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System Effect – “the Problem That Keeps On Giving”

System Effect – “the problem that keeps on giving”

Quality, Consequences and the Construction Industrial Complex (part 121) – All IMHO:

There is a mismatch between misdeeds and consequences in the building design and construction process. The people and firms generating problems do not typically pay the price. A good example of this is system effect. 

In North America we live in an air systems based world and VAV remains stubbornly ubiquitous with frequent examples of systems effect. Systems effect has consequences and is the problem that just keeps on giving. When the design and construction team are long gone, this problem will impact system performance and operating costs over its entire life span. Who’s pays? The owner and ultimately, the environment. 

Systems effect is the loss of capacity of fan volume attributed to poorly designed and installed ductwork fittings at the inlet and discharge of fans and AHU’s. 

In the field, system effect manifests as high noise levels, inconsistent airflow across controls sensors and coil faces, high duct static pressures and low system flow rates. 

What does it look like? I decided not to use any of my many photo examples in this post, to not embarrass projects I have worked on. However IMHO, it looks;

  • obvious; 
  • embarrassing; 
  • signals poor design and installation engineering. 

It really displays;

  • lack of professionalism; 
  • lack of knowledge; 
  • lack of expertise;
  • low accountability project culture.

There are many variables on good and poor ductwork configuration. Some basic diagrammatic examples follow:

System effect should be considered at design and installation stage. A wait and see approach leads to poor performing systems and high running costs. 

Post installation solutions to system effect typically consists of fan/motor speed increases leading to;

  1. increased static pressures;
  2. increased power consumption;
  3. increased systems air leakage; 
  4. lower equipment life; 
  5. increased noise generation;
  6. increased running costs.

To reduce system effect a uniform velocity profile is required at fan/AHU inlets and outlets. To achieve a uniform velocity profile, a 100% effective duct length must be used (source: ASHRAE). 100% effective duct length can be calculated as follows:

  • 2.5 duct diameters for 12.7 m/s (2,500FPM) or less
  • Add one duct diameter for each 5 m/s (1,00FPM)

To someone outside of our industry this all looks obvious and would generate “sceptical hippo” eyes that this is even a thing. Of course building systems are designed and installed to perform? Actually, I think they would be surprised if they really knew how the building “cake was mixed and baked”. 

Twitter: @BLDWhisperer

Related posts & links:

#88 – HVAC – The Low Consequences Game ( )

#105 – Bad Projects? No, Only Bad Leadership ( )

#99 – E&O’s, CO’s & Shame ( )

#115 – 4 Times Lucky? ( )

Edifice Complex Podcast


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Systems Effect Summary Diagram

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