So back in August of last year, I wrapped up a three-part series titled Back to Basics. Part three of that series was, for lack of a better term at the time, a collection of “shorts” that were just little thoughts, info, and insight that I’ve pondered over the course of my career. Since then I’ve been gathering more ideas on what would make good topics for these mini stories. Well, I came up with a whole lot of ‘em! That said, I present here the next installment of an ongoing periodic series of short stories pertinent to the HVAC / BAS business.

Analog, Binary, Digital, Oh My!

These three terms bring about a fair amount of confusion to the average HVAC professional. The first one was all we had to worry about prior to the “digital” age. Not that the other two weren’t around or didn’t have meaning, just not as much relevance as nowadays. Simply put, analog refers to continuous, real-world variables. Like temperature. Temperature is an analog value, continuously variable from absolute zero on up. Binary refers to a two-state value, whether it’s on/off, opened/closed, yes/no, what have you. And digital stems from the word “digit”, which in the world of digital electronics refers to a piece of electronic information, or “bit” being able to hold a digital value of one (presence of electrical charge) or zero (absence of electrical charge).

The fun begins when you start talking about digital controllers. The term digital as it pertains to digital controllers is a generalization used to describe the conversion of real-world information represented by analog signals (such as temperature) into digital information (zeros and ones) such that the microprocessor-based controller can process it. Analog inputs to the controller are control signals generated by sensors, and analog outputs from the controller are control signals used to drive variable end-devices such as motorized dampers and control valves. Binary inputs are two-state signals produced by switches, relays, etc. And binary outputs are two-state outputs used to control the state of two-state end-devices such as fan motors and two-position dampers and valves. The confusion starts (or continues) when the term “digital” is used in place of “binary” to describe inputs and outputs of this type. While not entirely incorrect, it does make things a little bit harder to understand. Hopefully this little primer helps to get it straight in your mind.

Duty Cycle Anyone?

Every now and then I come across a term, whether it be on a drawing or in a written specification, that I would consider “archaic”. Duty cycle is one that surely fits that mold. I suppose at one time this term had relevance, however certainly before my time. The term refers to cycling a piece of heating/cooling equipment on and off, with little to no regard for the heating/cooling needs of the space, in order to conserve on energy. Don’t know where this scheme of control might still be applied, but not something that I’ve seen in practical use lately (lately meaning the last twenty years or so!). Maybe I’m wrong, but another term that I feel fits the definition of archaic is “night setback”. Now I know there’s still such a thing, however these days it’s more generally referred to as “unoccupied mode operation”. You see, the term night setback stems from the notion of “setting back” the heating setpoint in the unoccupied mode, and has been generalized to include the process of “setting up” the cooling setpoint as well. Nowadays, it’s more common, and more meaningful, to refer to the setback/setup of setpoints in the unoccupied modes simply as “unoccupied mode operation”. At least in my opinion.

Thermostat Designations

Space thermostats, those that are designed to control heating and cooling equipment such as packaged rooftop units, and even residential systems, have wiring terminal designations that pertain to the function of each terminal. For instance, a two-stage heat/cool thermostat with fan control will have the following terminal designations: R for 24-volt power (from the unit to the thermostat), G for unit supply fan control, Y1 and Y2 for stages of cooling, and W1 and W2 for stages of heating. Beggin’ the question...why? In other words, why not label these terminals to give more insight as to their function? It’s as if someone a long time ago came up with these seemingly random letters to represent fan, cooling, and heating. I don’t know, maybe there’s a real reason behind it, significant at least back in the past. But for the twenty plus years that I’ve been in this business, these designations have been messin’ with me! Hence, I propose to rename these terminal designations as follows: F for fan control, C1 and C2 for cooling control, and H1 and H2 for heating control. Who’s with me?!

The Name Game

I love how these pieces of equipment that occupy our world of HVAC have been named. I mean, rooftop unit! How clever! Of course it doesn’t really give any insight to what it does, but at least you know where to look for it. Although I have seen one or two installed “on grade” throughout my career. More fitting names for HVAC equipment include make-up air unit, fan-coil, and air-cooled condensing unit. A make-up air unit is just that…a unit designed to “make up” the air that is leaving the space served, typically by some kind of exhaust system. In doing so, the unit brings in outside air, at a rate equal to the rate of air being exhausted, and conditions it (heats or cools it) as required. A fan-coil unit, again, is just that…a fan and a coil. Well, minimally anyway. The unit will have a fan, and will have a cooling coil or a heating coil, and maybe both. It also may have dampers that allow air to be brought in from the outdoors and to be returned from the space served. And an air-cooled condensing unit…well, this one requires a little more of an explanation. A condensing unit is a single piece of equipment that contributes to the refrigeration cycle (see next topic). In short, a condensing unit contains in its package the compressor(s) and condenser coil(s) required for refrigeration to occur. An air-cooled condensing unit is one that is located outside, and uses condenser fans to help reject heat from the condenser coil(s) to the outdoors (think residential central air…the thing that’s outside and makes all the racket). Contrast this with the term “air-cooled condenser”, which removes the compressor from the package, to be located elsewhere in the refrigeration cycle, and typically indoors. Which leads us to…

Refrigeration for Dummies

Every now and again I need a refresher on the refrigeration process, so a very long time ago I took the time to write up a quick reference description of the refrigeration cycle. And it goes like this:

[an error occurred while processing this directive]The refrigeration cycle consists minimally of a compressor, a condenser, an expansion device, and a evaporator. The compressor is the heart of the system, and is used to pump low-pressure refrigerant vapor from the evaporator and compress it to a higher pressure. This hot, high-pressure vapor is delivered to the condenser, where it rejects its heat to the ambient (outside) air. As the heat is removed from the refrigerant, it condenses, and turns into a high-temperature liquid. The refrigerant, now in its liquid phase, passes through an expansion device that substantially reduces the pressure and temperature of the liquid refrigerant. The cool, low-pressure liquid refrigerant is then delivered to the evaporator. The evaporator may be a finned coil inside a packaged rooftop unit, of which the rooftop unit’s supply air passes through. As the air passes though, it’s cooled down. The refrigerant absorbs the heat from the air, and boils. The refrigerant, now a vapor again, is drawn to the compressor, and the cycle continues…

Starters vs. Contactors

These terms are at times mistakenly used interchangeably. I’m here to set the record straight (at least as far as I’m to understand it!). A contactor is basically a relay. A three-pole contactor has three separate normally-open contacts. When the coil of the contactor is energized, the contacts change state, from open to closed, allowing electricity to flow through each contact. Three-pole contactors are used to control three-phase motors. But the story doesn’t end there.

Smaller motors, those of which are single-phase, fractional horsepower, typically have overload protection built into the windings of the motor, for if the motor were to be overworked and overheat, the internal overload protection would shut down the motor, until the motor cooled down.

Larger motors, those of which are three-phase, need to be protected externally from overload conditions. An overload device, when used in conjunction with a three-pole contactor, allows for both control and overload protection of a three-phase motor. Combine the two devices, contactor and overload block, and viola, you have a starter!

Tip of the Month: Starter for a Single-phase Motor?? While they do exist and you may need one on the odd occasion, a contactor plus overload block for a single phase fan is often “specified overkill”. As noted herein, small, fractional horsepower single-phase motors will have the overload protection built in to the motor. The dividing line is blurred however, because three-phase motors don’t necessarily start at 1HP. You will find 1HP motors (and larger) that are single phase, and you’ll find fractional HP motors that are (specified to be) three-phase. The key here is that, for the upper range of single-phase motors (3/4 HP and above, for instance), you need to check the motor specs to see if there’s built-in overload protection. If so, no sweat. But if not, you’ll be required to use a starter (contactor plus overload block), at least if you want to be “up to code”!

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