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VAV Control Sans Duct SP Sensor

VAV Control Sans Duct SP Sensor

Quality, Consequences and the Construction Industrial Complex (part 101) – All IMHO:

What is the most effective way to control a VAV system? 

I have been asked this many times by HVAC design engineers, which in itself, is quite distrubing. I have seen three solutions in the field that range from bad to OK to optimum;

  1. No system control i.e. no duct mounted static pressure sensor or modulating fan speed. The control is all via local VAV boxes and the AHU fan “rides its curve” as demand varies. I have seen this on many speculative office buildings with on floor AHU’s in North America. So to be clear, there is no effective system control plus high ductwork leakage rates due to over pressurized ductwork during mid and low cooling demand – If you are designing and installing VAV systems this way, we cannot be friends, please lose my contact info
  2. Duct mounted (2/3’s along the ductwork distribution network) static pressure sensor controlling fan speed based on aggregate system demand – This is OK, if the system is Commissioned correctly
  3. Using the BMS/BAS to monitor all VAV boxes and control fan speed based on, real time system demand – This IMHO, is optimum

In all energy models and estimates for building IAQ and energy consumption there is a basic assumption that the VAV system is optimized and efficient. If anyone believes that in the real world, I have a bridge I want to sell you in London, e-mail me!

The VAV system control point is critical for optimizing system performance (IAQ, comfort & energy consumption) and reducing energy consumption. Duct mounted static pressure sensor controlling fan speed based on aggregate system demand is a perfectly OK way of achieving optimum system control. I have written about this in a previous post (see link below) so I will not waste time on it here. 

However, “In a world” where controls are digital, ubiquitous and relatively inexpensive, VAV system control should IMHO, move away from electro mechanical (duct static pressure sensor) and be digital. We talk (talk is cheap) about “digital control” and “the internet of things”, well, why not actually do it in HVAC and buildings. 

For digital, fine resolution control the following is required:

  • Fully networked VAV system i.e. all boxes, control devices and sensors connected to the BMS/BAS system
  • BMS/BAS head end with detailed interactive systems graphics 
  • Collaborative, integrated design, installation and commissioning process

The key issue for optimum VAV system control is to ensure the “index” VAV box is always, just about “floating” at any given system load profile. Some definitions are required:

  • Index VAV Box – The VAV box within a VAV system which at any time receives lowest indicated percentage of design flow rate i.e. the least favoured VAV box due to the relationship between available air flow and system resistance
  • Floating – point at which VAV box is under control but not over pressurized i.e. it is not under served and fully open, nor is it over served with the primary air damper heavily shut to maintain design flow rate
  • VAV system control point – The optimum point at which all VAV boxes are floating at the lowest possible AHU fan speed whilst maintaining correct outside air and IAQ

Note: on large VAV systems the VAV index box can change based on building load profile throughout the day.

On a fully networked, digitally monitored and controlled VAV system the basic system control principal looks like this:

  • BMS/BAS system polls all VAV boxes every 5 minutes to see which boxes are not floating – note: this polling time will need tuning during commissioning to find the balance between latency and AHU fan “hunting”
  • BMS/BAS system identifies the index VAV box i.e. VAV box with the primary air damper fully open and lowest flow rate compared to design flow rate
  • BMS/BAS system increase AHU fan speed until the index box is just floating
  • Rinse & repeat 

From a system resilience and safety perspective the following should be implemented:

  • Loss of BMS/BAS comms raises visual and audible alarms
  • In event of power or network failure all VAV boxes fail to fully open
  • Electro/mechanical SP sensor on fan discharge which shuts down the AHU on high pressure alarm

The above is a simplification and requires a developed controls algorithm. However, why not do it this way?

Twitter: @BLDWhisperer

Related posts & links:

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

#89 – Single Point of Effectiveness & VAV Systems ( )

#71 – VAV Box – Sensitive, Install as Follows ( )

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

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