Making Buildings Work Better

The Units of Performance

People spend a lot of money on new buildings and often don’t get the performance they expect and deserve. As a commissioning authority, I can confirm that poor performance often stems from poor communication. Ambiguity, in particular, leads to assumptions which, let’s face it, are often wrong.

If we can improve communication, we can reduce the number of issues that occur, having a very real and positive impact to health and safety, comfort and operational costs (most noticeably smaller energy bills).

From Design to Operation

Mechanical designers use numbers and units to communicate the types and sizes of devices and sensors to be installed in a building. These decisions impact construction costs, capacities, control stability and operational efficiently. Designers also write sequences of operation to clarify how systems will be configured and programmed, which establishes appropriate limits and setpoints that can have an extremely large impact on operational performance.

If a contractor isn’t provided with enough information to implement a design correctly, should we really expect a designer’s intentions to be met?

In modern building automation systems (BAS), numbers are actually communicated with the use of electrical signals (in volts, milliamps, resistance or digital communication). As sensors and control devices can usually be selected or configured to act differently for the same electrical signal, people can’t make sense of the electrical signals until they have been translated into different numbers with more descriptive units (like airflow expressed in cfm). When units don’t get properly assigned—or even worse, when translation never occurs (ex: valve command expressed in volts), it gets much harder for building operators and engineers to understand how things are operating and to properly troubleshoot problems.

If a contractor implements an ambiguous design as documented or makes wrong assumptions for one or more pieces that were unclear, should we expect building operations and maintenance staff to fully understand the installed systems, effectively troubleshoot issues and keep a building running at peak performance?

What is a Unit?

When we deal with numbers, ‘units’ help provide a standard quantity to compare against—basically providing more information that ensures people are speaking the same language with regards to the meaning of a value. A size of a man who weighs 170 has very different meaning if measured in pounds or kilograms.

Numbers with missing or ambiguous units are one common source of confusion in buildings that are particularly easy to address. When these problems occur, an experienced person can usually read between the lines, but we should expect our design and construction teams to provide a professional-level effort to make sure bad assumptions are minimized.

Percentage is a ‘Quasi Unit’

Percentage takes the role of a unit, but still leaves room for interpretation. I call this a ‘quasi unit’.

Valve and damper positions are usually expressed in percent but what exactly is that communicating? For these kinds of devices, percentage usually refers to ‘% open’ but there are still many cases where they are actually programmed as ‘% closed’ or where the whole ‘open’/’closed’ concept simply doesn’t apply. The position of a 3-way valve could refer to a ‘percentage bypassed’ or ‘percentage recirculated’ but this is almost never clarified. When the intent isn’t clear, the likelihood of costly mistakes (like simultaneous heating and cooling) increases.

VFD speeds are also typically expressed by designers and manufacturers in the units of percent, but industry hasn’t come to agreement about whether this percentage is based on the high limit, controllable range or electric distribution frequency. Each of these strategies results in different actual speeds for the same signal. The table below illustrates how we’re not always speaking the same language when we communicate speed in percent.

Speed Signal (%) VFD Frequency (Hz)
(based on % of high limit) (based on % of controllable range) (based on % of elec distribution frequency)
0% 0 12 0
50% 45 41 30
100% 70 70 60

*Assuming VFD low limit set to 12 Hz, high limit at 70 Hz, and elec frequency of 60 Hz

This is more important than it seems. If a designer provides a sequence that asks for a fan to control down to 0% (presumably assuming % is based on a controllable range), the selected method could put the motor at risk of premature failure by operating at speeds where heat can’t be properly dissipated. The energy efficiency of different strategies can also vary significantly.

What’s the solution?

Improving performance has to start with a well-documented design. Reduce the number of bad assumptions by insisting that designers and contractors communicate more effectively. Make sure they consistently use units on numbers to fully communicate meaning.

Avoid the use of ‘quasi units’. If percent or another ambiguous unit is needed, ask for additional words to fully clarify what is really meant.

If the meaning is still unclear, make sure clarification is received early on… before equipment gets ordered and changes become difficult and costly.

Prove you’ve gotten what you asked for and what the designer intended. If you aren’t already using it, commissioning is a great way to help catch issues early, minimize risks to performance and maximize project success.

Benson headshotMr. Benson holds a degree in electrical engineering and is skilled in high-performance building design and control. He has a unique blend of experience with engineering, controls programming and fault detection. With this experience, he is able to bridge critical gaps that typically exist between design, implementation and ongoing performance. His work in new buildings includes fundamental and enhanced commissioning, measurement and verification (M&V) and facilitating energy-efficiency utility incentives. He is also adept in existing building services such as building performance assessments (including ASHRAE energy audits), existing building commissioning and HVAC controls upgrades. He consistently achieves significant energy savings and improvements to comfort from implementation of recommendations.

Category: Building Controls, Total Building Commissioning (Cx)

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As a leader in the building commissioning industry, Total Building Commissioning (TBC) is a facility consulting firm that specializes in the commissioning of mechanical, electrical, controls and all other major building systems as well as LEED™ certification consulting.

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