HVAC
HVAC stands for heating, ventilation, and air conditioning. It refers to the systems that
regulate and move heated and cooled air throughout residential and commercial buildings, from
homes to offices to indoor stadiums. Although there are many options when it comes to HVAC
systems, they all work similarly, taking in fresh air and using a mechanical ventilation system to
heat or cool it to a desired temperature. HVAC units also can control humidity levels and
improve air quality through air cleaners that capture bacteria, spores, and virus-sized particles.
At the most basic level, an HVAC system takes in air, cools or heats that air, and blows it
into an indoor space. That space could be a specific room or an entire building or structure, such
as a house, office, school, or even an airport. An individual component of an HVAC system may
serve dual purposes. For example, your home could have a separate air conditioner and furnace,
or a heat pump for both cooling and heating.
How Does an HVAC System Work?
HVAC systems are designed to move air, heating or cooling an indoor space in the
process. These systems generally have three major elements: a heating unit, cooling unit, and
duct to move the air. Natural ventilation takes air from open windows, doors, or cracks and uses
pressure differences to force the air through a building. With today's tightly sealed homes and
offices, this requires the use of fans, blowers, filters, and ducts.
Air Conditioning Systems
To cool air, the fan in an air conditioning unit draws the air in and forces it over the
evaporator’s coils. Filled with refrigerant, these coils draw heat from the air, cooling it. Next, the
air moves into the air handler, where a blower sends it into the ducts. As the cool air moves
through the ducts, any noxious gasses produced during the process are expelled through the flue.
When an air conditioner absorbs heat, the refrigerant vaporizes. To return to its natural state, the
refrigerant travels into the compressor, which pressurizes it, and onto the condenser, where it
condenses from a vapor back into a liquid. As it does, it expels heat that is blown out by a fan.
Figure 1 Air Conditioning Systems
Heat Pumps
A heat pump works similarly to a central air conditioner and is based on the same idea that heat
is always attracted to cold. To cool the air, heat is absorbed by the refrigerant and expelled out of
the building. To produce warm air, a heat pump draws cold outside air over even colder
refrigerant. Heat is then drawn into the refrigerant, which heats the coils. The air is drawn over
those heated coils, where it rises to the desired temperature and is blown into the home.
Figure 2 Heat Pump
Furnaces
To heat a building using a gas furnace, an HVAC system draws air in through the ducts
and forces it into the furnace. When the furnace is running, the combustion chamber heats the
heat exchanger to the designed temperature. Air is pushed through the exchanger, heated, and
blown by the motor through the ducts and back into the house.
Air purifiers, humidifiers, dehumidifiers, and other indoor air quality products can be
added to the air system to enhance the comfort of the home. These elements then deliver optimal
thermal comfort through the control of either a conventional or smart thermostat.
Figure 2 Furnace
Types of HVAC Systems
Split System
Also referred to as a forced-air system, a split system has one unit inside the home and
one outside. This setup can consist of a furnace and air conditioner, an air handler and heat
pump, or a furnace and heat pump. Which configuration is best for your home will depend
largely on where you live. For example, the furnace and heat pump setup works best in very cold
climates.
Hybrid Heat Pump
In this scenario, the central heating system includes an electric heat pump that works in
conjunction with a furnace. In milder times of the year like the spring and fall, the heat pump
heats the home. During weather that's too cold for a heat pump to operate effectively, the furnace
takes over. This hybrid system, also known as a dual-fuel system, saves money because it costs
less to heat the home with a heat pump than a furnace.
Ductless Mini-Split
A ductless mini-split system has an outdoor unit that contains the compressor and
condenser, and an indoor air handler mounted in the room that blows the cooled air directly into
the room. This type of ductless system is usually best for small spaces like garages and
workshops that don’t require a traditional split system. They aren't suited to whole-house
applications. Typically, these systems are easy enough for homeowner to install themselves.
Ducted Mini-Split
A ducted mini-split system uses tubes instead of larger ducts to move air into a room
from an outdoor compressor and condenser. This solution is best for homes where there isn’t a
lot of room for traditional ductwork. Compared to ductless mini-split systems, ducted mini-splits
have the advantage of better air circulation.
Packaged System
A packaged system contains the traditional components of a split system. However, all
units are housed outside. This solution works best for homes that don’t have the space for a
heating unit inside the home. Even if you have the space for a split system, you might consider a
packaged system because they are quieter (everything is located outside the home) and have
lower installation costs (there’s only one unit to install).
HVAC Loads
Heating and cooling loads are the measure of energy needed to be added or removed
from a space by the HVAC system to provide the desired level of comfort within a space. Right-
sizing the HVAC system begins with an accurate understanding of the heating and cooling loads
on a space.
Heat Quantity
Heat quantity is measured using two scales: Calories (cal) by most of the world and
British Thermal Units (BTU) in the United States.
One BTU will increase the temperature of one pound of water by 1 °F. One c will
increase the temperature of 1g of water by 1 °C.
Figure 1 Heat measurement
Heat Flow Rates
In air-conditioning design, four related heat flow rates, each of which varies with time,
must be differentiated
a. Space heat gain ----------------How much heat (energy) is entering the space?
b. Space cooling load -------------How much energy must be removed from the space to
keep temperature and relative humidity constant?
c. Space heat extraction-----------How much energy is the HVAC removing from the space?
d. Cooling load (coil) ---------------How much energy is removed by the cooling coil serving
various spaces plus any loads external to the spaces such as duct heat gain, duct leakage,
fan heat and outdoor makeup air?
Sensible and Latent Heat
Sensible heat - Heat which a substance absorbs, and while its temperature goes up, the substance
does not change state. Sensible heat gain is directly added to the conditioned space by
conduction, convection, and/or radiation. Note that the sensible heat gain entering the
conditioned space does not equal the sensible cooling load during the same time interval because
of the stored heat in the building envelope. Only the convective heat becomes cooling load
instantaneously. Sensible heat load is total of
a. Heat transmitted thru floors, ceilings, walls
b. Occupant’s body heat
c. Appliance & Light heat
d. Solar Heat gain thru glass
e. Infiltration of outside air
f. Air introduced by Ventilation
Latent Heat Loads - Latent heat gain occurs when moisture is added to the space either from
internal sources (e.g. vapor emitted by occupants and equipment) or from outdoor air as a result
of infiltration or ventilation to maintain proper indoor air quality. Latent heat load is total of
a. Moisture-laden outside air from Infiltration & Ventilation
b. Occupant Respiration & Activities
c. Moisture from Equipment & Appliances
Figure 2 Latent and Sensible Heat
Figure 3 Cooling Load Components
Why Is a Professional Load Calculation Important?
Choosing the wrong-sized equipment will negatively affect your home comfort and drive
up your energy expenses for as long as you own the system. A too-small system won’t deliver
the level of comfort that you expect on hot summer days. Because an undersized system strains
to reach the temperature set on your thermostat, frost can collect and clog the overworked
condenser, shutting the system down.
An undersized air conditioning system is just as problematic. A too-big system actually
cools your home too fast, cycling off before the air has had a chance to circulate and dehumidify.
This results in uncomfortable shifts in temperature and rooms that feel cold and clammy. You
wind up paying more than is necessary for a system that doesn’t deliver the level of comfort that
you expect.
References:
[1] Bitler, T. (2002, February 2). What is HVAC and How Does It Work? US News.
https://www.usnews.com/360-reviews/services/hvac/what-is-hvac#how-it-works
[2] Bhatia, A. (n.d.). Cooling Load Calculations and Principles. www.cedengineering.com.
https://www.cedengineering.com/
[3] Web Editor. (2017, December 29). What is an HVAC Load Calculation? Senica Air
Conditioning, Inc. https://www.senicaair.com/blog/what-is-an-hvac-load-calculation
[4] Aircon Contractor. (2020, August 12). Heat Load Calculations and Tons of Refrigeration
Paano nga ba? YouTube. https://www.youtube.com/watch?v=uxDEGXHQO34
[5] Burdick, A. & IBACOS, Inc. (2011, June). Strategy Guideline: Accurate Heating and
Cooling Load Calculations. U.S. Department of Energy.
