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TAB Witnessing Protocol

TAB Witnessing Protocol

Quality, Consequences and the Construction Industrial Complex (part 189).   

There is an element of “voodoo” about commissioning based entirely on the property industry’s lack of understanding of what it actually is. Consequently, there is a lack of clarity resulting in the same questions on auto-loop. Two of these perpetual questions are;

1. Who, witnesses, signs off and accepts Testing, Adjusting & Balancing (TAB)? 

2. Is it the Commissioning Authorities (CxA) job to witnesses, sign off and accept TAB? 

To answer these FAQ’s in short: 

1. The Engineer of record is responsible for witnessing, signing off and accepting TAB.

2. No, it is not the CxA’s job to witness, sign off and accept TAB.

Sometimes the specification or CxA’s scope of work does include some verification of TAB. However, this scope is normally vague and can lead to blame games. So for clarity:

  • Is verification, acceptance (see definitions below)? – No.
  • Who takes responsibility and liability for acceptance? – The Engineer of record. 

However, when I am project CxA I always sample witness TAB results because:

  • The TAB team know what is really going on and where the real problems lie. They are the best source of real time, project intelligence. 
  • If TAB is in fact compete and correct then the building is ready for systems functional performance testing. 

To ensure TAB witnessing is kept to the minimum effective dose, I use this protocol: 

1. Witness all Fan / AHU / Pump total flow rates.

2. Witness all AHU minimum and maximum outside air flow rates.  

3. Witness all strategically important system set-points such as VAV systems static pressure set point and variable flow hydronic systems differential sensor set point. These are empirically derived settings that tell me if the TAB and controls team have worked on the system together. 

4. Sample witness between 5% min to 10% max of terminal readings.

5. Always identify and witness the system “index” (see definition below).

6. Always identify and witness the most heavily regulated terminal. 

I recommend TAB witnessing protocols are discussed prior to accepting any commissioning order and the above TAB witnessing protocol is incorporated into the commissioning plan. I hope this helps. 

Twitter: @BLDWhisperer 


Validation – Verifying accuracy and efficacy.  

Verification – Investigative process which confirms compliance with specified criteria.

Acceptance – “Contractually” defined action that permits an activity to commence or conclude. 

Witness –  Affirm to be true. 

Index – Component, equipment, system section, grille, diffuser, valve which at any time receives lowest indicated percentage of design flow rate i.e. the least favoured due to the relationship between flow and system resistance.

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#120 –  How to Make $$$$ in TAB ( )

#111 – Information Quality & Transmission ( )

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