Dissatisfied HVAC equipment owners often complain of uneven levels of comfort between the different floors of their multi-story homes. Depending on the prevailing outdoor weather conditions, the temperature differential between the basement and the second story of a building can vary by as much as 20 degrees. The primary cause of this condition is a naturally occurring phenomenon called “air stratification.” While the physics are relatively complex, air stratification is a basic layering effect that allows large air pockets with different core temperatures to remain intact, regardless of whether the building’s central HVAC system is running.

Understanding the Stack Effect

Air stratification results from the influence of buoyancy and the stack effect. Heated air rises because it has a lighter density than colder air. During the winter, heat energy exits through natural and man-made pathways in the attic. In essence, the structure itself acts like a large chimney. Make-up air enters the building through penetrations and gaps in the perimeter envelop. In the summer, warmer air is drawn into the building as high pressure is attracted to low pressure.

In either case, the HVAC system is forced to cycle longer to meet the indoor load. This results in higher utility bills and can lead to premature equipment failure. When the system shuts off, the living area quickly becomes uncomfortable as the stratification effect is reestablished, especially in poorly insulated buildings.

Air Movement Strategies and Solutions

The airflow issues associated with multi-level homes usually originate with a poor duct design and improper equipment selection. There are a variety of strategies that can be used to counter the effects of air stratification and restore acceptable levels of comfort to every floor in the building.

* Repair or Replace the Ductwork: Prior to the installation of new equipment, a qualified HVAC contractor will design and install a duct system in accordance with ACCA Manual D standards. These calculations account for the unique characteristics of the building and each individual room. Static pressure and friction loss impact the velocity and quantity of air that travels through the system. Proper design accounts for the differences in ambient temperatures of buildings with multiple levels.

* Return Air Grilles: Regrettably, many production home builders reduce construction costs by eliminating critical HVAC system elements that affect comfort and efficiency. Return air grilles play an important role in providing a clear pathway for indoor air to return to the equipment for further conditioning. Reducing the size of a central return air grille may save on installed costs, but it can restrict the airflow and also contribute to nuisance air noise. Adding additional return air pathways can be extremely effective in reducing stale air pockets and equalizing the temperature throughout the building.

* Duct and Envelope Sealing: Ductwork leaks and loose building envelopes create a negative pressure that intensifies the effects of air stratification. As the unit draws outdoor air into the system, the capacity of the HVAC equipment is compromised. The indoor air temperature will tend to move in the opposite direction of the thermostat setting, and the system will continuously cycle in a futile attempt to meet the indoor load. Duct and perimeter sealing will improve efficiency, promote proper air mixture and help maintain a consistent temperature throughout the building.

* Zoning: Multi-story homes and offices present significant challenges in HVAC system design, primarily because of the stack effect. Builders looking to reduce costs often install a single system to service two story homes. In most instances, this results in comfort related complaints since the load varies significantly in the different zones. Dedicated systems can be installed for each level, but this is often a cost prohibitive solution.

Mechanical zoning relies on a single HVAC system and a network of motorized dampers, relays, zone controllers and communicating thermostats to address the effects of stratification layers. The dampers are installed in the various branches of the air distribution system. Each room has a separate thermostat, and both the dampers and thermostats are hard-wired to the control module. When a zone calls for heating or cooling, the damper opens and directs conditioned air into the targeted zone while the other dampers are unaffected. This allows the occupants on different floors to tailor the temperature to their personal preference.

* Air Balancing: Cheap supply registers lack a mechanism for controlling air volume and direction. Homeowners with comfort issues are advised to consider purchasing double deflection supply registers with an opposed blade damper for better air movement. Professional HVAC contractors have special equipment they use to adjust the registers in order to achieve the optimal balance of air flow throughout the building.

Consult an HVAC Professional

The stack effect was been largely ignored in HVAC system design until the introduction of ACCA Manual J and Manual D guidelines. If you are considering installing new heating and cooling equipment, always hire an HVAC contractor who will run a complete set of load calculations on your home or office before making an equipment recommendation.