forces on them. Fluid mechanics
can be divided into fluid statics, the study of fluids at rest; fluid kinematics, the study
of fluids in motion; and fluid dynamics, the study of the effect of forces on fluid
Heat transfer is a discipline of thermal engineering that concerns the generation,
use, conversion, and exchange of thermal energy and heat between physical
systems. As such, heat transfer is involved in almost every sector of the
economy. Heat transfer is classified into various mechanisms, such as thermal
conduction, thermal convection, thermal radiation, and transfer of energy by phase
In heat transfer, conduction is the transfer of heat energy
by microscopic diffusion and collisions of particles or
quasi-particles within a body due to a temperature
Convection is the concerted, collective movement of
groups or aggregates
of molecules within fluids(e.g., liquids, gases) and rheids,
either through advection or through diffusion or as a
combination of both of them.
Thermal radiation is electromagnetic radiation generated
by the thermal motion of charged particles in matter. All
matter with a temperature greater than absolute
zero emits thermal radiation.
The invention of central heating is often credited
to the ancient Romans, who installed systems of
air ducts called Hypocausts in the walls and floors
of public baths and private villas.
The use of water as the heat transfer medium is
known as Hydronics. These systems also contain
either duct work for forced air systems or piping
to distribute a heated fluid to radiators to transfer
this heat to the air.
The radiators may be mounted on walls or
installed within the floor to give floor heat.
Most modern hot water boiler heating systems
have a circulator, which is a pump, to move hot
water through the distribution system.
. This distribution system can be via radiators,
convectors (baseboard), hot water coils (hydroair) or other heat exchangers.
Ventilation is the process of changing or
replacing air in any space to control
temperature or remove any combination
of moisture, odors, smoke, heat, dust,
airborne bacteria, or carbon dioxide, and
to replenish oxygen.
Ventilation includes both the exchange of
air with the outside as well as circulation of
air within the building
"Mechanical" or "forced" ventilation is
provided by an air handler and used to
control indoor air quality. Excess humidity,
odors, and contaminants can often be
controlled via dilution or replacement with
outside air. However, in humid climates
much energy is required to remove excess
moisture from ventilation air.
In warm or humid months in many climates
maintaining thermal comfort solely via natural
ventilation may not be possible so conventional air
conditioning systems are used as backups.
An important component of natural ventilation
is air changes per hour: the rate of ventilation
through a room with respect to its volume.
For example, six air changes per hour means that
the entire volume of the space is theoretically
replaced with new air every ten minutes.
For human comfort, a minimum of four air
changes per hour is usually targeted.
The highest recommended replacement rates are
for crowded spaces like bars, night clubs, and
commercial kitchens at around 30 to 50 air
changes per hour
Air conditioning and refrigeration are provided through the removal of
Heat can be removed through Radiation, Convection, or Conduction.
A refrigerant is employed either in a heat pump system in which
a compressor is used to drive Thermodynamic refrigeration cycle, or in
a free cooling system which uses pumps to circulate a cool refrigerant
(typically water or a glycol mix).
Free cooling systems can have very high efficiencies, and are
sometimes combined with seasonal thermal energy storage so the cold
of winter can be used for summer air conditioning.
Common storage mediums are deep aquifers or a natural
underground rock mass accessed via a cluster of small-diameter, heat
exchanger equipped boreholes.
Some systems with small storages are hybrids, using free cooling early in
the cooling season, and later employing a heat pump to chill the
circulation coming from the storage.
An HVAC system is responsible for moderating the temperature of a building’s
interior and maintaining it at a comfortable level for the inhabitants.
During the hot days of
summer, the air
conditioning kicks in,
In the frigid days of winter,
the system supplies heat.
The furnace unit is typically fairly
large, requiring its own space within
It is often installed in the basement,
in the attic, or in a closet.
The furnace pushes the cold or hot
air outward into the ducts that run
through every room in the building.
Throughout the ducts, there are
vents that allow the warm or cool air
to pass into rooms and change their
THE HEAT EXCHANGER
Heat exchangers reside in the housing of
every furnace unit. When the furnace is
activated by the thermostat, the heat
exchanger begins to function as well.
Air is sucked into the heat exchanger, either
from the outside or from a separate duct
that pulls cool air out of the building’s rooms.
This type of duct is called a cold air return
When the cool air comes into the heat
exchanger, it is quickly heated and blown
out through the ducts to be dispersed into
If the furnace operates on gas, the heating is
accomplished by gas burners.
If it uses electricity, it is done via electric coils.
THE EVAPORATOR COIL
Like heat exchangers, evaporator coils are also
part of the furnace unit. However, they serve the
opposite function to that of heat exchangers.
They are also attached to a different part of the
Instead of being within the furnace housing, they
are installed inside a metal enclosure that is
affixed to the side or the top of the furnace.
Evaporator coils are activated when cool air is
needed. When triggered, the evaporator coil
supplies chilled air, which is then picked up by the
furnace blower and forced along the ducts and
out through the vents.
The internal design of an evaporator coil
resembles that of a car’s radiator.
Evaporator coils are connected to the HVAC
system’s condensing unit, which is typically
located on the exterior of the building.
THE CONDENSING UNIT
The condensing unit is installed outside the
building, separate from the furnace.
Inside the condensing unit, a special kind
of refrigerant gas is cooled through the exchange
of heat with the air outside.
Then, it is compressed and condensed into liquid
form and sent through a tube or a line made of
This tube runs straight to the evaporator coil. When
the liquid reaches the coil, a series of small nozzles
spray the liquid, lowering its pressure and allowing
it to resolve back into gaseous form.
During the evaporation of liquid to gas, heat is
absorbed, causing a sudden drop in temperature
and supplying cold air for the furnace blowers.
The refrigerant gas is then sent back outside to the
condensing unit, and the process is repeated
again to generate additional cold air.
THE REFRIGERANT LINES
The refrigerant lines are the metal tubes that carry the liquid to the evaporating
coil and return the gas to the condensing unit.
Refrigerant lines are usually made from aluminium or copper.
They are designed to be durable and functional under extreme temperatures.
The thermostat controls the function of the furnace.
It is directly connected to the furnace and includes
temperature-sensing technology as well as user
A thermostat is usually positioned somewhere within the
building where it can easily discern temperature and
remain accessible to users.
A large building may have more than one thermostat
to control different areas of the structure.
The inhabitants of the building can manually set the
thermostat to a certain temperature.
If the air in the room or building is too cold, the heat
exchanger kicks in and blows heat through the vents.
If the room is too warm, the condensing unit and
evaporator coil start to function, and the air
conditioning system sends cool air throughout the
building or to one particular section of the building.
Heating ducts are put in during the construction of a home
or a building.
They are often run through the ceiling.
In each room, at least one rectangular opening is cut into
the duct so that a vent or vents can be installed.
Vents are usually rectangular in shape. They
are placed in the ceiling, with their edges
corresponding to the opening in the duct
As warm or cool air pours through the ducts,
vents allow it to disperse into the rooms below.
Vents are usually made of metal, which can
handle a wide range of temperatures.
The vent is comprised of a rectangular edge or
frame, within which is a series of thin metal
The slats are angled to channel the air
Some vents also include a manual control that
lets users angle the air toward a different part
of the room depending on their preference.
TYPES OF FURNACES
Furnaces can be divided into two main categories:
Single stage furnaces
Both types of furnaces are further distinguished by their performance ratings.
The chart below explains the function of performance ratings.
Ducts collect and reuse 80 percent of the
generated heating or cooling energy
92% efficiency Ducts collect and redistribute 90 percent of the
energy created by the furnace unit
In furnaces with 80% or +92%efficiency, any energy that is not captured by
the ducts is lost, usually through the furnace housing or through vents
leading to the outside of the building.
The Coefficient of performance or COP of a heat pump is a ratio of heating or cooling provided to
electrical energy consumed.
Q is the heat supplied to or removed from the pump
W is the work consumed by the heat pump
Energy Efficiency Ratio : The EER is the ratio of output cooling energy (in BTU) to electrical input
energy (in Watt-hour)
𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 𝐵𝑇𝑈
𝑖𝑛𝑝𝑢𝑡 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑖𝑛 𝑊
Seasonal Energy Efficiency Ratio : It is the ratio of output cooling energy (in BTU) to electrical input
energy (in Watt-hour).
SEER is a representative measurement of how the system behaves over a season where the
outdoor temperature varies
𝑂𝑈𝑇𝑃𝑈𝑇 𝐶𝑂𝑂𝐿𝐼𝑁𝐺 𝐸𝑁𝐸𝑅𝐺𝑌 𝑂𝐹 𝐴 𝐵𝑇𝑈 𝐼𝑁 𝐴 𝑆𝐸𝐴𝑆𝑂𝑁
𝐼𝑁𝑃𝑈𝑇 𝐸𝐿𝐸𝐶𝑇𝑅𝐼𝐶𝐴𝐿 𝐸𝑁𝐸𝑅𝐺𝑌 𝐼𝑁 𝑊ℎ
Kilo-Watt per Ton (kW/ton)
The efficiencies of large industrial air conditioner systems, especially
chillers, are given in kW/ton to specify the amount of electrical power that is
required for a certain power of cooling. In this case, a smaller value represents
a more efficient system.
Another unit in use in the US is the horse power (HP). This is a unit of power
and typically is used to specify the size of motors. It may also be used to
specify the input power of an air conditioning system. One HP is approximately
Energy Star :
In the US, Energy Star is the Environmental Protection Agency’s (EPA’s)
indication for products that have high energy efficiency. it makes it easy for
consumers to identify and purchase products that have higher energy
efficiency than those products without such designation.