The Basics Of HVAC Controls | Optimizing HVAC Systems

HVAC Systems

Discussing controls for heating, ventilating, and air conditioning (HVAC) can sometimes get tedious because they cover quite the range of products, functions, and sources of supply.

When it comes to HVAC, we define control as the starting and stopping, and regulating heating, ventilating, and air conditioning.

The focal point will be devices and systems used to control much larger HVAC systems commonly used for commercial and industrial purposes instead of residential applications.

What controls are needed on HVAC systems? The answer would depend on the HVAC system being used. The most suitable types like pneumatic, electric, and analog electronics can fulfill and execute primary control sequences.

This article aims to discuss what HVAC is as well as a brief history of HVAC. It also aims to discuss how to implement and optimize them to fit the intended infrastructure best.

The Basics Of HVAC Controls

We cannot discuss the applications of HVAC controls without understanding the building infrastructure and the systems to be used and using spaces to be controlled and conditioned.

As stated above, the type of HVAC system used will determine the control sequence that will be executed. Furthermore, building infrastructure and its purpose or functions would influence the benefits or deficits you receive from a system.

What Are HVAC Systems?

HVAC systems fall into two categories which are self-contained unit packages or central systems. As the name implies, unit packages describe a single unit that converts a primary energy source, be it electricity or gas, and provides the heating and cooling effect to space.

Examples of these systems are rooftop HVAC systems, air conditioning units for rooms, and air to air heat pumps.

On the other hand, central systems are more complex because they utilize central supply and multiple end-use subsystems.

These end-use subsystems can be as simple as fan systems or terminal units. If the end-use subsystems are fan systems, their applications can either be single or multiple zone types. 

Unlike unit packages, the direct conversion of central systems takes place in a central location using fuel such as gas or electricity that creates thermal energy to be distributed throughout the building or facility.

There are many combinations of central supply and end-use zone systems. One most frequently used example of this system is the central hot and chilled water distributed to multiple fan systems.

These fans use coils which are water to air heat exchangers to provide either hot or cold air to space.

Another unique combination of a central supply and end-use zone system is a central chiller and boiled mainly utilized to convert the primary energy and a fan system that delivers the hot and cold air.

Multiple end-use zone systems use mixing boxes which are called VAV boxes.

Along with unit packages and central systems, various special-purpose systems are used in other, more specific cases. These are:

  1. Rejected heat or tower cooling from heat pump cycles on chillers
  2. Thermal storage
  3. Electricity and heat cogeneration

Background Of HVAC Controls

Controlled heating was done by draft dampers followed by thermostat control of said dampers at any one time. Mechanical stokers needed another step to achieving control when used for coal firing.

The concept of combustion safety controls became necessary once oil burners were introduced. This process involved the sensing and proof of flame in the correct sequence of introducing the draft, the fuel, and the ignition.

Usage of steam and hot water radiators led to zone control and individual room control (IRC).

Other forms of zone control include closed-loop controls using thermostats and open-loop controls with outside conditions dictating heat delivery rate into space. The common denominator of these forms was that they were used to regulate deliveries of heat.

The ways these methods regulate the heat include the following: Valves are used to control the flow of steam or water, controlling pumps circulate hot water, and controlling the boiler operation.

The central supply was maintained, and the radiator valves were controlled by room thermostats when the IRC was utilized.

Fans that are used to deliver ventilation and heated air are controlled by dampers that vary the source as well as the volume of air coming in.

Pneumatic controls are the typical control of unit ventilators that involve minimum outside air, discharge air, low-temperature air, and thermostats that feature lower night settings, activated by the compressed supply pressure level.

Ever since air conditioning has increased usage, more complex control sequences have led to more extensive systems to monitor and regulate.

The advent and surge of computers and microprocessors have had a significant impact on the HVAC controls industry. Minicomputers were utilized on jobs to collect data provision of direct, centralized control.

Microprocessors were then used for data gathering panels to gather readings and provide direct digital control. Computers have evolved into and are being used as on-site central controllers with interfaces and computer-assisted engineering (CAE) tools to design programs, databases, and documentation.

Microprocessors today are still used in gathering data but have branched out into small unit controllers and smart thermostats.

History Of HVAC Controls

Before World War II, companies that provided and promoted pneumatic controls were the leading HVAC suppliers in commercial buildings.

At the time, the predominant idea was that commercial building controls were too complicated for over-the-counter selling and had to be installed and supervised by a controls manufacturer, which was seen as tedious.

This concept further included having offices with only installers and service people.

Commercial buildings were modulating type controls for electric control systems. These systems were sold on a supervised and regulated basis. By the time other companies entered the commercial controls market with electric and electronic controls, their distribution method was through branches.

Some newcomers, who also started with electrical and electronic products, began expanding into pneumatic controls through their own development or association with foreign companies.

By the time computer-based supervisory control systems arrived at the market, some larger companies that also had computer-based products assimilated into the HVAC controls market but eventually gave up.

As companies became more open to international trade, thus leading the globe to international business development, the number of foreign-based controls companies expanded into the U.S markets directly or by association.

In the 1970s, several small companies evolved with more limited product lines for energy management functions. When the DDC became widely accepted, other small companies also developed microprocessor-based DDC controllers and supervisory systems.

Major players in the market, primarily full line control companies, stayed the same but now, with more competitors with limited systems.

Other major HVAC systems manufacturers have also acquired and developed control capabilities and have market packaged HVAC systems with supervisory control and controls systems.

Other companies started to provide produ9 for more specific applications. The criterion of a source of supply at the time should be considerate of its life cycle, needs and costs, along with the track record of said suppliers.

HVAC Basic Control

The amount of cooling or heating necessary to meet the load in a specific, conditioned space is regulated by basic controls. Minimum outside air is required for ventilation any time-space is occupied.

When outside air is freely available and suitable as a free cooling source, ventilation is controlled as needed at parameters more significant than the minimum.

The approach utilized by packaged unitary equipment is to employ space thermostats to control the generation of heat or cold inside a zone. The approach of central systems is to regulate the delivery of the heat and cold through the end zones to match the load inside the space.

The supply is often controlled and regulated to match the load imposed by every space. A well-known example of this is to use a room thermostat to control zones and discharge controllers to regulate central supplies.

The discharge temperature controllers regulate the rate of primary conversion (chillers and boilers). In contrast, the delivery rate of the pumps or fans that are in charge of distributing the central supply is controlled by the pressure controls.

Often, there are multiple boilers or chillers and pumps present, in which case they are all put online or offline as necessary.

HVAC Supervisory Control

The regulation of scheduling and interaction to meet all building needs is called the role of supervisory. Supervisory control systems can be called many different names depending on what function you want to emphasize.

Here’s a list of the most famous nicknames that all refer to the role of supervisory:

  1. BAS – Building Automation System
  2. EMCS – Energy Monitoring and Control System
  3. FMS – Facility Management System
  4. EMS – Energy Management System

DDC, also known as direct digital control, describes everything a computer or any microprocessor-based system does. The original use of this term provided closed-loop controls of local loops through a digital computer or microprocessors.

We utilize direct digital control in stand-alone panels in intelligent data gathering panels. Energy management programs, often in the central computer of a building automation system, are now placed in remote gathering panels.

Early management application programs are different from local loop controls and are named because of their specific function, for example, start or demand control.

The factors to consider about which energy management application should be used depending on the building it is being applied to and which HVAC system will be used. For example, a hospital that operates 27/7 is not suited for optimum start top programs.

Systems that supply heating and cooling simultaneously, like reheating systems and cold deck mixing box systems, are suited for load reset of supply temperatures.

Optimizing HVAC Systems

The primary concept of optimization of control is to more effectively regulate space conditions and do it in a similar manner that is efficient, effective, and flexible.

A strategy for optimization usually entails improving the efficacy of the primary supply equipment or reducing the losses of energy in the end-use systems. The size of the equipment often meets maximum loads, but they usually run below their limits.

It means that the part-load characteristics of the equipment determine the efficiency in meeting a given load. The optimal strategy for this would be to choose the most effective equipment capable of meeting the load at any given time if there are multiple chillers and boilers.

Furthermore, along with some types of end-use systems, the energy consumed and wasted by bucking heat against cold can be minimized simply by resetting supply temperature levels.

Using the thermal storage of a building to use energy stored at a low cost to be utilized when needed is yet another way to optimize control.

Moving the heat from one area of an enclosure to another is yet another form of optimization too. However, these principles are only possible through specific types of HVAC.

The primary variable in these circumstances would be the amount of heating and cooling needed and the control action to change the way the load is supplied.

This procedure has led to the use of the terms load reset and dynamic load control as a description to approach the idea of optimizing control. The selection of the combination of chillers to supply a cooling space is called the optimized chiller selection.

Room Thermostats

For many years, the industry standard has been that the thermostat must be mounted near the door of a room5 feet from the floor.

For just as long, this subject has seen its fair share of discussions. One problem that may arise from this is that if the room in question would be filled with people, the thermostat might not do its intended job.

There are times when installers are concerned about how their thermostats and humidistats should be mounted on the wall, that is, whether they should be upright or horizontal.

The answer to this is relatively simple; it doesn’t matter how you install it, but it’s essential to keep in mind that the thermostat is electric and has a mercury bulb switch contact.

The thermostat requires installing a certain way that is common in most residential and commercial buildings. Sometimes, the installer has to use a level to be able to mount the thermostat properly.

Thermostats and humidistats are essential in space because they regulate automatic valves and dampers in a room. These devices have sensors to determine whether a zone is too hot, too cold, too humid, or too dry.

An example of this would be a pneumatic thermostat with a bimetallic sensor and a relay. More often than not, the complete package can be installed under one cover on the wall, and the thermostat automatically gets to work for you. However, there are sensors mounted under cover of the room without actuators or relays.

Thermostats usually transmit the information to another device that is in charge of controlling the relays and so on.

This principle is often used in electronic control systems that involve a wire wound resistor mounted under the cover that translates the temperature in the space and transmits the information to a controller in the equipment room.

The advent of electronic control systems brought about the concept of a sensor under a cover in the room. Room sensors go hand in hand with other control systems and are often called transmitters.

In pneumatic controls, the transmitters use a sensor and a particular relay that sends a pneumatic air signal proportional to the medium being sensed.

One example of this is a transmitter under a room thermostat that sends signals based upon the room’s atmosphere.

An example of this would be a room transmitter with a range of 30 degrees Fahrenheit to 80 degrees Fahrenheit being sensed as 55 degrees Fahrenheit.

Here, the transmitter produces a signal of about 8 to 15 psi as it sensed the two extremes of the range. Transmitters are strictly not sensors because they cannot actually control anything by themselves but are sometimes called.

The Importance Of Dampers

Automatic dampers can be classified into two: parallel blades and opposite blades. The first one to be historically implemented was the parallel blade.

The air is controlled and regulated can be considered an incomprehensible fluid at pressures below 12 inches of water.

Calculations about anything above 12 inches should begin to factor incompressibility. Gases can bend, so the volume may not be affected and may not be controllable at all.

Gases easily stratify in a duct; therefore, a damper can be classified as a poor control device at best. However, at the same time, dampers can be just as capable as valves, provided they are appropriately sized.

Parallel Versus Opposed Blade Dampers

Parallel blades tend to bend the air during the first few degrees of their rotation while they go from fully open to close and only control the first 20 to 30 percent of movement.

These blades bend the streams rather than modulate them. However, there are also instances where mixing air streams are helpful too.

Opposite blades are often used to achieve more control of the airstreams and to prevent large amounts of stratification in the duct.

Fire And Smoke Dampers

The primary use of fire dampers is to help prevent the spread of fires and, ideally, confine it into one area of a system. Hence, they need to be made with heavy-duty material that is capable of withstanding heat.

These dampers are always held open by the linkage system that can be fused and is designed to melt and close once the temperature in an area reaches about 165 degrees Fahrenheit.

Smoke dampers, on the other hand, are not required in an enclosure. These dampers are manufactured by vendors that supply automatic dampers and can also be used as both control and smoke dampers.

Like fire dampers, smoke dampers also need to be approved by officials and will have to follow codes but applications about said codes are not as stringent as the fire dampers.

Just as their name implies, their primary purpose is to help prevent smoke from spreading, which can cause panic in an emergency.

Manual Dampers

Manual dampers are considered an essential part of a system because they can balance the system and assist the control systems to work better.

These dampers are used primarily to balance the amount of air distributed to various places of the system. Manual dampers can be of the splitter type or the more common closure type.

As their name implies, splitter dampers split the air and redirect it to various sections of the ductwork. Splitter dampers often help maintain the balance across the system by consistently ensuring the right amount of air gets to the right places.

Static Control Dampers

Static control dampers are unique because they maintain the static pressure inside the ductwork based on the static pressure controller reads. Since this process is rather complicated even for machinery, the parts needed to maintain this function must be immaculate.

Damper Motors

Unlike the name implies, damper motors are not actually motors for dampers. But the name has stuck and is widely called as such by the industry. A damper motor is in charge of operating all the dampers in a system with the help of a control signal device.


To sum it up, ventilation and air conditioning are essential to any space, be it a humble house or a towering skyscraper. Since these are two critical factors in creating a comfortable space, it is significant to learn the basics of ventilation.

Doing so will help you ensure a happy and comfortable environment in the comfort of your home or the ever-bustling office you are in.

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