Chapter 13: Return, Relief and Exhaust

13.1. Theory and Applications

13.1.1. Return and Relief Systems

13.1.2. Exhaust Systems

13.2. Commissioning Return, Relief, and Exhaust Systems

13.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test

13.3. Testing Guidance and Sample Test Forms

13.4. Supplemental Information

13.4.1. Special Exhaust Fans and Systems


13.1. Theory and Applications

Return, relief, and exhaust systems provide the network that moves the air delivered by the supply distribution system back to the air handling location. The air is then re-circulated or exhausted out of the building as required for ventilation purposes, building pressure control purposes, or to control contamination from processes. Many of the issues associated with the return, relief, and exhaust duct system are very similar to those associated with the supply duct system as outlined in Chapter 11: Distribution. The issues associated with the return, relief and exhaust fans and their drive systems are very similar to those discussed in Chapter 10: Fans and Drives. These chapters should be referenced to supplement the information in this chapter.

13.1.1. Return and Relief Systems

For air handling systems that can recirculate some of the supply air, it is necessary to provide a return system to move the air from the space served back to the air handling unit location. If the air handling system is equipped with an economizer cycle (see Chapter 3: Economizer and Mixed Air), it is also necessary to provide a relief path to allow the extra outdoor air introduced by the cycle to exit the building and prevent building pressure control problems. On some systems, this relief air is called exhaust air, but should not be confused with exhaust air that is provided for the purpose of control of contaminates and compliance with ventilation requirements.

In most cases, the return and relief path associated with an air handling system share the same network of duct and plenum space. The return and relief path are differentiated at the air handling unit location where the return path connects to the mixing chamber of the air handling unit and the relief path exits the building. In some cases, the relief dampers are located remotely relative to the air handling equipment to minimize the potential for recirculation into the outdoor air intake.

Ideally, the pressure drop through the return-relief network should be the same in the 100% return cycle and the 100% relief cycle. In doing this, the designer can minimize the potential for flow variation due to varying static pressure requirements in the different flow paths as the economizer system modulates from full return to full relief. Unfortunately, the configuration of many buildings and their mechanical spaces often dictates that one flow path is longer than the other. The difference in pressure drop between the return path and relief path can impact system performance in several ways:

       Building pressure relationships can vary with the position of the return and relief dampers.

       System balancing needs to occur with the system configured to operate with the worst-case pressure drop.

       Non-linearity can be introduced into the economizer cycle due to varying inlet pressures at the return and relief dampers. This non-linearity can be a destabilizing influence on the control loop associated with the economizer. The loop tuning can be affected as the seasons vary, and loop tuning can in general be more difficult.

       Non-linearity can be introduced into the return and supply fan volume control systems which can have a destabilizing influence on their control, resulting in loop tuning difficulties.

Return and relief air can be collected in ducted systems, plenums, or combinations of the two. Ducted returns usually add first cost to the project and require more coordination to fit the system with other systems located above the ceiling and in the mechanical space. Ducted returns provide the following advantages:

       Better opportunities for controlling sound

       Better methods for controlling pressure relationships

       Better resistance to tampering and contamination in areas where security is a concern.

Plenum systems make use of the existing ceiling cavity space to collect the return and relief air, and thereby have the potential to reduce construction costs and minimize coordination problems. However, plenum returns have the following disadvantages:

       Subject to leakage problems similar to those described for under floor plenums in Chapter 11: Distribution.

       Subject to cross talk and sound transmission between adjacent spaces without sound treatment on the return grills.

       More difficult to establish and maintain building pressure relationships with than ducted returns.

       More prone to security breeches.

High-rise buildings commonly use the mechanical shaft that houses the supply ducts and piping systems as a return pathway, eliminating the need for a return duct running the height of the building. When this occurs, it is important that the designer establish a shaft size large enough to keep the return velocities and pressure drops to an acceptable level based on the available free area after accounting for supply ducts, piping, conduit and other obstructions. In most situations of this type, the shaft will be fire rated and the penetrations into it from the ceiling return plenum or duct system will need to be protected by a fire damper mounted in the shaft wall.

Pipe, conduit and other materials installed above the ceiling in a return or relief plenum that are flammable or could generate smoke or noxious fumes in a fire must be U.L. listed as suitable for plenum use. This requirement often places some restrictions on the type of cable, pipe, and insulation that can be installed on a plenum-equipped project.

Air handling systems with a relatively short path from the occupied space to the air handling unit and relief locations can often be designed so that the supply fan can pressurize the space slightly, thereby controlling infiltration and providing the necessary force to move the air back through the return system or relief system. Systems with short return paths but long relief paths may require relief fans that operate when necessary to move air through the relief system on the economizer cycle. Systems with long return paths generally will require a return fan that is sized to handle the worst-case static pressure requirement for the return/relief system.

Control of the return and relief fans and dampers is generally integrated into the economizer control system, the building static pressure control system, and/or the supply fan volume control system.

13.1.2. Exhaust Systems

Independent, fan-powered exhaust systems are required in most buildings to ensure that the outdoor air ventilation rates are maintained, control moisture accumulation, and remove contaminates. Building mechanical codes and industry standards like ASHRAE Standard 62-2001 - Ventilation for Acceptable Indoor Air Quality typically set required flow rates. In most commercial buildings, a make-up air system must replace the air that is removed from the building in order to prevent building pressurization problems due to the exhaust flow. Make-up air functions are often combined with other air handling requirements in the air handling systems that supply air based on the building loads. Bringing the make-up air in with the main air handling systems often reduces the energy requirements associated with the make-up air due to energy recovery effects from return air. For a more detailed discussion, see Section 3.1.3 Building Pressure Control and Return Air Heat Recovery.

Exhaust systems need to be interlocked with their make-up air systems to ensure that both systems function together to prevent abnormal and potentially dangerous pressure relationships from developing. On large systems, this situation could easily occur if one system were started without the other system starting. Large systems may also require a specialized start-up sequence that ensures that both systems come up to speed at the same time.

Variable flow supply systems often require variable flow exhaust systems to maintain the desired pressure relationships. These systems can be expensive and time consuming to start-up and maintain, but the energy savings associated with minimizing make-up air and exhaust air flows justifies the complexity. However, if these more advanced approaches are employed, the initial and ongoing commissioning of the systems is critical to success. Without commissioning, the failures and problems that can occur often result in operating and other costs that can be far in excess of the costs associated with a more energy intensive but less complex design.

Large exhaust systems are often good candidates for energy recovery strategies. A well-designed and implemented energy recovery cycle can often recover a significant portion of the energy required to treat the make-up air from the exhaust stream, especially the preheat energy.

In order to maintain proper pressure relationships, the make-up air must be introduced into the building in a manner that allows it to provide ventilation and reach the exhaust system via a reasonably unrestricted path. Frequently, doors to areas that have exhaust paths require an undercut if no make-up air is supplied directly to the space. Janitorís closets and small bathrooms are good examples of this type of situation. Codes usually limit the amount of undercut that is acceptable for a variety of reasons. In situations where the undercut is not sufficient to allow the necessary flow without excessive pressure drops, transfer grills can be provided between the adjacent spaces to facilitate the flow. If the wall through which the air is being transferred is part of a fire or smoke rated assembly, the transfer opening will require protection in the form of a fire damper, smoke damper, or smoke curtain.

13.2. Commissioning Return, Relief, and Exhaust Elements

The following sections present benefits, practical tips, and design issues associated with commissioning an air handlerís return, relief, and exhaust systems.

13.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements lists practical considerations for functional testing. Key Preparations and Cautions address potential problems that may occur during functional testing and ways to prevent them.

Key Commissioning Test Requirements

Background information on return, relief, and exhaust systems is similar to the information associated with supply fans and drives (Chapter 10: Fans and Drives) and the supply side distribution system (Chapter 11: Distribution). Note that many of the following test requirements pertain to issues that should be addressed and checked during design, construction, or initial start-up and TAB in order to successfully test each system.

Pressures in open plenum returns (ceiling plenums) should be considered to ensure that the plenum pressures will be satisfactory in all operating modes in order to minimize infiltration and prevent unintended movement of ceiling tiles. (Satisfactory pressures may be positive with respect to outside in warm, humid climates, or negative in cold climates, but should not be excessively positive or negative.)

In addition to the acceptance criteria listed in Section 10.2 Commissioning Fans and Drives and Section 11.2 Commissioning the Distribution System, some exhaust systems can have acceptance criteria that are related to code, OSHA dictated hazard control functions, process cleanliness, or control functions. The system designer should be familiar with these requirements and specify compliance criteria in the contract documents.

Key Preparations and Cautions

In addition to the cautions listed in Section 10.2 Commissioning Fans and Drives and Section 11.2 Commissioning the Distribution System, working with or around hazardous exhaust may require that additional safety precautions be included in the test procedures. Typical exposure hazards may include:

a Viral, microbiological, or radioactive contamination in exhaust systems serving hospital and laboratory system or pharmaceutical processes.

b Acids or caustics in exhaust systems serving scrubbers.

c Fine dust and particulates in exhaust systems serving dust collection systems and dust generating processes.

d Explosive atmospheres in exhaust systems conveying explosive gasses or serving perchloric acid hoods.

e Non-breathable atmospheres and toxic fumes in process and laboratory exhaust systems.

MSDS sheets should be available onsite for any toxic substances used in the construction process or by the owner in their processes. Consult and coordinate with the system owners, designers, and the owner's Environmental Health and Safety Manager for all testing of potentially hazardous exhaust systems.

Differences in static pressure requirements between the return and relief path can introduce instabilities into economizer and return and supply fan volume control loops, making them more difficult to tune.

Backdraft dampers need to be tested for proper operation. Non-motorized dampers must open and close freely without binding. Motorized dampers must be connected to the DDC control system and verified that they are commanded open prior to fan operation.

Time Required to Test

The testing time requirements for return, relief, and exhaust fans are similar to the requirements associated with the supply fan and drive systems (Section 10.2 Commissioning Fans and Drives).


For return and relief systems, the functional testing requirements will be similar to those described for the supply fans and supply distribution system. Guidance on these topics can be found in Chapter 10 and Chapter 11, respectively. Guidance on the integrated operation of the return, relief, and exhaust system testing can be found in Chapter 3: Economizer and Mixed Air.

Hazardous and special exhaust systems require functional testing targeted at ensuring that special design requirements are met. Usually, these tests will by highly customized to the particular project. Testing often involves coordination with multiple parties and special preparations to execute. These considerations are similar to those associated with the testing of smoke control systems in Chapter 15: Management and Control of Smoke and Fire.

13.3. Testing Guidance and Sample Test Forms

Click the button below to access all publicly-available prefunctional checklists, functional test procedures, and test guidance documents referenced in the Testing Guidance and Sample Test Forms table of the Air Handler system module.

AHU Testing Guidance and Sample Test Forms

13.4. Supplemental Information

This section provides educational information that will be useful for solving problems identified during commissioning and design review.

13.4.1. Special Exhaust Fans and Systems

Some processes that occur in buildings have special requirements for the way that their exhaust is handled that are set by the nature of the area or process exhausted, the material or vapors entrained in the exhaust, and the velocities required to capture and contain the contaminates that are generated by the process. The exhaust from these processes may require special treatment prior to discharge including filtration or scrubbing (See Chapter 14 Scrubbers for additional information). These special exhaust systems may also require special materials of construction and wash-down and protection systems due to the vapors and materials entrained in the exhaust. Fans serving these systems may also need to incorporate these special materials and other features to safely handle the effluents. In most cases, the design requirements for these systems will be set by a variety of sources including the requirements of the process, building code requirements, and insurance underwriting requirements. There are several NFPA standards that relate to these systems. The following is a list of some examples of these special exhaust requirements.

       Kitchen Hoods Kitchen exhaust systems use exhaust flow to control odors, remove moisture and steam generated by dishwashing and cooking, and remove heat. The exhaust and make-up requirements for the areas served are often quite high, and the systems providing the air are often some of the most energy intensive in the building. Thus, these systems are good candidates for energy conservation and recovery strategies and worthy of ongoing commissioning targeted at maintaining them at peak operating efficiency. Often, the ducts, hoods, and other equipment associated with kitchen exhaust systems are constructed of stainless steel or welded black iron to minimize the potential for failure due to corrosion and to control a fire due to the accumulations of grease.

       Lab Hoods Laboratories use hood that require sufficiently high exhaust rates to ensure that odors and fumes generated in them are captured and removed from the occupied zone.

       Hazardous exhaust Frequently, these odors and fumes are hazardous and require special hood designs, system designs, materials of construction, and treatment of the effluent prior to discharge. Examples include acids, caustic vapors, noxious gasses, Volatile Organic Compounds (VOCís), biologically contaminated air, radioactively contaminated air, and dust. Many of the more exotic exhausts will only be encountered on production sites or at laboratory facilities.

1   Percholoric acid hoods are often encountered in teaching laboratories at schools and universities. Perchloric acid exhaust will generate a precipitate that is essentially a contact explosive, and thus requires special materials of construction, fans, and a wash-down and drainage system. Caution is required when working on this type of system to prevent injury due to an explosion triggered by a tool striking some of the precipitate.

2   Biologically, radioactively, and acid contaminated exhausts are often encountered in health care facilities and the laboratories serving them. Emergency rooms can be required to include special provisions in their HVAC systems related to tuberculosis, and the exhaust may be considered biologically contaminated. Similar conditions may arise with the exhaust from isolation rooms. Radiology departments can have radioactively contaminated exhaust from some of the preparatory areas as well as acid exhaust from some of the film processors that are employed. Some of these exhaust streams require filtration prior to discharge with HEPA filters installed in housings that allow the filters to be bagged and sealed as they are removed. These filters then must be disposed of as hazardous waste.

3   Ethylene Oxide is frequently used to sterilize equipment and materials in health care facilities and requires special handling.

4   Training shops in educational facilities often require special exhaust systems to handle dust and fumes associated with metal and wood-working operations and automobile body work. The exhaust from paint booths can be explosive in addition to being noxious and requires special treatment for both. The fine dust carried in a wood shop exhaust system can also be an explosion hazard.

Regardless of the exact nature of the special exhaust, the location of the fan relative to the termination point of the system is a very important feature in hazardous exhaust systems compared to a general exhaust system. In order to minimize the potential for the hazardous effluent to escape from the system, the exhaust fans serving hazardous exhaust need to be located at the termination point of the system so that the entire duct system is held at a negative pressure relative to the surrounding environment. This ensures that any leakage will be from the clean and safe area into the hazardous air stream. As a result, most fans handling this sort of exhaust need to be located on the roof or exterior of the building.

It is not unusual in a retrocommissioning environment to discover a fan system that violates this rule. When this occurs, it is important to bring it to the Ownerís attention because the fumes that may leak from the pressurized discharge duct can create life safety problems and cause corrosion in the building envelope and structural members located in the vicinity.

In most instances, the general arrangement and design of the fans used for exhaust systems are identical to those used elsewhere in the HVAC system, and the information found in Chapter 10: Fans and Drives applies. Some applications, like dust control systems, require special wheel designs that are capable of handling entrained materials without damage to the fan. In small hazardous exhaust systems, especially perchloric acid systems, an ejector type fan may be used. This fan employs the venturi principle to move the exhaust by injecting a high velocity airstream into the exhaust path, thus eliminating moving parts from the flow path.

Many applications also require special materials of construction for the fans and ducts including special coatings and finishes, explosion proof construction, non-sparking construction, and special bearing and drive arrangements. Scroll access doors are desirable in HVAC applications, but are critical for exhaust system applications to ensure that the internal components of the fan can be inspected and serviced. Some fans may be required by code to have a special rating for the intended service.

Roof-mounted fans should always have their discharge duct arranged to minimize the entry of water. This orientation can require a special approach to the design of the discharge if the fan must also discharge directly upward at a high velocity to ensure that the effluent is discharged safely to atmosphere and mixed with the ambient air. Roof-mounted fans should be equipped with a scroll drain to clear any water from the scroll that gets past the discharge duct rain protection. If the fan serves a hazardous exhaust stream, the effluent from the drain may be considered hazardous and require special treatment. Effluent from the wash-down systems associated with some fans may also be considered hazardous wastewater.

In some instances, the special materials of construction cause a ripple effect in special requirements. For example, some acid resistant duct materials are considered flammable by code authorities and insurance underwriters. As a result, the duct systems constructed of these materials require fire suppression systems, usually in the form of sprinklers. The sprinklers, being exposed to potentially corrosive exhaust, require special treatments and coatings to protect them and special installation connections (typically, flexible metallic hose) and arrangements to allow them to be removed for inspection and service. Since a sprinkler discharge inside the duct system will fill the ducts with water, the ducts need to be supported in a manner that could withstand the weight of the duct if it were filled with water. In addition, the duct needs to have drainage access in the event that the sprinkler system activates. The effluent from this drain will probably need to be treated as hazardous waste. And finally, the discharge connections of the duct need to be arranged to trap water in the duct so that a sprinkler discharge does not flood the expensive and delicate process machinery served by the system.