VAV Box Sizing – Rules of Thumb Vs Engineering

February 17, 2017
1 Comment

Quality, Consequences and the Construction Industrial Complex (part 94) – All IMHO:

The lack of consensus in HVAC engineering continues to surprise me.

How do you size (select) a VAV box?

Should be easy to answer this question considering they are everywhere, right? Actually no, answers range from “we use rules of thumb” to “thats the suppliers job”.

Short Answer (using “rules of thumb”)

V Max should be approximately 2/3’s to 3/4’s of VAV box V Nom
V Min should be approximately 1/3 to 1/4 of VAV box V Max

IMHO, rules of thumb are only useful to trigger further analysis. All rules of thumb should be considered inaccurate and dangerous. A longer answer is required.

Long Answer
For context, lets start with definitions:

VAV Box: 1970’s technology favoured by the North American design and construction supply chain.

Nominal: “existing in name only” (Dictionary.com) Engineering context, “measurement that matches the predicted value” (Wikipedi.com).

V: Volume flow rate.

V Nom: (Nominal): Theoretical maximum flow rate for a given box size, determined by manufacturer.

V Max: Design maximum (cooling) flow rate, determined by system designer.

V Min: Design minimum (heating) flow rate incorporating Bldg Code required minimum outside air, determined by system designer.

For good or bad, VAV boxes are everywhere. If they are selected incorrectly or not commissioned the consequences are;

inefficient system and building operation;
thermal overlap and high energy consumption;
high noise levels;
poor temperature & humidity control;
occupant discomfort.

VAV box selection matters and if mistakes are made they happen at scale, due to the high multiples of VAV boxes on a project.

IMHO, HVAC equipment sizing is 90% engineering and 10% art. The following criteria should be embedded considerations by the engineer with each VAV box selection:

I believe there is a basic assumption by clients that engineers size and select VAV systems and equipment. Or at the very least, engineers know how to do it.

When confronted with Commissioning results showing the VAV system just did not work on a recent project, the HVAC design engineer could not explain how the VAV boxes where sized to meet the OPR and specifically the NR criteria. This is what happens when engineers abdicate responsibility to suppliers for sizing VAV boxes. What can I say, size matters!

Twitter: @BLDWhisperer

Related posts & links:
#36 – How many people does it take to commission one VAV Box? – This is not a joke! ( https://www.linkedin.com/pulse/how-many-people-does-take-commission-one-vav-box-joke-muggleton?trk=mp-reader-card )

#71 – VAV Box – Sensitive, Install as Follows ( https://www.linkedin.com/pulse/vav-box-sensitive-install-follows-adam-muggleton?trk=mp-reader-card )

#37 – Project Managers beware! This is why VAV systems are not “start up and walk away” ( https://www.linkedin.com/pulse/project-managers-beware-why-vav-systems-start-up-walk-adam-muggleton?trk=mp-reader-card )

#64 – Respect the VAV System Static Pressure Sensor! ( https://www.linkedin.com/pulse/respect-vav-system-static-pressure-sensor-adam-muggleton?trk=mp-reader-card )