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Single Point Of Effectiveness And VAV Systems

Single Point of Effectiveness and VAV Systems

Quality, Consequences and the Construction Industrial Complex (part 89) – All IMHO:
I want to introduce into the property development and HVAC lexicon a new term, “Single Point of Effectiveness” (SPE). 
How does this matter? It is the difference between an HVAC system working efficiently or in some cases not working at all. 

Lets start with some definitions:

  1. SPOF – Single Point of Failure: “part of a system that if it fails, will stop the entire system from working”.
  2. Effective: “has an intended or expected consequence”.
  3. SPE – Single Point of Effectiveness: “degree to which the control point is successful in producing a prescribed result”.

Single Point of Failure is a well understood although not well managed concept in HVAC and data centre systems. However, Single Point of Effectiveness, not so much. If you do not believe me ask the engineer where in the HVAC system the SPOF or SPE is. Be prepared for a blank look. 

Dreams are free, so lets dream a little. “Image a world” where a HVAC system is designed, installed, and commissioned perfectly. Savour that thought for a minute. Now realize that the whole performance, effectiveness and efficiency of this system depends on a single point of control and probably a $200 sensor. 

This is the reality for all VAV and variable speed hydronic systems. For these systems to work correctly they require optimum system set points for pressure and temperature sensors derived from empirical systems testing. They also require maintenance and a policy of “non-interference” from the FM team i.e. not changing set points every time there is a temperature complaint. 

The main Single Point of Effectiveness devices in HVAC systems are;

  • Static pressure sensors
  • Temperature sensors
  • Differential pressure sensors

Lets look at a generic VAV system profile of effectiveness issues. There are a hierarchy of effectiveness issues culminating in one Single Point of Effectiveness for control. The diagram below summarizes the hierarchal issues and mitigation measures. 

VAV System SPE Hierarchy

Note: Diagram can be downloaded in PDF at:

For any VAV system to work as designed and tested in the energy model, the effectiveness issues need to be addressed. Clients and third party Commissioning professionals should investigate these issues prior to accepting completion of a VAV system. 

I personally would like systems SPE to be identified and discussed when each system, is design and commissioned. Why? Because the basic assumption is that systems are handed over working optimally, particularly in green and mission critical buildings!

Twitter: @BLDWhisperer 

Related posts & links:

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

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

#42 – The “Hot Potato” That Is The Controls Sequence of Operation  ( )

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

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

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