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VAV Box – Sensitive, Install As Follows

VAV Box – Sensitive, Install as Follows

Quality, Consequences and the Construction Industrial Complex (part 71) – All IMHO:

“Can we use flexible ductwork on the inlet to the VAV box?” Answer, Hell No! This is a question I have actually been asked.

Why we are still using the 1960’s technology of VAV boxes is another subject. However given that they are still massively used in North America and the Middle East we should at least install and Commission them to provide optimum performance. Quality is a million little things done well that add up to a high quality outcome. For high performance buildings, quality matters at every level. 

Basic as installation of a VAV box is, I still see many poor installations and regularly see 90 degree ductwork connections on the inlet of VAV boxes then everyone wondering why the box is not stable or operating at design. VAV box and ductwork installation quality matters because:

  1. VAV boxes cannot achieve stable control without the correct inlet airflow conditions.
  2. Between 10% – 30% of conditioned air is lost via leaky ductwork.
  3. System ductwork leakage effects system performance, occupant comfort, IAQ and energy consumption.

Wide availability of cheap, flexible ductwork is an issue. A ductwork contractor once said to me, “Adam, if I was not meant to use flexible ductwork everywhere why is it so cheap and easy to install?”. Sometimes we forgot that flexible ductwork should only be used to overcome “local” coordination and clash issues. Long runs of flexible ductwork installation are examples of poor engineering practice IMHO. 

The place to fix these issues is during design phase, as many of the problems experienced on a project are “baked in the IFC drawings & spec cake”. The specifications and drawings should be clear on ductwork leakage rates, sealing requirements and installation details. IMHO the watch points at each project stage are:


  • Include a typical VAV box detail in tender drawings.
  • Explicitly specify ductwork materials and leakage testing for high/medium pressure side ductwork to a SMACNA or DW143 standard. 
  • Explicitly specify minimum acceptable ductwork sealing methods on high/medium pressure and low pressure ductwork. 


  • Respect the ductwork inlet and outlet configurations required for stable airflow conditions (inlet ductwork10Ø’s and outlet ductwork 5Ø’s or widths).
  • Ensure the ductwork is sealed as specified to reduce conditioned air leakage.
  • No flexible ductwork ever on high pressure side of the VAV box.
  • Max 1 Meter (~ 3Ft) of flexible ductwork on VAV box low pressure ductwork. Install flexible ductwork as straight as possible to reduce airflow resistance and noise generation.
  • Ensure access zone is made available for Commissioning and Facilities Maintenance.


  • Commissioning Authority or Manager to sample witness and verify ductwork pressure leakage testing.
  • Commissioning Authority or Manager to undertake sample inspections of ductwork installation quality and report deficiencies on the Cx Issues log. 

Below is a VAV box installation detail that I sometimes issue for clarification. It can be downloaded in PDF format at

​If the above is not done, the risk of the infamous “light leakage test” is high. I have been on more than one project where the VAV ductwork was not tested or sealed because the drawings and spec where not explicit. They where vague and only called for “testing”. The contractors answer to “testing” was a “light leakage test”. What is this test I asked. It consists of a man crawling through and / or shining a light source down a duct with the building lighting off at night. If the light does not show at ductwork joints then the ductwork has passed a leakage test. 

When I resisted this method I was told it is an approved SMACNA test (not yet had any evidence of this, but would welcome anyone who could drop some knowledge on this) and I did not have the necessary language in the specifications to support my complaints. The result was a poor quality installation and years ahead of conditioned air being leaked and wasted.  

Related posts:
#36 – How many people does it take to commission one VAV Box? – This is not a joke! ( )

#64 – Respect the VAV System Static Pressure Sensor! ( )

#37 – Project Managers beware! This is why VAV systems are not “start up and walk away” ( )

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