HVAC Systems, Air Duct System, Supply and Return Ducts

The terms Air Handler, HVAC and Heating / Cooling Systems are basically synonymous. That is, they all do the same thing; the industry just gives them different names.

The term Air Handler, is normally associated with heating / cooling (HVAC) systems in commercial buildings. These are normally very large systems moving 2000 CFM to 10,000 CFM and higher. They may be mounted on the top of the roof or in large mechanical rooms located in the building. They often have an economizer or inlet damper that allows for a small amount of outside air or make-up air to be pulled in through the air handler.

Like most heating/cooling systems, Air Handlers require air filters to keep the blower/motor clean and to keep dust and dirt off the heating and cooling coils. Some of the older systems use roll filters, which is a very minimum duty air filter media that moves on a roll and maintains a clean air filter area continuously. Roll media filters are MERV 3 or less. In most air handlers today, a MERV 6 or 30% ASHRAE filter is used. These provide good air filtration to protect heating and cooling systems equipment without adding much in static pressure or pressure drop. Like all air moving devices, Air Handlers are affected by static pressure as well. The ductwork (both supply and return) as well as the air filter, cooling and heating coils and any dampering system will all affect the static pressure or pressure drop and thus affect the amount of horsepower required by the blower/motor to move the required amount of air through the system.

It should be pointed out, that all Air Handlers as well as HVAC systems are designed to heat and or cool a specific amount of air (as measured in CFM) at a designed static pressure or pressure drop. The amount of motor horsepower required to move this amount of air at the designed static load is then applied. As stated previously, static pressure or pressure drop is the greatest single factor affecting the performance of any air-moving device. And, air filters are a major part of that static pressure load.

All Air Handlers as well as HVAC systems are designed to heat and or cool a specific amount of air (as measured in CFM) at a designed static pressure or pressure drop.

The term HVAC or Heating/Cooling System is normally associated with the typical heating and air conditioning systems that are used in residential and light commercial construction. These systems are normally gas fired heating systems with an air conditioner/compressor system (electrical) for cooling. Some residential systems are Heat Pumps and some are geothermal systems. In all cases, these are ducted, fan powered units. Some homes use in-floor radiant heat (either electrical or heated fluid) that requires no fans or ducting.

Heating/cooling systems come in a variety of sizes and efficiencies. The size of the unit is normally selected to meet the heating and cooling demands of the occupied space. A 2,000 square foot home will have a smaller unit (in BTU’s and CFM) than a home that is 3,500 square feet in size. Many two-story homes have multiple heating/cooling system units, one for the lower level and one for the upper level. For more information on Heating and cooling systems and HVAC systems, click here.

Again, as with any air moving device, static pressure plays a key role in the overall design and performance of the system. All HVAC systems are designed to move a certain amount of air (in both the heating and cooling modes) at an approximate pressure drop. For example, a 3-ton heating/cooling system may be designed to move up to 1200 CFM at a total system static pressure of 0.5″ (w.g.) with a ½ Hp motor. This pressure drop includes the air resistance in both the supply and return ductwork as well as the resistance across the burner system and the A coil or air conditioning coil. This also includes the resistance through the throwaway or sponge washable air filter that comes with each system. It is also important to know, that most of the newer, high efficiency HVAC systems are all AIR-FLOW Critical units, meaning a certain amount of air must flow over the burner system or heat exchanger to perform correctly. Any reduction in air- flow can affect the performance.

So…the system static for ALL heating/cooling systems is designed with the minimally effective air filters. The air filter that comes with the HVAC system is only designed to keep the blower/motor assembly clean. It is not designed for any thing else. THEREFORE… when you add any other type of in-line air filter to the HVAC system, you MUST know the total system static pressure as to make sure the airflow is not affected by adding an air filter that is too restrictive. If, as we mentioned in the above paragraph, you add an air filter that increases the total system static to .6″ or .7″ you can reduce the airflow by 100 CFM to 200 CFM with a clean air filter. This additional static load increases as the air filter loads up, and decreases the airflow even further and decreases the overall efficiency of the HVAC system. IT IS IMPORTANT YOU UNDERSTAND THIS RELATIONSHIP BETWEEN STATIC PRESSURE AND HVAC PERFORMANCE.

Ductwork – Supply & Return

In order to effectively move the air through the occupied space, you will require some form of air duct system to both bring the warm/moist or cold air back to the HVAC system or Air Handler (thru return air ducts) to be properly cooled or heated. And you will require a system of supply ducts to take the cooled or heated air back to the occupied space. These systems of return and supply ducts are absolutely necessary for proper conditioning of the entire occupied area. For more information on supply & return ducts, click here.

The number of supply and return ducts is also a factor in proper heating and cooling. Having ample return ducts in any home or commercial facility increases the ability to more rapidly cool and/or heat the air. The faster you can cool or heat all the air, the less energy is used and a higher degree of comfort is achieved.

Conversely, having too few returns or supply ducts will create cold or warm spots or areas in the occupied space. In addition, the ability for the HVAC system to filter the air through the air filter will be reduced significantly when the number of return air ducts is reduced.

Alternate Heating and Cooling Systems

One of the other options to using forced air ducted heating systems, is to use in-floor or baseboard radiant heat. This type of system has been around for a long time and is making a comeback as a result of some newer technology. Since there is no ductwork used in this type of heating system you would not be able to use a traditional central air conditioning system. Often window air conditioners are used or, if cooing really is a necessity, then a central AC unit will be installed and some return or supply ductwork will be needed. Proper environmental design and planning is required when you use this type of heating design.

Introduction of Outside Air

This issue will be covered in more detail in the next section; however, we do want to address this aspect of environmental control. Due to energy conservation, homes, office and commercial buildings are now much tighter than ever. The lower air change rates (less than .25 Air Changes Per Hour) create some challenges in trying to keep the CO2 levels low and in maintaining enough fresh air exchange to keep the air from becoming stale.

Commercial buildings use Air Handlers that often have economizers or fresh air inlet dampers. These are used to introduce a certain amount of outside air, usually 5% to 15% depending upon the outside temperature and humidity levels.

Residential heating/cooling systems seldom have any mechanism for introduction of outside air. In many cases an HRV or ERV system is used to introduce conditioned, outside air. Also, the Pure Air Systems, Inc. HEPA Shield systems are used to introduce small amounts of outside, filtered fresh air.

Positive Pressure & Negative Pressure

If the occupied space is under negative pressure due to improperly balanced HVAC systems, more contaminants come into the indoor environment along with additional moisture. Positive pressure should be maintained at all times. This is covered in more detail in the following sections.