Understanding Return Air Bypass Systems: Benefits, Functionality, and Considerations

What is a Return Air Bypass System?  

A Return Air Bypass system is an available option in HVAC equipment that can help manage indoor humidity levels and maintain occupant comfort.  

It works by independently controlling three dampers—Return Air Dampers, Return Air Bypass Dampers, and Outside Air Dampers—to regulate airflow and provide dehumidification. This option is available for constant volume systems that handle spaces with moderate humidity loads, providing a cost-effective method for controlling indoor moisture without the need for additional complex dehumidification equipment. 

How Does a Return Air Bypass System Work?  

Below are the key elements of the feature’s operation: 

1. Independent Control of Dampers: 
   1. Return Air Dampers control the flow of air returning from the space. 
   2. Return Air Bypass Dampers direct the returning air around the evaporator coil, bypassing it for reheating. 
   3. Outside Air Dampers regulate the amount of fresh outdoor air entering the system. 
2. Dehumidification Mode: During dehumidification, the return air and outside air mixture is cooled and dehumidified as it passes through the evaporator coil (also known as the indoor coil). The air is then reheated using the return air bypassed around the evaporator coil. This method prevents excessive cooling while still ensuring that the air is dehumidified efficiently. 
3. Use of Return Air for Reheat: When the humidity ratio of the return air is lower than that of the mixed air (the combination of outside and return air), the system can use the return air for reheat. This eliminates the need for additional heating sources, utilizing the heat from the zone’s load rather than an auxiliary heating element. 
4. Low-Cost Dehumidification: The Return Air Bypass system is an ideal solution for spaces with moderate humidity loads, typically with a sensible heat ratio (SHR) of 0.7 or higher. This system can control this level of humidity without the need for more expensive dehumidification methods like modulating hot gas reheat, making it a cost-effective option. 

Benefits of Humidity Control  

1. Occupant Comfort: 
   1. Proper humidity levels create a more comfortable indoor environment, enhancing productivity and overall well-being. High humidity can lead to reduced comfort levels. By controlling humidity effectively, the system boosts comfort and supports better performance and focus among building occupants. 

1. Improved Indoor Air Quality (IAQ): 
   1. High humidity can foster condensation, leading to mold growth, wood rot, and deterioration of paper or other materials. It also increases the risk of mold, fungi, and dust mites, which are associated with allergies and sickness. By controlling moisture levels, the system improves IAQ, reduces the potential for microbial growth, and contributes to healthier indoor air conditions. 

Limitations of the Return Air Bypass System  

While Return Air Bypass systems offer several advantages, they are not without limitations. Below are some considerations for their use: 

1. Ideal for Constant Volume with Moderate Humidity: 
   1. Return Air Bypass systems work best for constant volume systems with moderate latent (humidity) loads. For spaces where both sensible and latent loads are high and occur simultaneously, additional dehumidification strategies, such as modulating hot gas reheat, may be required. This ensures that proper dehumidification occurs without overcooling the space.  
2. Energy Considerations: 
   1. Although Return Air Bypass systems offer energy savings in certain situations, they do require additional energy for operation, especially when handling latent loads. However, energy savings can still be achieved by adjusting the temperature setpoint of the zone based on the dehumidification process being used. This allows the system to balance both temperature and humidity control efficiently. 

FAQ Section 

What is a Return Air Bypass system in HVAC?  

A Return Air Bypass system controls humidity by adjusting airflow through independently controlled dampers to cool, dehumidify, and reheat air, ensuring comfort and energy efficiency. 

How does a Return Air Bypass system work?  

It cools and dehumidifies air as it passes through the evaporator coil, using return air as a reheat source to prevent overcooling and maintain comfort. 

What are the benefits of humidity control with a Return Air Bypass system?  

It improves occupant comfort, indoor air quality, and reduces the risk of mold and allergies caused by high humidity. 

What are the limitations of Return Air Bypass systems?  

They are best suited for spaces with moderate latent loads and may require additional dehumidification systems for areas with large latent loads or simultaneous sensible and latent load demands. 

When should Return Air be used for reheat instead of mixed air?  

Return air should be used for reheat when its humidity ratio is lower than the mixed air, ensuring more efficient dehumidification.