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Correspondence Lesson 8: HVAC Controls

Controls for heating ventilating and air conditioning (HVAC) cover a broad range of products, functions, and sources of supply. We define control as the starting and stopping or regulation of heating, ventilating, and air conditioning. Our concern here is control devices and systems to control larger commercial and industrial HVAC systems, not residential heating and cooling, except in a few cases

Controls for heating ventilating and air conditioning (HVAC) cover a broad range of products, functions, and sources of supply. We define control as the starting and stopping or regulation of heating, ventilating, and air conditioning. Our concern here is control devices and systems to control larger commercial and industrial HVAC systems, not residential heating and cooling, except in a few cases where residential controls crossover into light commercial controls.

The application of HVAC controls starts with an understanding of the building and HVAC systems, and the use of the spaces to be conditioned and controlled. The type of HVAC system determines the control sequence. Several types of control products such as pneumatic, electric, analog electronic, or electronic direct digital control (DDC) can then do the basic control sequence.

The way buildings are used determines the benefits you can obtain from additional controls.

Background. At one time, draft dampers (followed by thermostat control of the dampers) controlled heating. The use of mechanical stokers for coal firing required another step in the use of control. When oil burners were introduced, the concept of combustion safety control became necessary. This involved the sensing and proof-of-flame in the proper time sequence of introducing draft, fuel, and ignition.

The use of steam and hot water radiators led to the concept of zone control and individual room control (IRC). Forms of zone control included closed loop control using zone thermostats and open loop control with outside conditions setting the rate of heat delivery to the zone. Both of these forms of control were used to regulate the delivery of heat. The means of regulation included the following: Valves to control the flow of steam or hot water, controlling pumps to circulate hot water, and controlling boiler operation. When IRC was used the central supply was maintained and radiator valves were controlled by room thermostats.

The use of fans to deliver ventilation as well as heated air was controlled by dampers, which varied the source and volume of air. The typical control of unit ventilators was by pneumatic controls and included the following features: minimum outside air, discharge air, low-temperature lim, and thermostats with lower night settings activated by compressed supply pressure level. The increased usage of air conditioning led more complex control sequences in larger systems to central monitoring and control.

The development and use of computers and microprocessors has caused great changes in the HVAC controls industry. Minicomputers were installed on jobs to collect data to provide centralized control. Then, microprocessors were used for remote data-gathering panels to gather data and provide direct digital control. Computers are now used as on-site central controllers with operator interfaces and as computer assisted engineering (CAE) tools in the design of system programs, databases, and documentation. Microprocessors are still used in remote data gathering, yet also in small unit controllers and in smart thermostats.

HVAC systems. HVAC systems are classified as either self-contained unit packages or as central systems. Unit package describes a single unit that converts a primary energy source (electricity or gas) and provides final heating and cooling to the space to be conditioned. Examples of self-contained unit packages are rooftop HVAC systems, air conditioning units for rooms, and air-to-air heat pumps.

Central systems are a combination of central supply subsystem and multiple end use subsystems. End-use subsystems can be fan systems or terminal units. If the end use subsystems are fan systems, they can be single or multiple zone type. With central systems, the primary conversion from fuel such as gas or electricity takes place in a central location, with some form of thermal energy distributed throughout the building or facility.

There are many variations of combined central supply and end use zone systems. The most frequently used combination is central hot and chilled water distributed to multiple fan systems. The fan systems use water-to-air heat exchangers called coils to provide hot and/or cold air for the controlled spaces. Another combination central supply and end use zone system is a central chiller and boiler for the conversion of primary energy, as well as a central fan system to delivery hot and/or cold air. The multiple end use zone systems are mixing boxes, usually called VAV boxes. The typical uses of central systems are in larger, multistoried buildings where access to outside air is more restricted. Typically central systems have lower operating costs.

Besides packaged unitary and central systems, there are a variety of special-purpose systems. These include the following:

1. Heat pump cycles on chillers that use rejected heat or tower cooling.

2. Thermal storage.

3. Cogeneration of electricity and heat.

Basic control. Basic control regulates the amount of heating or cooling necessary to meet the load in conditioned spaces. Minimum outside air needed for ventilation is provided whenever a space is occupied. When outside air temperature is a suitable source for free cooling, it's controlled as needed at values greater than the minimum.

The approach in packaged unitary equipment is to control the generation of heating or cooling by space thermostats. The approach in central systems is to control the delivery of heating and cooling by the end use zones to match the load in the space. The supply is controlled to match the load imposed by all the zones. A typical method of doing this is for room thermostats to control zones, and discharge controllers to control central supplies. Discharge temperature controllers control the rate of primary conversion (chillers or boilers), and pressure controls determine the delivery rate of the pumps or fans distributing the central supply. In many cases there are multiple boilers and/or chillers and pumps, which are put on or off line as necessary to provide proper capacity. Those online are modulated as necessary to meet load needs. The controls to put units online and off-line would normally be applied to meet the system needs.

Supervisory control. The role of supervisory control is to control the scheduling and interaction of all the subsystems to meet building needs. Supervisory control systems have many names; each used for a particular emphasis. Among the names their acronyms are the following:

1. BAS: Building automation system.

2. EMCS: Energy monitoring and control system.

3. FMS: Facility management system.

4. EMS: Energy management system.

5. BAS: Building automation system. (The most generic of these terms.)

DDC (direct digital control), is sometimes used to describe everything a computer or microprocessor-based control system does. The original use of the term providing closed-loop control of local loops by a digital computer or microprocessor.

We implement direct digital control in stand-alone panels in intelligent data-gathering panels that are the remote panels building automation system. Energy management programs originally in the central computer of a building automation system are now placed in remote data-gathering panels; or even in stand-alone DDC controllers. This has led people to use DDC to describe all microprocessor-based control systems' functions.

Energy management application programs are different than local loop control and are named for their specific function, such as start or demand control. The considerations of which energy management application programs should be used rely upon the type of building and HVAC system. For instance, optimum start-stop programs are not appropriate for a hospital that has 24-hour operation. Load reset of supply temperatures is appropriate for systems that supplying heating and cooling simultaneously, such as reheat systems or hot and cold deck mixing box systems.

Optimizing. The concept of optimizing control is not only to control space conditions, but also to do it in a manner that minimizes the energy and costs when different forms of energy are available. An optimizing strategy is generally to improve the efficiency of primary supply equipment or to reduce the losses of energy in end-use systems. The sizing of equipment is to meet maximum loads, but the equipment is usually run at less than maximum load. This means that the part load characteristics of the equipment determines the efficiency in meeting a given load.

When there are multiple chillers or boilers, an optimizing strategy would be to choose the most efficient equipment that has the capacity to meet the load at any given time. Also, with some types of end use systems, energy wasted by bucking heating against cooling can be minimized by resetting supply temperature levels to be no more than is necessary to meet a given load condition. Another way to optimize is to use the thermal storage of a building to make use of energy stored at low cost and used when needed. Moving heat from one area of a building to another can be an optimizing opportunity as well.

These optimizing principles are used for specific types of HVAC. The variable in all of these circumstances is the amount of heating or cooling load and the control action to make some change in the way a load is supplied. This process has led to the use of the terms load reset and dynamic load control to describe this general approach to optimizing control. The selection of the most efficient combination of chillers to supply a cooling load has been called optimized chiller selection.

History of HVAC controls. Before World War II, the main suppliers HVAC controls in commercial buildings were companies that promoted pneumatic controls. The predominant idea at that time was that controls for commercial buildings were too complicated to sell over the counter and had to be installed and supervised by the controls manufacturer. This concept included having branch offices with installers and service people.

Electric control systems for commercial buildings were modulating type controls. They were sold on a supervised basis. When several other companies entered the commercial controls market with electric and electronic controls, some of their distribution was through distributors and branches. Some of the newcomers, who started with electric and electronic controls, expanded into pneumatic controls either by their own development or by association with foreign companies.

When computer-based supervisory control systems came to market, some larger companies with computer-based products entered the HVAC controls market; but eventually gave up. As international business developed and companies became multinational, some foreign-based controls companies expanded into the U.S. markets directly or through associations with smaller U.S. control companies. During the 1970s, some small companies evolved with limited product lines for energy management functions. When DDC became accepted, some small companies developed microprocessor- based DDC controllers and supervisory systems.

The full line control companies that started out as major players currently remain as major players but with more competitors that have limited systems. Some major HVAC systems manufacturers have acquired or developed control capabilities. They market packaged HVAC systems with controls and supervisory control systems. Some companies provide products for specific applications. The selection of a source of supply should consider the life cycle needs and costs as well as the track record of suppliers.

Room thermostats. The mounting of room thermostats and room humidistats has been the subject of much discussion, and for many years the industry standard has been for the thermostat to be mounted near the door of a room 5 ft from the floor. The problem is that if the room is full of children, the thermostat is not controlling the temperature where the occupants are.

It's important to study the location of the room thermostat or humidistat as to the effect of conditions at the thermostat. Remember, the thermostat responds only to what is going on at its location. If there is a ceiling diffuser blowing air at the location where the thermostat is mounted, there is going to be cycling of the system.

Sometimes installers and others are concerned about the way thermostats and humidistats are mounted on the wall; that is, whether they should be mounted in a horizontal or a vertical position. Generally, aside from writing on the unit's face, either horizontal or vertical mounting is fine. There is, however, one important exception: when the thermostat is electric and has a mercury bulb switch contact. This type is common in residential and commercial buildings and requires the thermostat to be mounted a certain way. Some of these require the installer to use a level to mount it properly.

Room thermostats and humidistats are devices that control automatic valves and dampers in a control system. These devices have built in sensors as well as moving parts that control the device. An example is a pneumatic thermostat that has a bimetalic sensor and a relay. Usually the complete package is under one cover on the wall, and all action takes place at the thermostat. There are, however, sensors that are mounted under the cover in the room that have no actuators (relays) under the same cover. They usually transmit the temperature information to another device at a remote location that does the controlling with relays, and so on. Normally this principle is used in electronic control systems involving a wire wound resistor mounted under a cover that reads the temperature in the space and transmits that information to an electronic controller in an equipment room.

Often, there is confusion with the terms thermostat and sensor. The concept of a sensor under a cover in the room is new and came about because of the advent of electronic control systems. Room sensors are used with other control systems and are sometimes called transmitters. In the case of pneumatic controls, the transmitters use a sensor and a special relay that transmits a pneumatic air signal proportional to the medium being sensed. An example is a transmitter under a room thermostat cover that transmits an air signal based upon the temperature being sensed in the room.

The transmitter may look like a thermostat, but it does no controlling by itself: it depends upon a receiver controller in a different location to take the action on the controlled device. The dials of these devices are only used for calibration, and are not moved once they are. These transmitters come in standard ranges and send out an a signal based on the medium being sensed. An example is a room transmitter with a range of 30 DegrF to 80 DegrF that sends out a signal of 9 psi the temperature being sensed is 55 DegrF. In this case, the transmitter produces a signal of 8-15 psi as its sensed temperature goes from a low of 30 DegrF to 80 DegrF. Transmitters are not strictly sensors but are sometimes classified as such, since they are devices that do not do the controlling themselves.

Dampers. Automatic dampers have two classifications: The parallel blade, and the opposed blade. The parallel blade types were the first ones used.

The air that is being controlled can be considered an incompressible fluid at pressures below 12 in. of water. Above that, compressibility should be considered. Gases (air) can bend so that the volume will not be affected, and may not be controlled at all. Air can easily stratify in duct. Therefore, a damper can be considered a poor control device at best. At the same time, dampers can be as good at control as valves, provided they are sized properly.

Parallel versus opposed blade dampers. Parallel blade dampers tend to bend the air during the first few degrees of rotation as they go from full open to closed, and thus do little controlling in the first 20%-30% of movement. They bend the streams, rather than modulate them. But bending there are instances where this bending of air is useful, as when mixing air streams.

Opposed blade dampers are usually used where better control of the airstreams are desired and we want to prevent large amounts of stratification in the ductwork. Some dampers are not used for control or to maintain comfort are used for safety. These are the fire and smoke dampers.

Fire and smoke dampers. Fire dampers are put in the ductwork to stop the spread of fires, and to confine any fires to one area of a system. As such, they need to be made of rugged material that can withstand heat. They are seldom pivoted in the middle like an automatic damper, although they can be similar. They are almost always held in the open position by a linkage system that can be fused and is designed to melt and close when the temperature reaches about 165 DegrF. The closure is accomplished by springs or weights, and the dam must never be of the type that can be easily opened during a fire situation. In all cases, the dampers must meet the requirements of such organizations as the National Fire Protection Association (NFPA) and Underwriters Laboratories (UL). The locations of the dampers are clearly spelled out in most codes that apply to a particular type of system.

Smoke dampers, on the other hand, are not required by all codes. They are manufactured by vendors that supply automatic dampers, and in some cases are used as both control and smoke dampers. With these dampers, as with fire dampers, there are codes that apply, but they usually are not as stringent as those for fire dampers. Smoke dampers to stop the propagation of smoke and the resulting panic in event of a fire. Generally, they are involved with the central control and monitoring systems.

Manual Dampers. Manual dampers are an important part of the overall system, in particular the ability to balance the system and assist the control system to work better. They are used primarily to balance constant volume systems correct amount of air is distributed to the various places at the rate. They can be of the splitter type or the usual closure type. Splitter dampers try to split the air and redirect it to various sections ductwork. Balancing dampers try to apportion the air to the different sections of ductwork so the correct amount gets to the correct p all times.

Static control dampers. Static control dampers are different from the usual control dam in that they maintain the static pressure in the duct work base the action from a static pressure controller. Since static pressure is usually difficult to control in a system, static control dampers need to be the best available for their applications.

Damper motors. Damper motors are not motors in the true sense of the word, but the term has been used and has stuck with the industry. Damper motors are the devices (both pneumatic and electric) that operate the dampers in a system from the control signal of a device.

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