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Balancing HVAC System to Avoid Hot or Cold Spots

One of the most common problems that the building HVAC engineers face is the complaint from occupants that the place that they are working is either too hot or too cold. The problem of hot spots and cold spots in an air-conditioned space is an issue that the HVAC operations team has been grappling with for ages. Approximately 45 – 55 per cent of calls to the building helpdesk by the occupants in an office are related to requests of increasing the temperature at the workstation or complaints of the space being too hot and a requirement of the temperature to be lowered. HVAC systems are typically designed for space cooling and ventilation and do not typically cater to the individual user in the work space. Thus, catering to these requests and complaints can put the HVAC system in disarray and hence, the HVAC operator must have an in-depth understanding of the system to be able to cater to the end user requirements.

To ensure that a building’s HVAC system works at the design point that was originally planned, a very crucial aspect of the process of commissioning as well as ongoing maintenance is the ‘Testing, Adjusting and Balancing’ (TAB) of the system. In common industry terms, this process is called air balancing but the process is more complex and covers various HVAC systems. There is a great deal of focus on commissioning of the main chiller and high side systems such as cooling towers and pumps as well as the AHUs. However, the process of testing the system if not robust enough in many cases which leads to the issue of hot or cold spots and other balancing related problems when the systems get operational.

What is TAB?

HVAC systems are designed on paper (electronically actually!) by consultants and specialist from various areas of the HVAC ecosystem. The testing of the components of the HVAC systems – fans, pumps, chillers etc are done at the factory and the ratings are as per standard test conditions laid down in various guides and codes. However, when the systems and components are installed and integrated into the overall HVAC system, the performance is no longer in an ideal factory environment or test conditions. There will be deviations from the ideal operating parameters due to various factors such as ambient temperature, system design, component degradation etc. The TAB activity is a process to assess how the system is performing in the field, in the actual conditions where the system must deliver. ANSI/ASHRAE Standard 111-2008 (RA 2017) – measurement, testing, adjusting, and balancing of building HVAC systems is a comprehensive document covering all aspects of this important activity in the commissioning phase of an HVAC system as well as during operations. Some of the benefits that a structured TAB activity brings to the HVAC O&M team are:

  • Availability of a baseline of the system at the time of installation or start of operations.
  • The baseline enables the O&M team to check for deviations during operations and take corrective actions.
  • Assess system efficiency and use the observations in energy conservation programs.
  • Assess change in operating parameters whenever modifications to the system are carried out.

While every system and component of an HVAC system needs to be tested and assessed for conformity to the original design specifications, it will be a very costly and time-consuming activity. Thus, the more efficient approach is to undertake the testing, adjustment and balancing of sub systems of the HVAC installation of a building. Typically, TAB is carried out for the following sub systems:

Air distribution system: The supply of the right quantity of air, at the right velocity and location is crucial for an efficient and effective HVAC system, one that can achieve the right temperature in the work space and provide a comfortable work environment for the occupants. Thus, a major focus of TAB activities is on the air supply system to the work space. Air balancing, as the activity is commonly called, is an important process in the commissioning of the HVAC system as the test of the system is at the delivery point. The air distribution system covers the ducting, air diffusers, supply and return lines and AHUs.

Hydronic system: A typical water-cooled HVAC system will have chilled water circulating through several Air Handling Units (AHUs) in a building. Hydronic or water balancing is the process of supplying the right amount of chilled water to the AHUs based on the load requirements at that point in time. Chillers consume the maximum amount of energy in the system and if the chilled water supply is not reaching the AHUs that need the desired heat load to be transferred, then there is a substantial loss of energy in the system. Thus, hydronic balancing is a very crucial aspect of the commissioning phase of the system and required specialist to ensure a correct balance of the system.

How to TAB? The processes and procedures involved in the carrying out TAB of air as well as Hydronic are quite exhaustive and detailed. There are established guidelines as per local codes such as ASHRAE 111 2008 (RA 2017) which TAB specialist use to assess the condition of the system, make necessary changes during the commissioning and then set the systems for an optimal balance of the system. A brief overview of the process of air and water TAB is as below:

Air side TAB: The aim of air balancing is to measure the air flow in the work space, use the ambient data along with the air flow and arrive at the heat transfer in that space. This is compared with the design values and where needed, changes to the air flow are made to bring the system as close to the design values. To obtain the air flow, the TAB engineer will install hoods on the supply grills of the workspace to assess the volume of air (cfm) passing through for a specified duration. The other parameters that the TAB engineer measures are the pressure difference between the supply and return sides of the AHU and the humidity in the area. The deferential air pressure is measured using a calibrated manometer.

Once the hot and cold zones have been identified through the testing process, changes to the air flow are made either by changing the fan speed or by adjusting the dampers for the selected areas. The air balance process is carried out again and through a series of iterations, the system is set to as close to design air flows as possible. There is a range of (+/- )10 per cent in the measurements due to the various system efficiencies which the TAB engineer needs to keep in mind while arriving at the final values.

Water Balancing: The aim of carrying out a check of the water flow at various points in the chilled water system is to see that the correct heat transfer occurs at the chiller as well as at the AHU coil interface. Water balancing is carried out after the air balancing has been completed. Prior to the water flow check, all system valves are opened fully, and the AHU bypass valves are closed. Thereafter, water flow (GPM) is checked at the chiller inlet and the temperature rise across the AHU coils are measured. The flow rate is adjusted at the AHU level to get the desired temperature drop and the process is carried out for all the AHUs. Once the adjustments have been done, the position of the flow values are marked, and the system is balanced.

Conclusion

An HVAC system that has been commissioned correctly will provide reliable and efficient service for a considerable duration of time if there are no changes in the HVAC system. However, in real life, there are hardly and steady state conditions. There could be a change in the use of the workspace, the occupancy may increase, meeting rooms may be concerted to work stations etc. Thus, the HVAC O&M team should have a process of TAB of the system whenever there has been a change in the HVAC system or if there are repeated complaints of hot or cold spots in the workspace. This will not only improve the comfort levels of the occupants but will also help lowering energy costs as the system is working at the design or close to the design point.



Aneesh Kadyan
Sr Director – Operations,
CBRE South Asia