The Complete Guide to HVAC Air Distribution Systems
Introduction: What is Air Distribution, and Why is it Important for Home Owners?
Air conditioning systems condition the air and distribute it. Mechanical ventilation distributes cool air from the air conditioner, air handler, or furnace to different rooms in a house. This process is essential because it helps maintain a comfortable temperature in the house.
Simple Air Distribution System
When a radiator is used, air distribution is simple. The heat exchange units are located on the outside walls. Heat heated air rises from the radiator along the wall during the heating season. It mixes with the cold air adjacent to the cold wall. Natural air currents then move the air mixture throughout the room.
Central Air Distribution Systems
A central HVAC system requires a more complex air distribution system. Ducts, vents, and registers achieve air distribution.
Ducts distribute cool air from the AC air handler to different rooms in a house. Vents on walls or ceilings allow cool air into certain rooms while keeping warm air out of those same rooms. Registers on walls or ceilings allow cool air into one room while keeping warm air out of that room.
Air Distribution and Indoor Air Quality
A sound air conditioning system delivers clean air to the space being conditioned. Mechanical ventilation systems can also help control humidity levels in the indoor air. Uncontrolled humidity from bathrooms, laundry rooms, and shower rooms can cause condensation on windows and walls. A proper air distribution system will also control odors from cooking, smoking, and other household activities. Filtration systems and air cleaners work together with air handling systems for improved indoor air quality.
How an HVAC System Works
An HVAC system is used in a building to provide thermal comfort and control. It is a distribution, heating, and air conditioning system that provides clean air and fresh air and eliminates pollutants.
The HVAC system includes the following components:
Air Handler
Heat Exchanger
Condenser
Ductwork or Mechanical Ventilation Systems
Types of HVAC Systems
A heat pump is a device that transfers thermal energy from one place to another or from one medium to another. A heat pump is similar in concept to an air conditioner but works in reverse. Air conditioners take heat from the inside of a building and transfer it outside, while heat pumps do both. They also take heat from the outside and move it inside during winter.
There are three types of heat pumps:
Air-source
Ground-source (Geothermal)
Water-source
Air-source systems use outdoor air as their heat source and can only be used in moderate climates. Ground-source systems use the energy contained in the earth’s ground as their heat source and can be used year-round in any climate. Water-source systems use water as their sole energy source or as an additional heating source for milder temperatures.
Heat pumps are typically paired with air handlers but can be paired with gas furnaces for a dual fuel application. Ventilation systems must be constructed to allow for the efficient operation of all attached HVAC equipment.
An Overview of Air Distribution Systems
Mechanical ventilation systems are a system of ducts, vents, and diffusers that distribute clean air to a building’s occupied spaces.
Forced air systems are typically classified as either supply or exhaust. Supply air is heated or cooled before being distributed to the occupied space. Exhaust air is taken from the occupied space and then exhausted from the building.
Air Distribution System Construction
Air ducts are passageways that circulate and distribute conditioned air to and from a space. They work on the principle of air pressure difference. If a static pressure difference exists, air moves from the highest-pressure areas to the low-pressure areas. The greater this pressure drop, the higher the airflow.
Duct Construction Materials
Ducts can be made of many different materials. They are usually constructed from sheet metal or some other non-combustible material. Air pressure in the ducts is slight, so materials with greater strength are unnecessary to avoid duct leakage. Air ducts were initially made of thin, tinned sheet steel. Later, galvanized sheet steel, aluminum sheet, duct board, and insulated flexible ducts were developed.
Flexible duct is the most common type. It can be used in multiple applications without losing energy efficiency. Also, the plastic duct liner on the interior surface isn't susceptible to corrosion like metal ducts. Plus, it is much more cost-effective to install.
Ducts can have round, square, rectangular, or oval cross-sections. Round ducts are more efficient because less material is needed for the same capacity as square or rectangular ducts. The round design also reduces resistance to airflow. Since round ducts have less surface area, the air inside the ducts is less prone to heat loss or heat gain.
However, square or rectangular ducts conform better to construction and air sealing. They fit into walls and ceilings better than round ducts. Also, it is easier to install rectangular ducts between joists and studs than round ducts with the same capacity.
Tables compare the carrying capacities of rectangular and round ducts. There are several equivalent round duct sizes from which to choose. The one selected depends on the one-side dimension desired. For example, ducts 11" high may be desired to improve appearance. They may be needed between joists. New construction planning will identify what duct to use.
Types of Air Distribution Systems
Air ducts deliver cooled air to a room or room. They then return the air to the heating or cooling equipment. There are several types of supply duct systems. Some installations are a combination of different types. Supply duct systems can be categorized as follows:
Radial duct system.
Extended plenum system.
Reducing trunk system.
Perimeter loop system.
SDHV (small duct, high-velocity) system.
Duct systems may be installed in basements, crawl spaces, attics, false ceilings, and other structural cavities. They may also be installed on concrete floors (slabs) of homes without basements. If the duct system is in a crawl space or attic, it is usually insulated. Otherwise, heat gain or loss would be too significant, depending on the season.
The central duct runs across basements and under the floor joists. Branch ducts are then run between the joists and the floorboards for three sides of the branch ducts (panned joist space). Then, the central return duct is run alongside the conditioned air duct.
Duct runs installed on a concrete slab are usually made of metal, plastic, or ceramic. The ductwork can be laid out, and the slab can be poured over it. The branch ducts usually connect to the perimeter duct. Diffusers are connected to the perimeter duct at intervals along the floor. Downflow furnaces are used with perimeter loop duct systems. In a downflow furnace, return air enters at the top, and supply air exits at the bottom.
Supply and return air ducts and branches are often secured to a building's structural components. This can be done in various ways, but any method should avoid constricting airways. Standard supplies used to secure ducts in place include metal, iron, woven polypropylene, or wire straps.
Some buildings are designed with unit ventilators. The ventilator automatically draws air from the conditioned space to remove older, stale air and replace it with fresh, oxygenated air. The ventilator simultaneously brings in makeup air from the outside. These two streams of air pass by each other, transferring heat into the makeup air as it enters the building.
False ceilings often have holes for concealed piping, wiring, heat exchangers, and ducts. The false ceiling may have holes to enable the movement of conditioned air into the room below. Diffusers may also be used. The space between the false and real ceiling may be used as a conditioned return air plenum chamber. Some systems use heated panels to provide radiant heating.
Room Air Movement
Air entering a conditioned space through ducts must circulate without causing annoying drafts. Air circulation depends on the number and size of the duct supply outlets and returns inlets, as well as the velocity of the air moving through the supply outlets.
Air delivered to the room from the supply duct is called primary air. Primary air pushes against and mixes with air already in a room. The outlet velocity is the speed of the air as it leaves the duct outlet. The distance the air travels from supply registers before it shows 50 fpm is called the throw.
The overall size of the supply outlet is not essential. The total area of the air openings determines the fixture's capacity. The spread of the air that leaves the fixture is necessary. Properly setting fan speed is critical to proper operation and comfort level. Variable-speed fans in high-efficiency systems can achieve proper air flow while limiting energy use.
Return Air Ducts
The pulling action of a fan or blower almost always causes airflow through return air ducts. If the return airflow matches the airflow into a room, the air flow will be adequately balanced. The return duct system must be sized to accommodate the flow capacity of the HVAC unit.
The room will have negative pressure if there is more return air than supply air. This condition is like being starved for air. Rooms that starved for air are cold during the heating season and warm during the cooling season. To correct the problem, more air must be supplied to the room. A cooling coil that is starved for airflow is likely to freeze.
Return ducts should be placed in the stratified air zone of a room. Return air ducts would be located near the floor during the heating season. During the cooling season, such ducts should be located near the ceiling. Ideally, there would be two return duct inlets, one near the ceiling and one near the floor. The higher duct would be closed during the heating season, and the lower one would be closed during the cooling season.
However, most homes have only one return duct inlet due to expense and practicality. Therefore, the type of construction determines the placement of the return duct inlet in a building or whether the primary usage will be cooling or heating. Return duct inlets should be located at the farthest distance from the supply duct outlets in all cases.
Basic Air Distribution System Requirements
Air normally contains about 21% oxygen. People need a certain amount of oxygen in the air, and how much you plan to spend on heating and cooling all play an important role in determining which system is best for you. Depending on how far below this level the oxygen content falls, people can experience a variety of symptoms, including discomfort, breathing difficulty, drowsiness, disorientation, unconsciousness, and even death.
If a room is tightly sealed, anyone in that room will slowly consume the oxygen. Carbon dioxide, water vapor, and various impurities increase as oxygen is consumed. As the oxygen content in the air decreases and the carbon dioxide levels rise, the room begins to get stuffy.
Continued exposure to oxygen-deficient air would cause people to experience progressive symptoms, beginning with drowsiness and ending in unconsciousness and death. For these reasons, living spaces must have air with sufficient oxygen content. Fresh air must be admitted to provide necessary oxygen, and the air must also be kept at a reasonable temperature.
In the past, fresh supply air entered the space by infiltration through door and window openings and cracks in the structure. However, modern construction techniques reduce this air leakage. As a result, in contemporary homes and workplaces, the air-conditioning system must furnish fresh air.
A modern HVAC system should have a controlled fresh-air intake. This fresh air is conditioned and mixed with the recirculated air before it reaches the room.
Fresh air can be brought into a building using a fresh air duct connected between the outdoors and the return air ducts. When the air handler's blower operates, fresh air can be drawn in and conditioned before circulating through the conditioned space.
A screen or mesh at the inlet to the fresh air duct prevents insects and other critters from entering. This fresh air duct connecting outdoors and motorized dampers, bypass dampers, or barometric dampers usually regulates the return air duct.
Another method of bringing fresh air from the outside is to use a heat recovery ventilator (HRV) or energy-recovery ventilator (ERV).
There are three main types of heating systems: forced air, radiant heat, and hydronic heat. Forced air is the most common type of heating system found in homes. It heats the air by blowing it through a duct system throughout the house.
Radiant heat systems use heated water coils or electric resistance wires to warm objects in a room without blowing heated air around. Hydronic heating systems use hot water or steam to warm your home’s rooms.
How an HVAC Air Distribution System Improves Indoor Quality
The Clean Air Act, a testament to our collective concern for good indoor air quality, was created to ensure we can breathe clean air. One of the most essential parts of this act is the development of an HVAC ventilation system. This system helps to distribute fresh, clean, conditioned air throughout our homes, offices, and other buildings.
This HVAC system is a reliable guardian of indoor air quality, diligently removing airborne particulates, pollutants, and contaminants from our environment. It also manages humidity levels, ensuring a comfortable and healthy environment in which to work or live.
Conclusion
In conclusion, the HVAC air distribution system is crucial in maintaining a comfortable and healthy indoor environment. Air distribution is essential for homes with air conditioning systems, as it ensures the even distribution of conditioned air to different rooms. Whether through simple systems like radiators or more complex central HVAC systems with ducts, vents, and registers, the goal is to achieve optimal thermal comfort.
The importance of air distribution goes beyond temperature control. It directly influences indoor air quality by delivering clean air and effectively controlling humidity. A well-designed air distribution system can mitigate the impact of pollutants, allergens, and odors, contributing to a healthier living space.
Understanding the components of an HVAC system, including the air handler, heat exchanger, condenser, and ductwork systems, is fundamental to appreciating how these systems work together to provide thermal comfort and air quality. Various HVAC systems, such as air-source, ground-source, and water-source heat pumps, allow for flexibility in meeting different climate and energy efficiency requirements.
Duct construction materials, including sheet metal, aluminum, duct board, and insulated flexible ducts, offer choices based on strength, insulation, and cost considerations. The type of ducts used, whether round, square, rectangular, or oval, further impacts efficiency and installation ease.
Integrating fresh air intake is crucial to ensuring a continuous oxygen supply and preventing the buildup of pollutants in indoor spaces. Modern HVAC systems contribute to maintaining indoor air quality, whether through a controlled fresh air duct, motorized dampers, or heat recovery ventilators.
As technology continues to advance, the development of more efficient and environmentally friendly air distribution systems is a promising sign for the future of indoor air quality. A well-designed and properly maintained HVAC air distribution system not only enhances comfort but also promotes a healthy and sustainable indoor environment.
