The NEBB TAB Procedural Standard states that there are two main conditions which need to be fulfilled to consider any HVAC system correctly balanced:
- All measured airflow/hydronic flow quantities are within ±10% of the design flow quantities unless there are reasons beyond the control of the CF.
- There is at least one path with fully open-air damper/balancing valves from the air-moving equipment/pump to a terminal device.
However, are those two conditions enough to achieve the main purpose of the HVAC system? The answer simply is NO. Those two conditions are only a way to standardize the procedures of testing and balancing work and make it easily and systematically processed by anyone.
To better answer the question, we need to go back to the fundamentals of HVAC science and ask ourselves about the main purpose of HVAC systems. HVAC systems are a way to control the climate inside a certain space to achieve occupant comfort regardless of what the outside climate conditions are.
This then raises a new question: what does comfort mean? And what are its parameters?
The term “comfort” takes you, at first glance, to the thermal comfort zone basics mentioned in ASHRAE Standard 55. Humans generally feel comfortable between temperatures of 22°C to 27°C and a relative humidity of 40 to 60 percent, but this is not the complete story!
Throughout the history of ASHRAE Standard 55, the definition of comfort and its parameters have continually developed. The newest update, 2017 ASHRAE Standard 55, includes a new element that can take into consideration the change in occupants’ thermal comfort from direct solar radiation, in addition to the existing scope, requirements, conditions, and parameters. The four pillars of the ASHRAE Standard 55-2017 are:
- The six environmental and personal factors taken into account are temperature, thermal radiation, humidity, airspeed, activity level (metabolic rate), and occupant clothing (degree of insulation).
- In order to comply with ASHRAE 55, all of these factors must be accounted for in combination.
- The thermal conditions that ASHRAE aims to achieve are applicable to healthy adult occupants, up to an altitude of 3,000 meters, where occupancy time must surpass 15 minutes.
- This standard does not take into consideration factors such as air quality, acoustics, illumination, or contamination.
As an engineering rule of thumb, a higher level of perfection tends to mean higher costs and a less economic system. That’s exactly why testing, adjusting, and balancing (TAB) was founded. TAB is the tool which links the theoretical and the practical parts together.
When we apply TAB procedures, we are trying to close the gap between design requirements (the theoretical part) and the system implemented in real life with all its manufactured tolerances and human errors (practical part) as much as possible.
Designers can create a self-balanced duct, but to do that, they need to size it right. Let’s imagine, for example, the duct size needs to be 22 and a quarter inches. Yes, it is possible to manufacture 22 and a quarter inch duct, but it will be so costly and not economical, not to mention the tolerance of the machine used to manufacture the duct, too. The quarter inch, which makes the duct balanced, can be substituted by duct-mounted volume dampers to compensate for the pressure loss.
The same is applied on hot/chilled water systems. For a certain period, that was enough, but nowadays, it is not. As comfort parameters become more sophisticated, systems also become more and more advanced to try to fulfil such parameters. In the past, it was assumed that by achieving correct air and water flow rates, all comfort parameters would be fulfilled automatically. At that time, those assumptions were correct because parameters were few and easy to achieve.
Such sophisticated systems need more professional hands to deal with them – hands which are not only specialized and trained, but also have the skills to find solutions for any obstacle which may be found in real life. With this, standardizing the profession becomes mandatory by developing standards and procedures to be followed by professionals. NEBB has been a pioneer in this since its establishment in 1971.
One of the most well-known obstacles facing TAB engineers and technicians is the diversity of comfort for every human being. That’s why, in some cases, we need some minor adjustments to achieve the above comfort conditions. Those minor adjustments need to be coordinated mainly with the owner, consultant, commissioning firm, and operator.
The coordination process itself needs to be done through a commissioning firm and executed by a testing and balancing firm. Documentation of such minor adjustments is mandatory to preserve the history of the project and use it as a benchmark in the future.
Usually, such minor adjustments are applied on the air side by increasing/decreasing flow rates from design values by more or less than the allowable tolerance to achieve airspeed, air change, noise level, temperature, and relative humidity. Do not forget that nothing is perfect in real life.
In some cases, a major change needs to take place to achieve comfort conditions. Such cases are typically due to either a fatal mistake in design or a change in the usage of the space. The testing and balancing firm is NOT involved in this case.
In conclusion, many sciences and discoveries throughout history were, and still are, aimed at human comfort throughout all fields. The science of air conditioning and adapting the weather is no different. Humans have been constantly developing the concept of comfort to comply with emerging science and technology that enables control of the surrounding environment.
Testing, adjusting, and balancing, as a part of air conditioning science, has also evolved from just a tool to adjust the performance of central air-conditioning systems to an essential process which is now indispensable. That’s because TAB has become the link between the theoretical and the practical. Applying TAB procedures correctly frames the effort of all parties on a project, as well as serves as the final result that is seen and felt by occupants.
