Single-zone variable air volume (SZVAV) roof-top units

Published: January 13, 2014


Single-zone variable air volume (SZVAV) HVAC systems have the ability to simultaneously vary heating and cooling capacity and supply airflow while maintaining a zone's temperature at a constant setpoint. The ability to lower supply airflow rates when a zone is not fully occupied, and/or when heating or cooling loads are relatively low, can result in significant energy savings. This is because the supply fan's energy use is proportional to the third power of its rotational velocity, so running a fan at a high speed when it is not needed results in a large amount of wasted energy. This occurs in traditional, constant air volume systems; ventilation fans run at the same speed regardless of the zone's occupancy or heating or cooling loads.

Single zone variable air volume

SZVAV technology has been on the market for sometime; however, until recently this technology has been limited to the largest single-zone applications. These typically included school gymnasiums and auditoriums. Because of increased interest in energy efficiency, VAV technology has been adapted for smaller single-zone applications such as classrooms, conference rooms and retail stores. Updates to state energy codes now require that two-stage or variable speed fans be used in certain applications so that fans run at lower fan speeds when a system is operating under part-load conditions.

Key components

Variable air volume (VAV) equipped rooftop unit

Packaged rooftop units (RTUs) for variable air volume systems differ from constant volume RTUs in that they incorporate features that allow the unit to vary cooling capacity and air flow, including control software, variable frequency drives and compressor unloading features. Retrofit packages are available for existing RTUs.

Technologies coupled with SZVAV units for more efficiency

Outdoor air economizers: Outdoor air economizers introduce more outside air into the space when outdoor weather conditions are favorable. For example, if the ambient outdoor temperature is lower than the zone's temperature setpoint, then an economizer would introduce the relatively cooler air from the outside instead of running the system's compressor.

Energy recovery ventilators (ERVs): ERVs use return air from the zone to pre-heat or pre-cool outdoor ventilation air. On a hot summer day when indoor temperatures are lower than outdoor temperatures, the ERV will pre-cool ventilation air using the zone's relatively cooler conditioned air. On the flip-side, during the winter months the same ERV will use the zone's relatively warmer air to pre-heat the much colder outdoor ventilation air.

Heat recovery ventilators (HRVs): HRVs are similar to ERVs in application and how they save energy. However, HRVs only use the sensible heat from return air to pre-heat or pre-cool outside air and retain no moisture before exhausting air to the outdoors. ERVs transfer both temperature and moisture to incoming outside ventilation air. ERVs may be the preferred choice for some applications during the cold and dry winters of the Upper Midwest because they will conserve more moisture from the zone's conditioned air, which may have accumulated moisture from its inhabitants.

Occupancy sensors: Occupancy sensors can be used to tell the RTU whether or not the space is at full or partial occupancy. One type of occupancy sensor that is often used in this application is a CO2 sensor.



Implementation costs

Commercially available retrofit packages range between $200 and $600 per ton.

Potential energy savings

Typical savings achieved by installing a retrofit package are approximately 50%.

Market barrier(s)

lack of technical knowledge

SZVAV systems are well-suited to a variety of applications throughout the Midwest and in Wisconsin. The best applications are buildings with zones that frequently transition from full to partial occupancy and are only partially occupied for long periods of time. A prime example would be a school gymnasium, which might have high ventilation and cooling loads when fully occupied, but then little to no loads when unoccupied.

Schools: Typically schools are occupied at capacity only during well defined times during the weeks when school is in session. At other times (evenings, weekends and during the summer) occupancy is much lower resulting in lower space conditioning and ventilation requirements. During these times, ventilation fan speeds can be reduced to realize energy savings. In the past, VAV technologies have been used in the largest spaces (auditoriums and gymnasiums). More recently, manufacturers have scaled this technology to condition smaller spaces, like classrooms. Units are designed to appropriately condition individual classrooms as students move around the school throughout the day.

Retail: Individual retail outlets have their own heating and cooling requirements. Store traffic varies depending on the time of day and the day of week. SZVAV units can modulate fan speeds and operate at a lower cooling stage when store traffic is low. As more customers enter the retail space, the zone's cooling load may increase and the RTU's compressor engages to maintain the zone's temperature set point.

Hotels and conference centers: SZVAV systems may be well suited for smaller to medium sized conference rooms located in hotels, convention centers and office buildings. Between meetings and sessions, the zone's RTU will operate below design capacity. VAV technology allows fans to operate at lower speeds during these times.


Lower fan speeds = lower energy: Most energy savings result when a space is sparsely occupied and ventilation needs are well below designed capacities. During these times, ventilation fan speeds are reduced resulting in energy savings. In fact, the relationship between fan energy savings and speed reduction is not a 1 for 1 relationship. Generally, fan energy use increases at a rate equal to the cube (or third power) of a change in the fan's speed. So cutting the fan speed in half results in 1/8 of the energy used at the higher speed.

SZVAV can cycle on/off less frequently: Cooling compressors of SZVAV units operating at a lower fan speed may cycle on and off less frequently. Each time a compressor cycles on, an inrush of current results. The amount that the inrush current exceeds the normal operating current varies with each RTU. Regardless, compressors that cycle more frequently will experience periods with higher current and will tend to use more energy over time.

A quieter space: Most fans produce more noise when operating at higher speeds. Since fans in SZVAV systems run at lower speeds for more time, the amount of noise introduced into the zone is reduced.

Passive dehumidification: SZVAV systems can maintain a constant zone temperature by varying fan speed while keeping compressor(s) running. For example, if the zone calls for less cooling, the SZVAV system may keep its compressor(s) engaged and simply lower the fan speed in order to maintain the zone's temperature setpoint. The supply air temperature remains low, even though it passes over the coils at a slower rate. More dehumidification occurs because the cooler supply air holds less water.

In contrast, in constant volume systems the same amount of air is forced passed cooling coils regardless of whether or not the cooling compressor is running. When the sensible cooling load of a space decreases, a constant volume of supply air is forced past the relatively warm cooling coils while the compressor is off, delivering warmer air to the conditioned space. Warmer air can hold more moisture than colder air so less dehumidification of the supplied air occurs.

Challenges and market barriers

cost: Variable air volume rooftop units cost more per ton than do constant volume rooftop units.

technical knowledge: There are a number of considerations in determining the suitability of an application for an SZVAV. These include:
• Is the heating and cooling load uniform throughout the zone?
• Is the air distribution system designed to accommodate variable airflow?
• Is there adequate ventilation as supply airflow changes?
• Will the system be used for both cooling and heating?

Statewide energy savings

We took a high-level look at the potential energy savings in Wisconsin from single zone VAV systems. The estimate is meant to provide a sense of scale showing the impact this technology might have on Wisconsin energy customers.

To estimate statewide impacts, we assumed that this would be a retrofit opportunity to install single zone VAV technology for where applicable in commercial applications. We applied a technical savings rate of 50% to HVAC energy in commercial buildings. We assumed an applicability rate of 30%.

All data used for these estimates are from the Wisconsin Energy Statistics (2012) and Department of Energy's Commercial Building Energy Consumption Survey (2010).

Financial incentives

There are not currently financial incentives available for the single zone VAV technology specifically. However, Focus on Energy offers custom incentives for the Commercial building custom projects and this tecynology may be applicable under that program.

SZVAV technology can be implemented when an existing rooftop unit is replaced or as a retrofit option for existing units.

Retrofit option for existing units

Several manufacturers offer retrofit packages for existing rooftop units. Typically, the retrofit packages are focused on adding variable speed capacity to existing supply fans.

Available products:

Transformative Wave's Catalyst

A multi-component solution including a variable frequency drive, integrated economizer controls and remote monitoring and control. The Catalyst was selected for PNNL's Advanced Rooftop Control (ARC) Retrofit Field Study.

Bartell Drugstore Chain, Washington: This large scale fielding of the Catalyst system allows the drugstore chain to control and monitor over 124 rooftop units servicing 42 facilities in the Puget Sound area. Annual HVAC energy savings are estimated at 59 percent. MORE

IKEA Store, Renton, WA: The project integrated fan-only energy savings benefits of the Catalyst because the store's building management system already had many energy saving features. The estimated payback period for the project was 1.8 years, which includes a 25 percent subsidy from the local utility. MORE

Whole Foods, Santa Clara, CA: Five RTU's were retrofitted using the Catalyst system, resulting in approximately 29 percent energy savings during a four month monitoring period. MORE

Enerfit's V1 and V2

The Enerfit retrofit package offers a VFD for the supply fan, DCV and integrated economizer controls.

Bes-Tech's digi-RTU

The digi-RTU system modulates a rooftop air conditioning unit's supply fan AND compressor to better match a building's current cooling loads.

Vermont Country Store, North Clarendon, VT: Six RTU's were retrofitted and metered at a 170,000 square foot distribution center in Vermont. Estimated electricity savings were 68 percent, based on two weeks of pre- and post-retrofit monitoring. MORE

Office and Distribution Center in Omaha, NE: 19 Digi-RTU's were installed at the facility, resulting in an estimated payback of 1.8 years and savings of 85 percent. MORE

Understanding Single-Zone VAV Systems

summary Published by Trane, an equipment manufacturer that offers SZVAV products, this Engineers Newsletter provides a concise summary of the benefits and challenges surrounding the implementation of SZVAV systems. The summary offers illustrative diagrams of SZVAV system configurations and equipment control strategies. The newsletter describes strategies for ensuring that sufficient ventilation air is provided to the zone when the supply fan is operating at lower speed settings. Potential benefits of SZVAV systems are explored, including energy savings, noise reduction and passive dehumidification. Finally, the newsletter presents application considerations when pursuing SZVAV technology.
citation Murphy, J. Understanding Single-Zone VAV Systems, Engineers Newsletter, Vol. 42-2, Trane, 2013.

Single Zone VAV

summary This technical brochure summarizes SZVAV technology and system components. It describes the benefits of SZVAV systems, including fan energy savings; increased comfort through precise temperature control and passive dehumidification; reductions in compressor cycling; reduced noise and ease of installation and maintenance.
citation Single Zone VAV - discover how to save money, reduce energy consumption and lower sound levels, AAON corporation, R97020, (2013).

Advanced Rooftop Control (ARC) Retrofit: Field-Test Results

summary Although not explicitly about SZVAV, this comprehensive report presents the results of a recent field study about advanced rooftop control retrofits, technologies that closely resemble the features of SZVAV systems. The field study examined the operation of 66 rooftop units when operating using both constant air volume and demand controlled variable air volume control strategies. Energy consumption for the 66 systems was tracked over a 12-month period while each system's controls alternated between pre-retrofit (CAV) and post-retrofit (VAV) modes. The retrofit included adding variable speed fan control, integrated economizer controls and CO2-based demand controlled ventilation (DCV). Savings ranged between 22 and 99 percent with an average of 57 percent for all RTUs. The report presents the combination of conditions (RTU capacity, average daily run time and utility rates) that are most likely to result in a simple payback of less than 3 years based on the results of this field testing.
citation Wang, W. et al., Advanced Rooftop Control (ARC) Retrofit: Field-Test Results, Pacific Northwest National Laboratory, July 2013.

Up On the Roof - Recent advances in rooftop unit controls

summary Slide presentation made to the City of Fort Collins by ESource. Provides an overview of SZVAV systems. Also provides details about retrofit options for existing units.
citation Stein, J. Up On the Roof - Recent advances in rooftop unit controls, presentation to City of Fort Collins