Control devices and control circuits are used to start and stop load devices such as compressors and motors in HVAC systems. Service technicians must understand how these devices and circuits function so that they can successfully troubleshoot them when problems arise. The room thermostat is a temperature-actuated switch and is the primary control used to turn HVAC equipment on and off. In its simplest form, a thermostat consists of a bimetal sensor element that warps when heated. Attached to the bimetal are switch contacts that make or break an electrical circuit. The bimetal is often coiled, and the switch contacts may be encased in a glass bulb containing mercury. Thermostats are typically used in low-voltage (24V) systems because the components are less expensive, and there is less risk of electrical shock or fire. Heating-only thermostats will call for heat when the temperature drops low enough to cause the contacts to close. As the room warms, the contacts open, and the furnace ceases operation. To prevent temperature overshoot, an adjustable resistor called a heat anticipator applies heat to the bimetal during furnace operation. This causes the thermostat to open before the temperature reaches the setpoint. Cooling-only thermostats operate opposite to heating thermostats and close when room temperature rises. As the room cools, the contacts open, stopping the air conditioner. A cooling compensator is a fixed resistor in parallel with the thermostat contacts. During the compressor off-cycle, it adds a small amount of heat that causes the contacts to close slightly sooner, preventing the room from getting too warm. Combination heat-cool thermostats combine the functions of heating and cooling thermostats in one device. Manual switches select the desired mode of operation. The heating and cooling switch contacts are located in a common mercury bulb. In one bulb position, heating contacts are made. In the other position, cooling contacts are made. When manual switching between modes is inconvenient, an automatic changeover thermostat can be used. The thermostat will sense which mode is needed and automatically turn on the required equipment. All room thermostats have a built-in cut-in and cut-out temperature difference, called the differential. It is normally about 2°F above the setpoint. For example, a thermostat set for 70°F shuts off at 72°F. To prevent simultaneous operation of heating and cooling, automatic changeover thermostats have a minimum interlock setting, or deadband, of at least 3°F. Some HVAC equipment can provide heat or cooling in stages. To allow this, two-stage or multi-stage room thermostats are available. Heat pumps often use multi-stage thermostats. Typically the compressor provides the first stage of heat. If it cannot satisfy the demand, another thermostat bulb energizes an electric heater.
A programmable thermostat saves energy by automatically raising or lowering the thermostat settings to conserve energy. It contains a timer and other standard thermostat features. Microprocessor-based electronic programmable thermostats contain several features that enhance comfort and provide excellent flexibility. Line voltage thermostats directly control higher voltage (up to 240V) appliances such as baseboard heaters. They can be controlled by a bimetal sensor or by a hydraulic sensing bulb. Line voltage thermostats often suffer from a mechanical condition called droop that causes the thermostat control point to drift away from the selected setpoint. A thermostat should be located in the space it is to control and be mounted on a wall plate or sub-base about five feet above the floor in an area that gets good air circulation. Thermostats, especially mercury-bulb types, should be mounted plumb and level on the wall. Thermostats, especially electronic thermostats, require special care and handling to avoid damage. Room thermostat wiring and thermostat terminal designations for most equipment manufacturer’s products follow a standardized code. For example, R generally denotes 24V power. All thermostat wiring should conform to national and local codes. The thermostat heat anticipator is an adjustable device found on the thermostat. It is set to match the current draw of the furnace control circuit. The current value can be found in the furnace installation literature or on the gas valve. If the heat anticipator current value is not known, the current draw of the control circuit can be measured by wrapping 10 turns of wire around the jaws of a clamp-on ammeter. The other ends of the leads are attached to R and W of the furnace control circuit. The measured current is divided by ten to obtain the actual current. The heat anticipator is adjusted to the current value measured. If the anticipator is incorrectly set, too much or not enough heat will be generated by the anticipator, resulting in a burner-on cycle that is too long or too short. Instead of a traditional heat anticipator, electronic thermostats have a cycle rate adjustment that does the same thing as the heat anticipator. Once installed, all functions of the thermostat and the heating and cooling equipment it controls, must be checked out. Be aware that some cooling equipment may have a compressor start-delay or short-cycle timer that must time out before the compressor will start. HVAC systems operate automatically once the desired conditions are established. Controls and control systems perform the automatic functions.
Relays, contactors, and motor starters are widely used as control devices in HVAC equipment. Relays stop or enable the flow of current, or can reroute the flow of current in a circuit. They can be hard-wired in a circuit or plugged into a relay socket. A relay consists of an electromagnetic coil that moves switch contacts when the coil is energized. Contacts can be normally open or normally closed. On schematic diagrams, relay contacts are shown with the relay de-energized. Relay contacts are shown on schematic diagrams with the relay coil de-energized. The positions of the contacts would change when the relay is energized. The schematic symbol for a single-pole, single-throw relay shows the coil and a single set of contacts, either normally open or normally closed. An electro-mechanical depiction of the same relay shows how the coil controls the contacts. The coil of the relay is typically energized by 24 volts, but the relay contacts often carry higher voltages up to 240 volts. A single-pole, double-throw relay contains one closed and one open set of contacts. When the relay is energized the closed contacts open and the open contacts close. A double-pole, double-throw relay contains two pairs of contacts with each pair containing a normally open and normally closed set of contacts. Electronic or solid-state relays contain no moving parts. Instead, an electronic switching device like the TRIAC is used to control current flow through the relay. Electronic relays are more reliable than conventional relays because they have no moving parts. A contactor is basically a heavy-duty relay and is used to control large motors and compressors. Contactors do not provide overload protection. A motor starter is used to start and stop motors and to provide overload protection to the motor.Greater efficiency can be achieved by varying the speed of electric motors. A variable resistor called a potentiometer can be used in combination with a TRIAC to control motor speed. In another variation, a temperature-controlled bellows adjusts a potentiometer that controls a motor that opens and closes a damper. Microprocessor-based controls are being used with variable-speed blower motors on furnaces and fan coils for better management of airflow. A lockout control circuit prevents the automatic restart of a motor or compressor if unusual or damaging conditions occur. A high-impedance relay is placed in series with protective devices. It energizes and locks the system out if a protective device opens.
Time delay relays delay the energizing of a load such as a motor or compressor for a set length of time. They are commonly used in multiple-compressor equipment to prevent a large current surge if all compressors are started at once. An electro-mechanical or electronic compressor short-cycle timer is used to allow time for system pressures to equalize before attempting to restart the compressor. High-pressure and low-pressure cutout switches (HPCO and LPCO) shut the system down if damaging high or low refrigerant pressures are present. A variety of problems can cause refrigerant system pressures to be abnormally high or low. Specialty thermostats called freezestats are used to prevent evaporator coil icing by opening the compressor circuit when temperatures drop to near freezing. Outdoor thermostats are designed to control equipment operation based on outdoor temperature. Furnace controls include a bimetal fan switch or fan control that turns the indoor fan on only after the heat exchangers have warmed up. The limit switch is a safety device that stops burner operation if the plenum temperature reaches approximately 200°F. A thermocouple generates a small voltage when heated by the pilot. It is used to energize a safety circuit in the gas valve and will prevent the gas valve from operating if the pilot goes out. On furnaces that use an inducer motor, an inducer pressure switch proves inducer operation by sensing the pressure created by the inducer fan. The sequence of operation of a system must be known before the control circuit can be designed. A very simple control circuit for an air conditioner consists of an on-off switch and a thermostat. With power on, the thermostat opens and closes, turning the compressor and fan motor on and off. A more complex control circuit would include a contactor to turn on the compressor and outdoor fan and a fan relay to control the indoor fan. The most complex control circuits contain a number of different controls performing various functions. For example, a combination heat-cool unit contains controls for heating and for cooling. In many combination heat-cool units, the compressor and outdoor fan operate at the same time whenever the contactor is energized. Troubleshooting is an organized process that enables an HVAC service technician to quickly isolate a problem so it can be repaired. An important first step in the troubleshooting process is to interview the customer to gain valuable clues that might be useful in isolating the problem.
A thorough inspection of the system can reveal clues to the problem. Examples include dirty filters, oil stains, or strange noises. A basic system analysis consists of seeing if the equipment appears to be operating as it is supposed to, checking any logs or records, and taking various temperature and pressure measurements. The equipment manufacturer may have troubleshooting aids available such as label diagrams, troubleshooting tables, and fault isolation diagrams. Label diagrams are often attached inside the equipment. They normally show a component arrangement diagram, wiring diagram, legend, and notes pertaining to the equipment. The component arrangement shows the physical location of components in the cabinet. Simplified schematics called ladder diagrams may also be found on the label diagram. Some manufacturers supply troubleshooting tables or fault isolation diagrams with their equipment. The electronic controls used in newer equipment often contain self-diagnostics in the form of coded flashing lights. Some manufacturers provide dedicated test modules that plug into the equipment. More difficult equipment problems require a step-by-step process to isolate the specific cause. Safe work practices will greatly reduce or eliminate your chances of receiving an electrical shock. Additional procedures for locking out and tagging equipment are outlined by OSHA. The purpose is to insure that power is off, tagged, and locked so that power cannot be turned on while equipment is being worked on. As a last step, an attempt should be made to turn the equipment on to confirm that the equipment is, in fact, locked out and tagged. After the equipment is repaired, move all personnel to a safe distance, remove tools and reinstall any guards. Remove lockout/tagout devices and restore power to the equipment. When troubleshooting electrical equipment, it is often necessary to look for the underlying cause of the problem. For example, a compressor cycling on and off might appear to be a refrigeration system problem when in fact, it is being caused by low voltage. Electrical troubleshooting can be simplified if the equipment can be divided into input power distribution circuits, load circuits and control circuits. Input power distribution and load circuits are considered high voltage circuits. Control circuits control the operation of the loads and generally operate at 24 volts. For that reason, control circuits are often considered low-voltage circuits. The process of elimination can be used to isolate a problem. For example, if a heat pump functions normally in heating but not in cooling, components such as the compressor and outdoor fan and their related circuits can be eliminated since they are common to heating and cooling. With that knowledge, the focus can be on the components used only in cooling.
Isolating the problem to the component can be accomplished using a voltmeter. One lead is placed on the common side of the circuit and the other lead is used to probe for voltage. HVAC equipment is designed to operate within a range of ± 10% of the specified voltage. Operating the equipment outside the voltage range can damage the equipment. High or low voltage can overheat motors and cause eventual failure. The supply voltage should be measured where it enters the equipment. Voltage measured should be within the ± 10% range of the nameplate voltage. For example, measurements between 207V and 253V would be acceptable for a 230V motor. A voltage imbalance between the phases of three-phase equipment can cause a current imbalance that causes overheating or motor failure. The voltage imbalance between any two legs of a three-phase system should not exceed 2%. To calculate voltage imbalance, first measure and record the voltage across all three legs. Then, use a formula to calculate the imbalance. The current imbalance in any one leg of a three-phase system must not exceed 10%. To calculate current imbalance, measure current in all three legs and use a formula to calculate the imbalance. Fuses can be checked with an ohmmeter. Remove the fuse with an insulated fuse puller. If continuity is observed, the fuse is good. No continuity indicates an open (blown) fuse. With the unit disconnected, fuses can be checked with the power on. Place voltmeter leads on the line side and then the load side (unit side) of the fuses. Voltage measured on the load side indicates good fuses. A circuit breaker can be checked with the breaker on by placing voltmeter leads across the input and output terminals. The measured voltage should be the same on each side. If the output voltage is lower than the input voltage, check for loose wiring or overheated terminals. Any breaker that appears to have overheated should be replaced. A clamp-on ammeter placed around a wire connected to the circuit breaker can check the current flow through it. If the breaker trips at a current level above or below the current rating of the breaker, or does not trip at all, it should be replaced. Resistive loads in HVAC are usually heaters. Inductive loads include motors and the coils of relays, solenoids, and contactors. Both types can be checked with an ohmmeter. Before measuring the resistance of any load device, it must be removed or isolated from the rest of the circuit. Any measurable resistance indicates the device is probably good. Zero resistance indicates a shorted load, while infinite resistance indicates an open circuit. Relay or switch contacts in a de-energized circuit can be checked with an ohmmeter. A measurement of zero ohms indicates closed contacts. Infinite resistance indicates open contacts.
Transformers should be checked to see that the primary and secondary voltages are within the ± 10% voltage range. If voltage is measured at the primary windings but not at the secondary windings, the transformer may be bad or the fuse or circuit breaker in the transformer secondary may be open. Open windings can be confirmed with an ohmmeter. Electromechanical room thermostat switch functions can be checked by bypassing the switches with an insulated jumper wire. The fan switch can be checked by placing a jumper across R and G. If the fan operates with the jumper in place, it indicates a faulty fan switch. Cooling operation can be checked by placing a jumper across R and Y. If the condensing unit operates with the jumper in place, it indicates a problem with the thermostat cooling circuit. Furnace operation can be checked by placing a jumper across R and W. If the furnace operates with the jumper in place, it indicates a problem with the thermostat heating circuit. Permanent split capacitor (PSC) and capacitor-start-capacitor-run (CSR) motors are widely used in HVAC to power single-phase compressors and indoor and outdoor fan motors. Both the PSC and CSR motors require capacitors to aid in starting and running. The run capacitor is always in the circuit when the motors are running. The start capacitor provides a temporary boost to get the motor turning and is then removed from the circuit while the motor continues to run. Multi-speed PSC motors are used in furnaces and fan coil blower assemblies to allow for airflow adjustments. Speed changes are accomplished by selecting the appropriate speed tap on the motor. Three-phase motors are generally used in industrial and commercial applications where high starting torque is required. Three-phase motors do not require start or run capacitors. Capacitors can hold an electrical charge even after the power is off. The terminals of hermetic compressors are under pressure and can blow out under certain circumstances. A capacitor analyzer can be used to measure the capacitance value in microfarads. The measured value should be no greater than ± 20% of the stated value for start capacitors and ± 10% for run capacitors. Start relays remove the start capacitor from the motor circuit by using the voltage developed across the start winding of the motor to energize the relay. Start relays usually fail with their contacts closed, allowing the start capacitor to stay in the circuit. This can damage the start capacitor and the motor. To test a start relay, apply a clamp-on ammeter around the wire that connects the start relay to the start capacitor. At start-up, there should be a momentary surge of current that drops to zero, indicating normal operation.
A steady current reading on the clamp-on ammeter indicates the start relay contacts are stuck closed. No deflection and no current reading indicates the relay contacts are stuck open. The start thermistor is a compressor start-assist device. It is installed in the compressor circuit the same as a start capacitor and start relay. It initially offers a low resistance to current to assist motor starting. Current through the thermistor causes its resistance to rapidly increase to the point that current no longer flows in the start circuit. Thermistors should be removed from the circuit for checking and should be tested cold. Cold resistance should be 100% to 180% of the stated value. An ohmmeter can be used to identify the unmarked terminals of a single-phase compressor motor. Measure resistance across the terminals until the two terminals with the highest resistance are found. The remaining terminal is the common terminal. Measure resistance across the remaining terminals and common. The terminal with the highest resistance is the start winding terminal. The remaining terminal is the run winding. Motors should be checked for open, shorted, or grounded windings with the power off and the motor disconnected from the circuit.Open or shorted windings can be found with an ohmmeter set on the lowest scale. Apply the meter leads across the motor leads. Any measurable resistance probably indicates a good motor winding. Infinite resistance indicates an open winding, while zero ohms indicates a shorted winding. Check for grounded windings with the ohmmeter on the R × 10,000 range. Attach one lead to the metal frame of the motor (ground) and touch each of the motor leads with the other meter lead. An infinite or very high resistance reading of not less than 1000 ohms per volt is normal. For example, a 230-volt motor should have a resistance to ground of at least 230,000 ohms to be acceptable. The minimum resistance to ground is determined by multiplying the motor voltage by 1000. The higher the voltage, the higher the acceptable resistance to ground becomes. A simple hydronic or hot water heating system consists of a boiler, circulating pump, aquastat, expansion tank, radiators, and a relief valve. The main function of the aquastat is to prevent the boiler from overheating. Some aquastats contain a feature that prevents the water temperature in the boiler from getting too low. Oil-fired boilers use a special aquastat that is combined with the oil burner primary control. For enhanced energy savings, boiler water temperature can be controlled based on the outdoor temperature through the use of a reset controller. Potentially explosive steam pressure can build up in a boiler if the water level gets too low. A low-water cutoff prevents burner operation if this happens. The circulator pump in a hydronic system circulates warmed water through the piping and radiation.
Zone valves allow different temperatures to be maintained in different areas by directing heated water to the different zones. Pneumatic controls use compressed air to operate the valves, motors, and relays used in industrial and commercial HVAC systems. A pneumatic control system consists of an air source, air lines, controlling devices such as thermostats, branch circuits and actuators such as valves and motors. Pneumatic thermostats respond to temperature change by changing air pressure in the branch line. A direct-acting thermostat increases air pressure when temperature increases. A reverse-acting thermostat decreases air pressure when temperature increases. Bleed-type thermostats use a bimetal control to help control pressure. Non-bleed thermostats use air only when branch line pressure is being increased. Pneumatic damper motors contain a bellows that expands or contracts in response to air pressure and are commonly used to control dampers. A normally open damper is installed so that it moves toward the open position as pressure in the damper motor decreases. A normally closed damper is installed so that it moves toward the closed position as pressure in the damper motor decreases. Pneumatic-electric and electric-pneumatic relays provide a crossover between air devices and the electrical system. Pneumatic-electric relays are simply pressure switches in which a pneumatic signal causes an electrical change. Electric-pneumatic relays are three-way solenoid valves used as interlocking devices where a circuit is enabled under certain conditions that are signaled electrically. A pneumatic control system can control both the room temperature and the temperature of the air as it enters the room. Pneumatic systems coupled with electronic controls allow for the modulation of temperature, humidity and air volume. A sail switch installed in a duct verifies that air is actually flowing. Air pressure on the sail causes it to move, making or breaking the switch contacts. Microprocessor controls and advances in digital technology allow complex building management systems to be controlled for optimum energy savings and security. Direct digital control (DDC) allows a central computer to manage and control HVAC systems to a degree that is almost impossible to achieve with conventional pneumatic or electro-mechanical control systems. Digital and analog signals are developed by various control devices. Switches and relays provide digital signals, while analog signals are provide by devices such as thermistors and motor controls.
Analog signals change in small increments and cannot be directly processed by digital devices. An analog to digital converter changes the analog signals to digital form so they can be used. A digital control system can control a complex constant volume system consisting of several control loops, economizer control, heat-cool sequencing, and humidification-dehumidification sequencing.