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Variable Air Volume Ventilation

Variable Air Volume Ventilation

Austin BAROLINTravis English

A Variable Air Volume (VAV) system can help decarbonize hospital buildings by reducing reheat energy through its ability to adjust the amount of airflow and cooling/heating capacity based on the actual demand.


Executive Summary

Standard practice in California and the US for many years has been the use of constant volume systems, which leads to over-ventilation. As a default, many areas of a hospital are constantly reheating a lot of air all year long. Those areas can be operated as variable air volume (VAV), reducing the amount of air that is conditioned and thus reducing the amount of reheat energy used. It is estimated that converting from constant air volume (CAV) to VAV can reduce 30-40% of the natural gas in a hospital.

In other states, VAV is already a requirement for hospitals which follow ASHRAE 90.1. (Map of state adoption of ASHRAE 90.1) ASHRAE 90.1 confirmed that VAV is a requirement in hospitals in an official interpretation. However, in California, there is no such requirement in the state’s Energy Code (Title 24, Part 6: California Energy Code) for hospitals, as they were exempt from the code entirely until 2019 and only now must abide by minimal efficiency standards for lighting.

Variable Air Volume Ventilation

Technical Description

The path of least resistance

For many years, in California, hospital HVAC systems were designed as constant volume (CAV) systems by most practicing HVAC engineers, not because of any evidence-based reason or perceived increased health benefit, but because it was the path of least resistance.

CAV was the design norm. California hospitals were seen as “exempt from Title 24”, and, as such, exempt from the need to diligently pursue energy efficiency. Until a few years ago, hospitals were completely exempt from the California Energy Code, and still have no specific HVAC requirements that are based on energy efficiency.

There was also no code requirement in California to specify VAV systems, as is still the case; (see “Barriers” section below).

Until very recently, the California Mechanical Code required return air VAV boxes for each zone with supply air VAV, which added both cost and complexity to the systems. This led to many VAV systems being value-engineered out of the final designs.

More ventilation, more of the time = more energy

Conditioning the space in healthcare buildings is an incredibly energy-intensive process. Outdoor air is brought into the building, and in some cases, mixed with return air, then cooled down to remove heat and moisture, and then delivered to each space at a specified temperature, often 55 degrees. Once at the terminal unit, the air is reheated (typically using a hot water coil, but electric resistance coils also exist in some facilities) to the desired zone temperature setpoint if the load is insufficient, which without reheating would result in overcooling. When minimum ventilation rates increase, the building uses more cooling and reheat energy. In an average hospital, reheat energy is generated by burning natural gas in a boiler to heat water (or make steam, which is then used to heat water), which is then distributed to each reheat coil. This reheat energy accounts for 40-45% of the overall hospital energy consumption. More information on the end-use energy breakdown in hospitals can be found in this summary of an NREL report.

Ventilation rates are set to provide enough airflow to comply with code and maintain comfort. This is accomplished by cycling enough air through the space to:

  1. Remove any airborne contaminants such as particulates, carbon dioxide, or airborne infectious particles.
  2. Meet the cooling and heating loads for the space

In a constant-volume system, the ventilation rates are set to meet the peak cooling load in the space, which occurs only a small fraction of the time, as shown below in Figure 1. As such, in a constant volume system, many spaces are over-ventilated almost all the time.

Figure 1: Air Requirements in Sample LDRP or Patient Room in Los Angeles, CA

Designing a VAV system

In existing buildings, CAV to VAV conversions require redesigning the room ventilation schedule. In many cases, this can be a controls-only redesign. If supply terminal units with modulating dampers and VAV capability exist, implementing the measure requires only reprogramming of control setpoints.

The terminal box supply minimums should be reprogrammed to the minimum level allowed by code. In CA, Title 24, Part 4, Chapter 4, Table 4-A lists the minimum air changes allowed per space type. In the rest of the country, ASHRAE 170, Table 7.1 lists the minimum air changes allowed per space type.

There have been updates to many of these codes in the past few years.

The redesign must ensure all space pressurization requirements are met as per applicable code.

Setup the return or exhaust (if 100% outside air) fan to control to a fixed offset from supply air flow. The fixed offset may need to be adjusted to ensure that negative pressure is maintained where it is required. In a new building, these spaces would have a separate exhaust system, possibly with air valves, to ensure that negative pressure is maintained, while allowing the remainder of the system to track demand.

It is also possible to provide additional control on the return or exhaust side to track supply air flow, such as providing control damper(s) by suite, group of rooms, or floor. This results in better control, but adds cost and complexity.

A final air balance should be completed after all ventilation setpoints have been changed to ensure minimum air changes per hour (ACH) and pressurizations are met.

Figure 2: VAV System Design Diagram

Control strategies

VAV control strategies are well known in the industry. The best practices are well known and documented in ASHRAE Guideline 36.

Once a VAV system is implemented, revise the control strategy for the air handling unit serving the area to include both supply air temperature reset (SATR - see section “Retro-commissioning”) and supply air static pressure reset. The former reduces both cooling and reheat energy. The latter reduces fan energy. These strategies only “work” for VAV systems.

Setup occupancy schedules in the VAV controls, so that space ventilation can be significantly reduced and temperature deadband widened when the space is unoccupied. This strategy further reduces cooling, reheat, and fan energy in spaces with significant unoccupied hours.

Best practices

A white paper was developed, an offshoot of California Energy Commissioning research project “Advanced HVAC Technology Demonstration Project to Reduce Natural Gas Use in Hospitals”, [1] where existing constant-volume systems were converted to VAV. During the project, the design process for variable volume systems had to be articulated. Once articulated for the project, we formulated it into a generic design process, which is presented here. The white paper describes the best practice method for designing a VAV system in both existing building and new construction applications. The following seven steps is a process for design of variable volume central air handling systems in US hospitals:

        1. Define the peak and neutral conditions (refer to paper for definition of neutral condition),
        2. Divide the space into zones,
        3. Load calculations (peak condition),
        4. Peak condition room balance schedule (RBS),
        5. Neutral condition room balance schedule (RBS),
        6. Operating performance prediction, and
        7. Acceptance criteria for commissioning.

How does this decarbonize?

Per the ASHRAE Advanced Energy Design Guide for Large Hospitals, reheat is often the largest single energy consuming process in large healthcare facilities because of the requirements for high air change rates and the need to both cool spaces and maintain acceptable relative humidity rates. The baseline model in the ASHRAE guide shows that reheat represents 20-30% of the total energy use of the building in all climate zones, but that number could be higher in some facilities. One of the best ways to minimize reheat is to select a ventilation system that inherently reduces reheat. CAV systems use the most reheat energy, on average 30-40% more energy intensive than VAV systems, and they are not recommended. VAV reheat systems reduce reheat energy, especially in unoccupied periods when air change per hour (ACH) requirements generally do not apply, but reheat is still often the largest component in energy models of these systems. VAV systems reduce ventilation levels based on each space's demand. Reduced ventilation means less air needs to be reheated and less overall demand for boiler heating.

A 30-40% reduction in reheat energy means the amount of natural gas needed to provide a comfortable environment for occupants will be significantly reduced leading to direct reduction of the facility’s Scope 1 emissions. Indirectly, it reduces the energy needed to meet the heating load overall, which improves the possibility of using heat pumps or other heating plant alternatives.

In systems that recirculate air (not 100% outside air systems), reheat energy is a false load added to the building. When air is reheated, delivered to the space, then recirculated and returned to the air handling unit, that heat must then be removed by the cooling coil. In addition to significant fossil gas energy savings, reducing reheat through proper use of VAV systems also reduces cooling energy, which will have an impact on Scope 2 Purchased Electricity emissions.


Barriers: Codes

      1. Currently in California, hospitals are exempt from the requirement for VAV zone control. Any zone in a hospital is permitted to be constant volume. This is not true in the national standard ASHRAE 90.1, where all zones must include controls to turn down to their code-minimum ventilation. California code does not prevent use of VAV, neither does it require VAV.
      2. VAV zone controls reduce airflow when the full cooling is not required, to reduce reheat of previously cooled air. Commercial buildings in California have a prescriptive requirement prohibiting simultaneous heating and cooling of air (reheat) (California Code of Regulations 2016). However, there are exceptions for spaces with humidity or pressure controls. The exceptions affect some hospital zones, particularly critical spaces. But critical spaces are a small fraction of hospitals’ total footprint.
      3. Historically, California's code added cost. Up to the 2019 California Mechanical Code, return boxes were required for VAV systems in hospital spaces. Return boxes are a costly addition and will not help the payback for retrofit applications. Recent changes to the 2022 California Mechanical Code have removed that requirement and only require return boxes for rooms with specific pressurization requirements.

Barriers: Culture

      1. The traditional design standard for constant volume was likely caused by the increased costs of designing a VAV system and a lack of attention to the excess energy consumption of constant volume systems. Code barriers to VAV implementation helped to solidify CAV design in California hospitals. This is a common design practice utilized by the engineering community - primarily in California.
      2. Rather than designing hospitals to historical standards, engineers must take advantage of the opportunities afforded by new codes and design all hospitals with VAV.

Barriers: Cost

      1. Facilities without modern terminal units will not be able to implement VAV in the existing facility as it is. It will require the installation of VAV terminal units with digital controls, which will add significant cost to a retrofit project and increase the project payback. See the Financial Analysis section below.


      1. Existing buildings: Without the need to install return boxes in most spaces, conversion to VAV can be as simple as changing the control programming on a terminal unit or as involved as replacing pneumatic or mechanical terminals with Direct Digital Controls (DDC) VAV terminals on the supply side. The Kaiser South Bay site only required reprogramming the terminal box minimum ventilation levels followed by an air balance.
      2. New construction: VAV should absolutely be designed in every new facility and controlled via the central building automation system. Any additional cost of a VAV system will be offset by operational energy cost savings in under 5 years.
      3. The implementation of this measure varies based on whether or not the facility is located in California. Outside California, VAV ventilation design is widely implemented as per ASHRAE 90.1 and as permitted by ASHRAE 170. However, within California, VAV is not required. And before 2023, the California Building Code inadvertently discouraged VAV with a requirement for additional terminal boxes on the return side. This requirement added significant cost to the construction of hospitals and resulted in many CAV systems being designed and built.

Financial analysis and business case

There could be two levels of implementation costs for retrofitting existing hospitals.

Retrofit controls upgrade: If the project only requires reprogramming control setpoints and performing an air balance:

        1. The estimated project cost could range from $1-3/sf.
        2. The project cost will include an engineer to redesign minimum setpoints per current codes, a programmer to program new setpoints, and an air balancer to perform pre/post balancing.
        3. Simple payback could be 2 years or less.

Infrastructure upgrade: For a facility with pneumatic controls, the work may involve adding DDC controllers to existing terminal units. For facilities without terminal units - just reheat coils, or with outdated terminal units, the project will require installation of new VAV terminals in existing ductwork. That cost can increase by up to 10x.

        1. The estimated project cost could range from $20-$30/sf.
        2. The project cost will include the above items as well as possibly any of the following: redesign and replacement of terminal units, controller hardware, sensors, and control panels.
        3. Simple payback could be upwards of 10-15 years depending on what needs to be replaced.

Even if your facility requires an infrastructure upgrade, it’s likely that old terminal units and controls are nearing the end of their useful life and the capital replacement cost could be part of the facilities capital improvement budget, removing this cost from the energy payback equation. Performing a VAV conversion with DDC controls will unlock many other decarbonization and energy reduction opportunities within the facility by significantly lowering the amount of heat (reheat) required to satisfy each zone.

Case Study: Kaiser South Bay

Kaiser Permanente South Bay Medical Center, Harbor City, CA

A demonstration project was completed to convert a CAV system to VAV while monitoring IAQ throughout the facility to ensure air quality was maintained at acceptable levels for occupant safety and comfort.

While it was predicted lower ventilation rates and VAV systems would increase airborne contaminant levels, contaminant levels experienced little or no change. Airborne contaminant levels were low, before and after project work. The profile and scale of airborne contaminants were comparable before and after changes. Contamination levels from space to space were also comparable. Airborne contamination was found to be primarily episodic, with episodic behaviors, or behaviors occurring at irregular intervals, unrelated to ventilation rate. The project data showed no relationship between ventilation rates and contaminant levels for two air changes per hour (ACH) to 12 ACH, and down to 0.5 ACH in administrative areas. This result is surprising, given that high ventilation rates in hospitals have long been assumed to be related to controlling airborne contamination.

The project cost to implement VAV in a hospital with existing terminal units includes design, reprogramming of minimum ventilation setpoints on each box completed by a controls contractor, and a pre and post air balance done by a TAB contractor. The cost of the Kaiser South Bay project was approximately $290,000.

As a result of the reduced airflow levels, the savings were 95,000 therms (21 percent) of natural gas, 2.8 million kWh (25 percent) of electricity. Carbon emissions reductions were more than 2,480 metric tons of CO2 in the first year after implementation.

This project was funded by the California Energy Commission. The final report will be available on the CEC website in 2023.

Case Study: Kaiser Baldwin Park

Kaiser Permanente Baldwin Park, Baldwin Park, CA (CEC Project PIR-19-008)

A model-based optimal control for a building HVAC system was proposed to minimize total HVAC system energy consumption by performing integrated energy efficiency measures in a building systems approach instead of individual building components.

CAV to VAV: Overnight turndown was incorporated into all zones to achieve day/night VAV. System turndowns for each AHU are shown below.

Air Handling System

System Maximum

System Minimum

Turndown Capability

















Case Study: Vallejo MOB

Vallejo Medical Office Building, Vallejo, CA

215,000 square foot Medical Office Building with VAV boxes and advanced control sequences to minimize reheat energy consumed. Results from the project led to heating energy reduced by 55%, overall EUI reduction of 49.5 kBtu/sqft, and over $200,000 saved per year.

Case Study: Kaiser Westside

Kaiser Westside Medical Center, Hillsboro, OR

This project, the 1st LEED project for Kaiser Permanente, involved converting an existing 265,000 sf hospital constant volume air distribution system to VAV and performing maintenance and adjustments to existing heat recovery systems. The facility operations team performed a programming only VAV conversion.

The results showed a 60% annual decrease in HVAC energy after the project was completed.

The VAV conversion in all non‐clinical spaces on D&T floors of the hospital has produced considerable electricity savings in the building. This is primarily attributed to fan power reduction in the AHUs. Recovered heat from the heat recovery chillers is now also connected to the MOB low- temperature heating hot water loop. This greatly enhanced the heat recovery system efficiency and reduced the amount of natural gas consumption for the boilers.



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