Understand system pressure and residential area controls
Understanding system pressure and its effect on system performance is an important consideration when designing and installing zoned forced-air systems. It is true that system pressure can be higher in zoned forced air systems, but the way in which zoned systems are designed by their manufacturer and installed by the local contractor has a definite impact on how the system works and the level of comfort enjoyed by the occupants of the house. There are a number of different methods to provide pressure relief when using forced air zoning to control conditioned air distribution by means of pneumatic or motorized dampers. Many installation contractors have been reluctant to use zoning due to the additional pressures on the duct system associated with closing these zone gates and the effect this has on the system. This article will help you understand the alternatives available when it comes to providing pressure. relief.
With forced air zoning, the amount of system static pressure will vary depending on the size and number of zones that require air conditioning at any given time. How this pressure is relieved will be based on the size of the ductwork and the method of pressure relief that the contractor has designed into the system.
The Oversized Duct Option
The oversizing method of providing pressure relief refers to drastically oversizing the ductwork in the system. This method involves sizing the supply duct so that each zone handles approximately 75 percent of the system’s total airflow. While this method maintains adequate airflow through the system’s evaporative coil or heat exchanger, it also increases ductwork costs, reduces reach (velocity) when multiple zones are calling, and can be prohibitive due to space available to system components.
The bypass gate option
A more common method of providing pressure relief is to size the ductwork as if a zone control were not being used and include a barometric or motorized style bypass damper. The bypass damper is designed to direct excess airflow to a “dump zone” (an area where occupant comfort will not be compromised) or to the return duct.
When using this method, a lot of thought must be given to where to discharge the air conditioner. Some common areas to use as drop zones include: hallways, docks, lobbies, restrooms, or machine rooms. An unloading room or zone can be any non-critical area where conditioned air can be mixed with a larger volume of air so that the overall temperature of that area is not adversely affected. The use of a discharge zone also avoids the negative and possibly harmful effects of diverting conditioned air to the return. The biggest drawback associated with using a drop zone is the typical drop in overall system efficiency. By dumping air into a non-critical area, the energy used to condition this air is mostly wasted. As a result, the energy saving benefit in this application is significantly reduced or even eliminated.
When diverting excess air to the return, it is best to introduce the conditioned air into the return duct as far upstream of the heat exchanger/cooling coils as possible. This allows the air to mix well with the return air before it enters the air handler. Air that is too cold or too hot that is diverted to the return reduces the temperature differential in the heat exchanger or coil A, which causes a reduction in the efficiency of the equipment. Also, excessively shorting the A/C can cause the heater to shut off at the high limit or cause the A/C coil to freeze. A freeze control must be used to prevent the coil from freezing by diverting cooled air to the return.
An innovative option
A unique method of zoning that provides the energy-saving benefits of a return-type bypass system, equipment protection, and proper airflow characteristics involves the use of purpose-designed dampers with built-in controlled pressure relief. By slightly oversizing the ductwork to manufacturer’s specifications, these dampers will relieve pressure by diverting a small amount of air into enclosed areas. The combination of duct size and preset dampers ensures a minimum of 85% design airflow through the evaporative coil/heat exchanger at all times, but not enough to affect the temperature of a non-call zone in more than 1°F. The ASHRAE standard ( https://www.ashrae.org/ ) confirms that a person does not react to a 2°F change during a gear cycle by affirming. There are no restrictions on the rate of temperature change if the peak to peak is 1.1°K (2°F) or less. Therefore, the comfort of the occupants is not compromised.
The benefits of air circulation within the no call zone(s) include:
Humidity Control – Allows humidity to equalize more quickly throughout the building when using a whole house humidifier during the heating season. Clean Air – Allows for more air movement to carry particles from the living space back to be trapped in a high-efficiency air filter.
Negative Pressure Minimization – Allows a small amount of air to enter the area that is closed off. This will help reduce the possibility of an area being subjected to greater negative pressure compared to other means of pressure relief.
energy savings
In addition to the comfort advantages of forced air zoning, there is an additional benefit, energy savings. As long as the system is designed with the proper size of ductwork and bypass relief, a typical homeowner can save up to 20 percent on energy costs. However, following a good partitioning strategy and only conditioning the areas of the house that are occupied provides the most energy savings that can be achieved. It only makes sense to condition areas of the house when necessary.
An easy to use guide
Duct sizing calculations are based on ACCA ( https://www.acca.org/ ) and SMACNA ( https://www.smacna.org/ ) recommendations of 900 fpm maximum velocities in main ducts and 650 fpm in the duct branch to avoid air noise. What this means is that a motorized or barometric bypass damper is not required, reducing system cost and installation time. A design guide provides step-by-step instructions through the duct sizing process. Savings achievable with this type of system design range from 25 percent on a three-zone system to 33 percent on a two-zone system, compared to a zone system using a bypass damper .
