Understanding system pressure and its effect on system performance is an important consideration when designing and installing forced-air zoned systems. It’s true that system pressure can be greater in zoned forced-air systems, but how zoning systems are designed by their manufacturer and installed by the local contractor have a definite impact on how the system operates and the level of comfort the home occupants enjoy. There are a number of different methods for providing pressure relief when utilizing forced-air zoning to control air distribution of conditioned air by means of motorized or pneumatic dampers. Many installing contractors have been reluctant to use zoning because of the added duct system pressures related to the closing of these zone dampers 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 upon the size and quantity of zones calling for conditioned air at any given time. How this pressure is relieved will be based upon the way the ductwork is sized and the method of pressure relief the contractor has designed into the system.
The Oversize Ductwork Option
The oversizing method for providing pressure relief refers to drastically oversizing the system ductwork. This method involves sizing the supply duct for each zone to handle approximately 75 percent of the total system airflow. While this method does maintain adequate airflow across the evaporative coil or heat exchanger of the system, it also increases ductwork costs, reduces throw (velocity) when multiple zones are calling and can be prohibitive due to space available for system components.
The Bypass Damper Option
A more common method of providing pressure relief is to size the ductwork as if no zone control is being used and include either a motorized or a barometric style bypass damper. The bypass damper is designed to direct excess airflow to either a “dump zone” (an area where the comfort of the occupants won’t be compromised) or into the return duct.
When using this method, a lot of thought should be put into where to dump the conditioned air. Some common areas to use as dump zones include: hallways, basements, foyers, bathrooms, or mechanical rooms. A dump room or zone can be any non-critical area where the conditioned air can mix with a larger volume of air so the overall temperature of that area won’t be adversely affected. The use of a dump zone also avoids the negative and possibly damaging effects of bypassing conditioned air back into the return. The greatest drawback associated with using a dump 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 bypassing excess air back into the return, it is best to introduce the conditioned air into the return ductwork as far upstream from the heat exchanger/cooling coils as possible. This allows the air to mix well with the return air before entering the air handler. Air that is too cold or too hot that is being bypassed to the return reduces the temperature differential across the heat exchange or A-coil, causing a reduction in equipment efficiency. Additionally, excessive short circuiting of conditioned air can cause the furnace to shut off on high limit or cause the air conditioning coil to freeze. A freeze control should be used to prevent coil freeze-up when bypassing cooled air into the return.
An Innovative Option
A unique method of zoning which gives the energy-saving benefits of a return-type bypass system, equipment protection, and proper air flow characteristics involves the use of dampers specifically designed with controlled pressure relief built in. By slightly oversizing the ductwork based on the manufacturer’s specifications, these dampers will relieve pressure by bypassing a small amount of air into the closed zones. The combination of duct sizing and preset dampers assures a minimum of 85% of design airflow across the evaporative coil/heat exchanger at all times, but not enough to affect the temperature of a non-calling zone by more than 1°F. ASHRAE ( https://www.ashrae.org/ ) Standard confirms that a person does not react to a change of 2°F during an equipment cycle when stating. 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.
Benefits of circulating air within the non-calling zone(s) include:
Humidity Control – allows for humidity to
equalize more quickly throughout the building when using a whole-house
humidifier during the heating season. Air Cleaning-allows for more air movement
to carry particulate from the living space back to be trapped in a
high-efficiency air cleaner.
Minimizing Negative Pressure – allows a small amount of air into the zone that is closed. This will help reduce the possibility of a zone being put under greater negative pressure when compared with other means of pressure relief.
In addition to the comfort advantages of forced-air zoning there is an added benefit, energy savings. Provided that the system is designed with proper duct sizing and bypass relief, a typical homeowner can save up to 20 percent on energy costs. However, following a good setback strategy and only conditioning the areas of the house that are occupied provides the major portion of the energy savings that can be realized. Only conditioning the areas of the home when it’s needed just makes good sense.
An Easy-to-Use Guide
Duct sizing calculations are based on ACCA ( https://www.acca.org/ ) and SMACNA ( https://www.smacna.org/ ) recommendations of maximum velocities of 900 fpm in the main ducts and 650 fpm in the branch ducts to avoid air noise. What this means is a barometric or motorized bypass damper is not required, thus reducing the system cost and installation time. A design guide provides step-by-step instructions through the process of sizing the ductwork. The savings that can be realized 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 utilizing a bypass damper.