AutomatedBuildings.com Column - Terms & Terminologies

Oftentimes we in the HVAC controls business talk in code, throwing around trade names and model numbers, abbreviations and acronyms, and high-tech terms and terminologies, as if it were plain English. It’s a wonder anyone else ever knows what we’re talking about! But when we’re talking to each other, we seem perfectly at home using these terminologies. Sometimes we even take for granted each other’s knowledge and comprehension when engaged in discussions regarding stats and sensors, DDC and BAS, and the like. For those of whom are not particularly well-versed in the lingo (and even for those who are), the following are some terms that are subject to interpretation and misuse, given the everyday context in which they are used.

These days the two terms are seemingly as interchangeable as flapjacks and pancakes! In order to understand the fundamental differences between the two terms, we need to go back in time a few years and explore the origins of these words. Let’s not and say we did, but had we done it, we’d find that the term thermostat has been broadened over the years, to cover many different types of temperature controllers, from single stage space temperature controllers, to proportional duct mounted temperature controllers. Whether it’s right or wrong to generalize the meaning of this term to include proportional controllers is probably a matter of opinion. In the purest sense, the term refers to a single stage or multistage temperature controller that is located in a space. And the term sensor refers to a device that can sense or measure a controlled variable (such as temperature or humidity), but has no capability to control something on its own. The perceived interchangeability of these terms is exposed with statements such as “That sensor over there on the wall controls this rooftop unit”, or “The VAV box for this space is controlled by this here thermostat”. In reality the “stat” in this case is likely nothing more than a sensor with some built-in adjustment capability, feeding into a digital controller up at the VAV box. For all that its worth, there is a difference between the two terms.

Interchangeable acronyms, right? Well…let’s begin by defining what they stand for. DDC stands for Direct Digital Control, and BAS stands for Building Automation System. All BAS are DDC, however the inverse of this is not true, as all DDC-based “systems” are not necessarily Building Automation Systems. Consider for instance the notion that a single digital (DDC) controller can be employed to operate an air handler in a stand-alone application. A single digital controller does not a BAS make. Now take a project that is made up of many digital controllers operating many different systems and pieces of equipment, network them together, throw in a front-end operator workstation, and you have yourself a BAS! Again, the lesson learned is that, while all BAS are DDC, all DDC are not BAS.

A digital (binary) input or output on a controller is a two-state point. The term digital stems from what is meant in digital electronics by the ability of a piece of electronic information, or “bit”, to assume one of two states, or “digits”. The term binary is similar in definition, and is often used interchangeably with digital, in the context of two-state inputs and outputs.

The term “digital” in Direct Digital Control refers not to the digital type inputs and outputs found on the controller. A digital controller is not necessarily one that is solely made up of two-state inputs and outputs (although it can be). In this context, the term digital actually has more meaning in reference to the analog types of inputs and outputs found on controllers, those that are not two-state but are continuous in nature, as are sensor inputs and proportional output signals. With a DDC or microprocessor-based controller, a microprocessor exists, virtually between the inputs and the outputs of the controller. In order for the “brain” to process information from the inputs, it must be converted to “digital information” (0s and 1s), and in order for the brain to convey its decisions to the real world, it must be converted back to “analog information”. This is essentially true for the proportional type, or analog, inputs and outputs. Analog information is converted to digital information, by means of an analog to digital (A/D) converter. The brain processes the digital information, and makes decisions that ultimately result in actions taken upon real world devices. However, the decisions must be turned into analog values before presented to the controller’s outputs. This is done with a digital to analog (D/A) converter. The lesson learned? Please don’t ever call a digital controller a binary controller!

Auxiliary (Aux.) Contact(s) – A set of dry contacts furnished as an option or add-on accessory for a motor starter. The contacts change state when the starter is energized. The contacts can provide information about motor status, or can serve as an interlock or control point for a separate (related) control sequence.

End Switch - A set of dry contacts furnished as an option or add-on accessory for a valve or damper actuator. The contacts change state at some point in the actuator’s stroke, and is often adjustable. The contacts can provide information about valve or damper status, or can serve as an interlock or control point for a separate (related) control sequence.

These two terms are most often used in the context of pumping systems, whereby typically more than one pump is provided for a single system.

The term primary/backup implies system redundancy. In simpler terms, it implies that two pieces of equipment are each sized to handle an application, with one operating at a time, and the other serving as a “backup” in case of failure of the “primary”. A primary/backup pumping system is that in which two pumps are installed and piped in parallel, each pump sized for the full system GPM, with the intention of having only one pump run at any given time, while the other serves as a backup in the event of failure.

The term lead/lag implies staging, where stage one is the lead, and stage two is the lag. It implies that two (or more) pieces of equipment are sized so that the sum of their capacities equals the maximum system requirements. This term is better suited to equipment fitting this description, such as: two boilers, each sized at half capacity, or two compressors that are part of the same chiller or condensing unit. The term is widely confused with the term primary/backup, but is distinctly different, as we see here.

This is a fun one, seeing as these terms have different meanings when talking about different devices. As was done earlier, let us simply define each of these terms in their proper context.

Normally Closed – When referring to a set of electrical contacts, it means that the contacts will allow current to flow when in their “normal” position. Typically refers to the contacts of a relay or contactor, in which “normal” means that the coil of the relay is unpowered. When referring to a control valve or damper, it means that the device will not allow fluid or air to pass through when in its “normal” position. In this case, “normal” means that the “spring return” actuator of the valve or damper is unpowered.

Tip of the Month: When it comes to our business, we seem to have our own language. The terminologies used throughout the industry are far from standardized, and nowhere is this more prevalent than with controls. Terms are used and abused, and one individual’s or manufacturer’s lingo may be quite different than another’s. The tip here is to be aware of the terminologies, and understand their use and their context. While it may be acceptable to talk in simple concepts and “technically” misuse common terms, it must be done so with each side understanding the general ideas behind the terms, so that misinterpretation is avoided and expectations are met!

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS). You can visit his website at www.pcs-engineering.com.

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.

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