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+/- 10%?

+/- 10%?

Quality, Consequences and the Construction Industrial Complex (part 97) – All IMHO:

I cannot stop myself continuing down the rabbit hole of UK Vs USA approaches to building services engineering. Who cares? I care, because I am a total nerd.

So, this weeks nerd fest is about Testing, Adjusting & Balancing (TAB) tolerances……really…. yes really. How is this not the same everywhere? HOW? There is an assumption it is the same everywhere, but it is not. 

Most people assume +/- 10% for TAB tolerances. However this is not adequate particularly with CHW systems or high performance rooms. Also the cultural differences in approach between the UK and USA add further confusion. 
Firstly, instrumentation accuracy is an issue. Site conditions are not laboratory conditions, therefore assume site instrument accuracy is at best +/-5%. Also a DRV fully open has an accuracy of +/-5%, if it is regulated 50% or more, the accuracy decreases significantly. 

Secondly, it is the HVAC design engineers job to specify TAB tolerances for each system type. 

CIBSE is clear; “It is the responsibility of the designer to specify flow rate tolerances and to ensure that these are appropriate to the particular design, installation and application”

Why different tolerances for different systems type? Because building services engineering is an applied science and application matters to performance. 

What must be specified? Lets start with some definitions then tables with UK & USA TAB tolerances:


  • Tolerance: Permissible deviation from a specified design requirement.


  • Comfort Cooling (Low Performance Effect): Provision of cooling to improve comfort without humidity control.


  • Air-Conditioning (Medium Performance Effect): Provision of cooling and heating to control indoor environment via temperature and humidity control.


  • Close Control Air-Conditioning (High Performance Effect): Provision of cooling and heating to control environment precisely via air cleanliness, temperature and humidity control within process driven tolerances.

Air Systems TAB Tolerances

Water Systems TAB Tolerances
PDF versions of these tables can be downloaded at

TAB is not an exact science, it is a technique for distribution of flow rates throughout systems to ensure performance. Also;

  • TAB to unnecessarily close tolerances results in high TAB costs with no advantage, it can also result in running cost penalties.
  • TAB to appropriate tolerances is necessary to ensure systems performance. Owners please take note!
  • TAB verification must recognize that prevailing conditions and instrument accuracy mean identical results are unlikely. A deviation of +/-5% is expected. 

What to do? 

HVAC System Designers

  1. Do your job – specify TAB tolerances based on system type
  2. Ensure MEP & TAB contractors are clear on required tolerances
  3. Witness TAB results

Commissioning Managers

  1. Ensure TAB tolerances are specified during design
  2. Raise any necessary RFI’s during construction phase to ensure TAB tolerances are specified
  3. Ensure qualified TAB firms undertake the work

We can put a man on the moon yet the delivery of HVAC systems optimized for performance remains unusual. I can’t complain, this is why I have a job!

BTW: I welcome any evidence based corrections to the tables, this information was actually difficult to gather. 

Twitter: @BLDWhisperer

Related posts & links:

#70 – USA & UK, 2 Engineering Cultures Sharing 1 Language ( )

#84 – TAB Certifications, Why they Matter  ( )

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