Air Balancing In Existing Buildings
The most common approach of circulating air across the work spaces is by way of ducting, usually mounted from the ceiling and concealed by the false ceiling for aesthetic reason. Ducting thus is an important component of air conditioning systems and a lot of emphasis is laid to the ducting system during the design stage...
Human comfort is the primary reason for air conditioning of office buildings. In cold countries, the air is heated, while in the hot countries, the air temperature is brought down to a level that makes it comfortable for the occupants of the buildings to perform the work that the building is intended for. Conditioning of the air also includes control of the humidity, quality, direction and velocity, which are all important in terms of human comfort.
The most common approach of circulating air across the work spaces is by way of ducting, usually mounted from the ceiling and concealed by the false ceiling for aesthetic reason. Ducting thus is an important component of air conditioning systems and a lot of emphasis is laid to the ducting system during the design stage. The performance of the air conditioning plant is closely linked to the air distribution systems in central plants, and hence it’s critical from the operating cost perspective as well.
Air circulation via the ducting system is designed for a specific work space arrangement, which is planned at the design stage. Air balancing of the system is the process by which the commissioning team ensures that the right quantity of the air reaches the work space so as to maintain the desired comfort level of temperature of the space.
This is a complex task, as there are many variables to consider such as heat load of the space, location, size of the work space (Cabins, open spaces, work stations etc.), noise levels etc. Due to the multi system interaction of the ducting, the process of air balancing often takes up a considerable amount of time.
Improperly balanced systems, despite the focus provided to the process at the time of commissioning are not uncommon. Last minute changes, in the layout, loads etc., impact the ducting design and ultimately lead to uneven distribution of air. The impact that an unbalanced air delivery system can have are many, such as hot and cold spots, poor thermal comfort, condensation on ducting etc., which ultimately will have an effect on employees’ productivity.
There is also the related increase in operating costs as the system operates off design. The issues that effect air balancing at the start of the building’s life get compounded as the building ages and have an even more detrimental impact on the occupants.
There is thus a need to assess the health of the ventilation system – and the review the air balance periodically to give the occupants the best possible environment.
Air balancing overview
To provide healthy air to the occupants and also to provide a positive pressure inside the building, fresh air from outside is introduced to the workspace through the Air Handling Unit (AHU). The positive pressure helps in keeping out non air-conditioned air from entering the building. The fan of the AHU delivers a specified flow rate to the workspace and develops a positive pressure through centrifugal force. This pressurised air is directed to different areas of the floor through the ducting and exits through diffusers. The air from the work spaces returns to the AHU on the suction side of the fan and the circulation continues. There are thus two aspects of air distribution that air balancing takes care of:
• Overall building envelope static pressure
• Air flow at multiple locations within the building for occupants’ comfort
Air balancing or as the process is referred to in practice as Testing, Adjustments and Balancing (TAB) of new buildings is governed by industry standards such as National Environmental Balancing Bureau (NEBB) or Sheet Metal and Air-Conditioning Contractor’s National Association (SMACNA). Standard procedures are defined and reporting formats are specified. Air balancing of existing buildings does not have standards as the condition of the work space and building envelope changes and it is not possible to isolate the building for considerable periods of time as required by the new build standards. The balancing of such buildings is then done based on the requirements and a base line that is referenced from the original commissioning reports.
Air balancing is carried out by collating data about the various components of the air distribution system, and undertaking measurements of pressure or/and air flow at various points of the work space. The system is considered as ‘balanced’ if the measurements are within +/- 10% of design operation points.
Why is air balancing important?
Any system working off design is a cause of concern to the building operations’ team. As the system is designed to cater to specified operational requirements such as heat load, area etc., if there is a significant deviation; the operational requirements will not be met. Some of the detrimental effects of inadequately balanced work spaces are as follows:
• Poor Indoor Air Quality (IAQ): If the static pressure is not adequate, outside, untreated air will enter the work space, leading to poor indoor air quality problems, which will ultimately affect the productivity of the work force using the space.
• Poor comfort levels: Due to air flow imbalances, certain sections of the work space will receive more than the desired air flow while others will consequentially be supplied with less air flow. This results in creation of ‘hot’ and ‘Cold’ spots in the work environment leading to more complaints from the employees as well as inefficient operations due to the frequent changing of the AHU set points to address these issues if not diagnosed correctly.
• Higher operating costs: Poorly balanced air delivery systems result in more frequent running of the AHU, and also operations at higher speeds. This combination of more time and speed results in higher energy usage by the AHU.
• Noise levels: Higher noise levels in the system are often linked to imbalances in the system that creep in due to changes in the ducting that occur over the period of use in a building. The higher noise levels impact employees predictably, as well as have long-term health effects.
Air balancing can thus be said to have direct impact on the health of the employees as well as their productivity and hence it is of utmost importance.
Causes of air imbalance
The air balance is certified at the time of building commissioning, so one would naturally ask why should there be any change in the air circulation as the air distribution system (AHU, Ducting/diffusers etc.) is not disturbed during the course of the building operations. Unfortunately, the situation is very different when the building becomes operational. Many factors impact the original design intent and lead to the system operating far away from the desired operation points.Some of the operational factors that impact the air balance are mentioned below and the system related causes are listed in table 1.
• Changes in office layout: This is the most common cause of system imbalance. Cabins and meeting rooms getting converted to open spaces or the opposite, addition of partitions etc., lead to changes in the air flow patterns resulting in variations in the air flow parameters.
• Closing of diffusers: It is not uncommon to see diffusers covered by paper in many office spaces because the occupants are feeling too cold. While the cause of the lower air temperature may be many such as wrong set points at the AHU, blocking off the diffuser results in distortion of the air flow and impacts air balancing.
• Inadequate air conditioning discipline: When a system is designed for only minimal openings of the entrance doors, if it is operated when the door opens more frequently or is left open, the positive pressure is lost – and that impacts the air balancing of the whole space.
• Increase load: This is another common problem in office spaces, where there is a densification of the workspace as office work increases. More people in the same space lead to higher heat loads – and also mandating higher air flows that in many cases are tackled by enhancing speed of the AHU. While this approach increases flow, the ducting may not be designed for the changed flow characteristics leading to deterioration of air balance
• Deterioration of ducting: Fouling of the ducting over a period of use leads to higher friction losses in the ducting, leading to lowering of the air flow at many locations.
Air balancing process
Air balancing of existing buildings can be a difficult and complex process as the work space has occupants working, and there is a constant change in the system operating conditions. Air balancing is however critical, especially when there has been a major change in the office layout or other design changes. The typical process for air balancing (For constant volume system) in existing buildings is shown in figure 1. It is also recommended to carry out air balancing at least once in 5 years to assess the health of the system. The key steps involved in the air balancing process are listed below.
• Pre air balancing: The TAB team first assess the current state of the air distribution system by obtaining data of the various equipment such as motors, fans, AHU characters, duct dimensions etc. These can be obtained from the commissioning data whereever available. In case the original documents are not available, a lot of data can be obtained from OEM data sheets that are available from public sources.
• System walk through: Basic system defects and deviations can easily be identified by walking the floor and observing changes made to the system such as new outlets or blocked diffusers.
• System readiness for testing: Prior to testing the distribution network, the TAB team ensures that the system operation is as per the original design as far as possible. This involves opening all Grills, Registers and Diffusers (GRD, Setting dampers to the design points, removing all temporary covers of diffusers etc.
• The total system volume is obtained using air flow measurement at the fan outlet and compare with design values, fan curves etc.
• With the fan at the design rpm, measure the air flow at each of the GRD/Outlets of a branch. Calculate the total air flow of the branch and compare with the design air flow. This process is followed for all branches.
• To carry out the adjustment, the air flow is varied by adjusting the dampers in the system. The process starts by identifying the branch with the lowest percentage of design flow. No change is made to the damper adjacent to this branch. The next damper with the second lowest percentage variation is adjusted till the two dampers have the same deviation from design. These two dampers and branches are in balance with each other.
• This process is continued and dampers with reducing deviations are adjusted till all have nearly the same percentage variation in design flow.
• Once all dampers have been adjusted, the air flow across all GRDs on a branch is similarly adjusted to achieve branch level balancing.
• On completion of the adjustments, the air flow across all GRDs is measured again to verify correct balancing.
HVAC systems consume the major portion of the energy of a building. Within the HVAC systems, the ventilation components are in use constantly, and hence this is an important aspect of the operations. Air balancing is usually undertaken at the time building commissioning is done under the TAB process.
There is however a number of factors that can lead to the original air distribution design points to change due to changes in the work space and nature of work as well.
A ventilation system working off design will lead to not only discomfort to the occupants, but also higher operating costs. Thus, air balancing should be undertaken once in 5 years at a minimum – and adjustments should be made to accommodate the new load and occupancy requirements.
Reference: 1 Procedural Standards for Testing, Balancing and Adjustments of Environmental Systems, NEBB
AUTHOR CREDIT AND PHOTOGRAPH
Director - Operations
CBRE South Asia Pvt Ltd.,
Asset Services - India