Long, long ago, man discovered fire. And it was good. The discovery of fire, however surely accidental, nevertheless brought forth some very important amenities for mankind. One was the ability to cook the food that man ate. Another was to keep man warm during the colder months of the year. By building a fire, man was able to regulate and maintain his comfort level. If man became too warm from the fire, he could take a step back and satisfy his preference of warmth. If man became too cold, he could put another log on the fire, again to maintain his preference.

This crude form of satisfying man’s comfort level by implementing a fire is no doubt among the first attempts at temperature control. Although manual in the sense that man had to regulate his comfort level by either stepping back or adding logs, it is a good example of the term “temperature control”. When we think of temperature control in this day and age, we think in terms of sensors, setpoints, and controllers. When we think of the caveman and his fire, we can relate these present-day terminologies to the caveman’s version of temperature control. As far as the sensor goes, that would be caveman’s sense of feel. The setpoint is the caveman’s preferred comfort level. The controller is the caveman himself, as he performs control actions in an effort to achieve and maintain his setpoint.

Simple Electrical Control

About a billion years after man discovered fire (give or take!), man discovered electricity. A while after that, man invented air conditioning. Along with these two developments came the evolution of the thermostat, which turned manual control of maintaining comfort levels into automatic control.

While that may not seem like a revolutionary concept in this day and age, if you think about it in terms of fundamentals, you can begin to see just how important automatic temperature control is to mankind. Man invented the air conditioner, and suddenly we were able to provide environmental control for indoor spaces. In short, we were able to cool down spaces that become too warm for human comfort. Yet without some means of automation to the process, we’d be forced to do what the caveman did with fire. We’d be forced to continually intervene with the cooling process, to manually engage it when we became too warm, and disengage it when we became too cold.

The thermostat automated the process. The simple thermostat has the capability to measure or sense the surrounding temperature, and engage and disengage a process based upon that temperature varying above and below a predetermined value, or setpoint. More specifically, the electric thermostat can make and break an electrical circuit as a function of the temperature surrounding it, and the setpoint. If the temperature rises above setpoint, the thermostat completes the electrical circuit and allows the electrically empowered air conditioning process to operate. Once the temperature falls back below the setpoint of the thermostat, the thermostat breaks the circuit to interrupt electrical power to the air conditioner.

Simple “two-state” electrical temperature control as described here is as old as air conditioning itself, yet the fundamentals are in wide use even today. A classic example is the through-the-wall air conditioning unit, or “window shaker”, as we sometimes call it. These units all typically have built-in electric thermostats regulating their temperature control processes. Turn your A/C unit on, and the fan begins to run. Quite likely at the same time, the compressor energizes, which begins the refrigeration (air conditioning) process. As the room cools down, an internal thermostat monitors the temperature of the air recirculated by the fan, and once the temperature drops below either a fixed or a user-adjustable setpoint, the thermostat drops out the compressor, leaving the fan running. The temperature in the room begins its ascent, above the setpoint of the thermostat, and the compressor kicks in. And the cycle continues, all in the name of maintaining a stable and consistent room temperature, without the need for manual intervention.

Pneumatic Control

For simple systems such as the one described under the last heading, simple controls were (and still are) sufficient. Yet as systems grew more complex, there came the need for more complex and more precise methods to control these systems. Prior to the invention of the transistor and the advent of electronic control, pneumatic control filled the bill for many of the larger and more complex HVAC systems of the past century.

Pneumatic control uses compressed air as the control medium (as opposed to electricity). An air compressor regulates a system pressure that is suitable for control purposes, and copper and/or plastic tubing distributes the compressed air out to all of the control devices. The compressed air is utilized by controllers, processed to perform specific outcomes, and delivered to end devices such as pneumatic-electric relays, pneumatic control valves, and pneumatic damper actuators.

Inherent with pneumatic control is the capability to vary a process. We think of a varying or proportional control process as one that is not limited to one of two states (such as on-off control), but can be implemented in varying degrees. An example of proportional control would be that of a radiator hot water control valve being utilized to maintain a desired temperature setpoint within a space. As the space temperature drops below setpoint, a proportional application would dictate that the control valve does not open fully all at once, but opens as a function of the deviation in temperature from setpoint. In simple terms, the colder it gets in the space, the more the valve will open. The more the valve opens, the more hot water is allowed to flow through the valve and through the radiator.

A receiver-controller is a pneumatic device that can be used to implement the above scheme of control. As its name states, the device has the ability to receive a sensing signal, and to control an end device. It also has, as all controllers, the ability to accept a setpoint. The receiver-controller receives a signal from a transmitter, the signal being in the form of a varying air pressure. The signal represents the medium that the transmitter is sensing, whether it be temperature, pressure, humidity, etc. The receiver-controller processes the signal, compares it with the established setpoint, and puts out the appropriate control signal, which is also in the form of a varying air pressure. This control signal is sent to a pneumatic end device such as the control valve in the preceding paragraph. The control signal sent out to the end device is a function of the signal received by the controller, and of the established setpoint. This signal, for instance, may increase as the temperature drops below setpoint, as determined by the receiver-controller. The control signal is used to directly actuate the end device; the higher the pressure of air that is sent to the control valve actuator, for example, the more the control valve is open, and the more hot water flows through it.

Pneumatic control systems are still in wide use in the HVAC world. Perhaps the primary reason is that there are a lot of these heritage systems still in operation that were originally installed in the heyday of pneumatic HVAC control. No reason to tear something out if it’s in proper working condition, right? Other reasons for its continued use in the HVAC industry is its strong actuation capabilities, as well its ability to be easily and safely applied in hazardous and corrosive environments. The inherent safety of compressed air based control systems make them a natural choice in certain industrial and process control applications, which is where you’ll find pneumatic control alive and well. The HVAC industry can still benefit from pneumatic control, as this paragraph illustrates.

Of course, pneumatics has its shortcomings as well as its benefits. For one, the compressed air system itself is a point of continual maintenance. The air compressor is an electromechanical device, one which uses electricity to power a compressor, and is subject to failure, both mechanical and electrical. When the air compressor is down, the entire control system is “dead in the water”. Other issues, relating to maintenance, are for the requirement of clean and dry air, and for the perpetual requirement of controller calibration.

[an error occurred while processing this directive] Electronic Control

The invention of the transistor revolutionized the electronic industry, and many other related industries as well. One of those being the HVAC industry. Many (most?) of the complex control strategies possible using pneumatic control were able to be replicated using transistor-based and integrated circuit-based electronics, at a lesser cost and with more reliability. Don’t quote me on that…I’m sure that it took some time for electronic controls to evolve to that point. Anyway, at some point they did, and along with that came the inherent stability, consistency, and maintainability of electronic control, something that pneumatic control lacked.

Basic to electronic control is the ability to sense the controlled variable electronically, as opposed to using mechanical or electromechanical means. A sensor or transmitter replaces the old gas filled sensing bulb or coiled up bimetal that are common to electrical controls devices that were more popular in the earlier days. An electronic temperature controller can replace the function of the simple electric thermostat, and at the same time offer some advantages over the latter, in the areas of accuracy, consistency, adjustability, and reliability. With no moving parts, the controller is inherently immune to the rigors of physical and mechanical stress, and thus not subject to mechanical breakdown. The term “solid state” describes, in the field of electronics, the concept of “no moving components”, the term born from the invention of the transistor, and the subsequent phase-out of the vacuum tube.

The ability to perform more complex schemes of control, such as those once only achievable with pneumatics, is another advantage of electronic control. For instance, replacing the receiver-controller is the proportional electronic controller. Like its predecessor, the electronic controller has the ability to receive a sensing signal, accept a setpoint, and proportionally control an end device. The sensing signal however is from an electronic device such as an electronic sensor or transmitter. The electronic controller processes sensor information, compares it with setpoint, and sends an appropriate electronic control signal out to the electronic end device. The control signal is an electronically processed calculation relating the sensed condition and the setpoint. The signal is proportional to the deviation in sensed condition from setpoint. The greater the difference between these two values, the greater the control signal sent out to the end device. In the end, the function of the proportional electronic controller is equivalent to its pneumatic counterpart. Yet the advantages of electronic control over pneumatic control are numerous, and the ever-decreasing size and cost of electronic components and devices make electronic control the favorable choice in applications that were once only attainable using pneumatic controls.

Tip of the Month: Actually, a clarification, and I do this to prevent receiving emails from fellow gearheads. I mentioned that the invention of the transistor facilitated the phase-out of the vacuum tube. For those of you old enough to know what a vacuum tube is, you would agree with me. However, for you modern-day musicians, you may know that tubes are very much alive and well even in this day and age. Yes, the transistor replaced the tube in all but the music industry, where it still thrives in the form of amplification for the electric guitar.

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