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Project Managers Beware! This Is Why VAV Systems Are Not “start Up And Walk Away”

Project Managers beware! This is why VAV systems are not “start up and walk away”

​Quality, Consequences and the Construction Industrial Complex (part 37) – All IMHO:

Following on from last weeks post about Commissioning one VAV box, lets assume we have Commissioned all the VAV boxes on a 800 box system. How do we ensure the whole VAV system is commissioned and operating as efficiently as possible? 


Firstly let’s acknowledge that VAV is not a “start-up” and walk away system. There are several third party components that need Commissioning in their own right as well as several empirical set & operational control points that need to be determined, set, locked and recorded. Whenever I walk on site and see a VFD running at 50Hz all day, I know the VFD has been “started up” but not commissioned. This failure on its own will negate many of the perceived benefits of VAV! However lets not rant, lets outline a logic process to get a VAV system Commissioned. By the way, all logic processes start and finish with a logic diagram IMHO. 

Therefore the Cx process in summary bullet format is:   

Installation and pre-commissioning tasks – whole VAV system

  1. Install the whole system including the entire ductwork distribution network, all AHU’s, power, VFD’s, controls plus other system interfaces 
  2. Quality control checks, ductwork pressure testing, controls programming plus uploading of controls “code” and system start up 

Dynamic commissioning tasks – whole VAV system

  1. Start up and preliminary commissioning of the VFD’s
  2. Controls point to point testing – VAV boxes and AHU’s
  3. Commission all VAV boxes – see previous post
  4. Apply system diversity 
  5. Proportionally balance return air system
  6. Test and determine the optimum set point, with diversity applied, for the static pressure sensor (located 2/3’s along the ductwork distribution network) 
  7. Set up the minimum outside air (to ensure IAQ and optimum energy usage) at Vmax and test proportionality between Vmax and Vmin
  8. Test and commission heating, re-heating and cooling
  9. Test and commission humidifier
  10. Test and validate all controls sequences of operation i.e. morning start-up, Vmax, Vmin, CO2 alarm, free cooling, safeties, fire interlocks etc.
  11. Test and validate freeze stat
  12. Simulate AHU filter dirty conditions and set filter dirty alarms
  13. Test, determine, set and record VFD Vmax speed, Vmin Speed and trip speed
  14. Set and verify supply and return air fan tracking between Vmax and Vmin
  15. Set parameters and alarms for “set points out of range” on the BAS

So how many and who are the players required to commission a VAV system?

  1. The main contractor / GC – to coordinate the specialist sub-contractors, provide access etc.
  2. The Commissioning engineer / technician / authority – please don’t call him or her an “agent”! Please, it sounds so unprofessional. 
  3. The mechanical contractor – typically the installation is their responsibility
  4. The TA&B specialist – to measure, test and determine empirically the optimum system set points
  5. The controls specialist – probably the most important player
  6. Specialists from vendors such as the VFD, Humidifier, AHU (sometimes) 

It should be noted that the empirical setting up of the VFD and static pressure set & operational control points (TA&B, CxA and Controls people working together at the same time) are critical if there is any hope of achieving the energy targets for the building. The set & operational control points that are required to be determined, set, locked in and recorded (with clean AHU filters) are as follows:

  • Optimum static pressure set point with diversity applied @ Vmax
  • VFD Hz @ Vmax – Noted for record with clean AHU filters
  • OA Damper position @ Vmax
  • RA Damper position @ Vmax
  • EA Damper position @ Vmax
  • VFD Hz @ Vmin – Noted for record
  • OA Damper position @ Vmin – Noted for record
  • RA Damper position @ Vmin – Noted for record
  • EA Damper position @ Vmin – Noted for record
  • Panel filter dirty alarm set point
  • Bag filter dirty alarm set point
  • Supply air temperature set point
  • Supply air temperature set point @ system Vmin (set back, on low load)
  • Time schedule 

Call me old fashioned, but I believe building systems should be set-up for optimum operation to meet the design intent. This is the basic working assumption by building owners when they employe design and construction teams.

How many systems do you know of that have been set to work to this level of testing and optimization?

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