HVAC Air Distribution

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. Air distribution is the process of distributing cool air from the air conditioner air handler or furnace to different rooms in a house. This process is important 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. During the heating season, heated air rises from the radiator along the wall. 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. Air distribution is achieved using ducts, vents, and registers.

Ducts are used to distribute cool air from the AC air handler to different rooms in a house. Vents are used on walls or ceilings to allow cool air into certain rooms while keeping warm air out of those same rooms. Registers are used on walls or ceilings to allow cool air into one room while keeping warm air out of that room.

Air Distribution and Indoor Air Quality

A good air conditioning system delivers clean air to the space being conditioned. Air distribution 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.

New air handler and ductwork

Written by Jeremy James

Jeremy James is a veteran of the U.S. Navy who has grown his family's HVAC business into one of the most successful home service businesses in Knoxville, TN. More than 20 years of experience in the HVAC and Engineering industries and passion for what we do has translated into our customer service, professionalism, and quality. LinkedIn

How an HVAC System Works

An HVAC system is used in a building to provide thermal comfort and control. It is a system of distribution, heating, and air conditioning that provides clean air, and fresh air and eliminates pollutants.

The HVAC system includes the following components:

  • Air Handler

  • Heat Exchanger

  • Condenser

  • Ductwork 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 either as their sole source of energy or as an additional heating source for milder climates.

Heat pumps are typically paired with air handlers but can be paired with gas furnaces for a dual fuel application. Air distribution systems must be constructed to allow for the efficient operation of all attached HVAC equipment.

An Overview of Air Distribution Systems

Air distribution systems are a system of ducts, vents, and diffusers that distribute clean air to the occupied space in a building.

Air distribution 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 out of the building.

Air Distribution System Construction

Air ducts are passageways that circulate and distribute the conditioned air to and from a space. Ducts 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 small, so materials with a greater deal of strength are unnecessary to avoid duct leakage. Air ducts were originally 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 are. 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 building 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 it is to install round ducts with the same capacity.

Tables are available that compare the carrying capacities of rectangular and round ducts. There are several round duct equivalent 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 in between joists. New construction planning will identify what duct to use.

New American Standard air handler and ductwork

Types of Air Distribution Systems

Air ducts deliver cooled air to a room or rooms. 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. Ducts may be installed on concrete floors (slabs) of homes without basements. Ducts are usually insulated if the duct system is in a crawl space or attic. Otherwise, heat gain or loss would be too great, depending on the season.

In basements, the main duct is run across and just 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 main 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 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 of the furnace, and supply air exits at the bottom of the furnace.

Supply and return air ducts and branches are often secured to a building's structural components. This can be done in a variety of ways. Any method used should avoid constricting airways. Common supplies used to secure ducts in place include straps made of metal, iron, woven polypropylene, or wire.

Some buildings are designed with unit ventilators. The ventilator removes older stale air and replaces it with fresh oxygenated air by automatically drawing air from the conditioned space. The ventilator simultaneously brings in makeup air from the outside. These two streams of air pass by each other. In doing so, heat is transferred into the makeup air as it enters the building.

False ceilings are often used to conceal piping, wiring, heat exchangers, and ducts. The false ceiling may have holes to allow movement of conditioned air into the room below. Diffusers may also be used. The space between the false ceiling and the 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. It also depends on 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 important. The total area of the air openings in the fixture determines the fixture's capacity. The spread of the air that leaves the fixture is important.

Return Air Ducts

Airflow through return air ducts is almost always caused by the pulling action of a fan or blower. If the return airflow does not match the airflow into a room, the flow of air will not be properly balanced. The return duct system must be sized to accommodate the flow capacity of the HVAC unit.

If there is more return air than supply air, the room will have negative pressure. 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 supply air must be delivered 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. During the heating season, return air ducts would be located near the floor. 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 closed during the cooling season.

However, due to expense and practicality, most homes have only one return duct inlet. Therefore, the placement of the return duct inlet is determined by the type of construction in a building or whether the primary usage will be cooling or heating. In all cases, return duct inlets should be located at the farthest distance from the supply duct outlets.

Wall-mounted AC Unit

Basic Air Distribution System Requirements

Air normally contains about 21% oxygen. People need a certain amount of oxygen in the air too 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. As oxygen is consumed, the amounts of carbon dioxide, water vapor, and various impurities increase. As the oxygen content in the air decreases and the carbon dioxide levels increase, the room begins to get stuffy.

Continued exposure to the 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. 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 modern 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 is operating, fresh air can be drawn in and conditioned before being circulated 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 the return air duct is usually regulated by motorized dampers, bypass dampers, or barometric dampers.

Another method of bringing in fresh air from the outside uses 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 that runs throughout the home.

Radiant heat systems use heated water coils or electric resistance wires to warm up 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

Air quality is a big concern for many people. This is why the Clean Air Act was created to ensure that we can breathe clean air. One of the most important parts of this act is that it has led to the development of an HVAC air distribution system. This system helps to distribute fresh, clean, conditioned air throughout our homes, offices, and other buildings.

This HVAC system improves indoor quality by removing airborne particulates, pollutants, and contaminants from our environment. It also manages humidity levels so that we have a comfortable environment in which to work or live.

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