In heating, ventilation and air conditioning (HVAC) systems, airflow reported at the variable air volume (VAV) box is actually a measure of total pressure. It gets converted into an airflow by a multiplier that is generated during testing, adjusting and balancing (TAB). This multiplier is commonly called “k-factor”, but can also be called flow multiplier, pickup gain, etc. This value is important to commissioning HVAC systems.
Correlation of airflows to a pressure reading is usually done at a single airflow (often cooling max). This is called single point calibration.
We care about k-factor because:
- it ensures the VAV box performs at the EOR’s capacities (i.e. minimum and maximum airflows), satisfying outdoor air (OA) requirements and cooling/heating loads.
- it maintains space pressurization (esp. in the case of offset airflows for labs).
- it’s evidence that balancing has taken place.
Because balancing is so important, and it is often not done (or done poorly), we check during VAV testing that k-factors have been entered into the control system. Many VAV controllers have pre-programmed k-factors, each for a different box size. However, installation can make these moot. A k-factor for a 4” box installed at the end of a long duct run will be different than a box installed close to the main with a 90° elbow immediately before it.
The k-factor can either be adjusted, or an additional calibration factor can be piled on top of it. This could be adapted to retro-commissioning (RCx) by polling the k-factors and validating that they are different.
Single point calibration is cheap and efficient for spaces that don’t require great accuracy or have an unbalanced* return airflow (i.e. offices with plenum return systems). For labs that maintain pressurization with an offset between the supply and exhaust airflows, single point calibration is not enough.
As our experience at a County Morgue demonstrated, calibration at a single point can lead to inaccuracies at low airflows. When the boxes went to minimum, errors in calibration forced the decomp autopsy room to become positively pressurized, spreading noxious smells throughout the building!
For commissioning practices, here’s what to do:
- Make sure the system is balanced before functional testing.
- Ask for the balance report and review it.
- Don’t look/ask for k-factors on the TAB report – they won’t be there because it is not standard practice.
- Document k-factors during functional testing.
- They should all be different.
- They can be the same for boxes of the same size only as long as there’s an additional calibration factor.
- Check low airflows during functional testing for lab spaces that maintain pressure with offset airflows.
- The lab should maintain pressurization.
- If pressurization switches, the lab fails testing!
- For RCx projects, check k-factors where balancing is thought to be an issue
- If they are all the same based on box size, look for an additional calibration factor.
Proper commissioning is critical for energy efficiency and optimized building operation. If you have any questions or need assistance commissioning your next project, please contact us any time.
For more information on k-factor, see this article: https://www.hvacbrain.com/blog/k-factor-in-hvac/
For further reading on the drawbacks of single point calibration, see this excellent NCBC paper from 2013: http://www.facilitydynamics.com/assets/1/15/VAV_Airflow_Control_Simens2013.pdf
* unbalanced = any airflow that is not adjusted by a TAB technician
This post was originally published in our internal Commissioning services group newsletter and authored by Lyn Gomes.