HVAC Control Questions

Control Questions - 1

• When checking a room thermostat for accuracy with a thermometer, "make" (close) the contacts at no more than 2° above the temperature indicated by the thermometer.
• Place a reliable thermometer as close to the sensing element as possible and allow it to remain there for at least 10 minutes to check for a faulty thermostat.
• A relay is a positional type and must be properly mounted for satisfactory operation.
• The voltage used in the control circuit of transformers is 24 volts because it is safer.
• The fan controls are used on heating systems to start and stop the fan motor on an increase or decrease in temperature inside the furnace and are manufactured in adjustable and non-adjustable types.
• The relay used to de-energize the condenser fan motor and energize the hot gas solenoid for the defrost and slab release cycle, energizes the condenser fan motor and de-energizes the hot gas solenoid at the end of the slab release cycle, which is the defrost control relay.
• NEC requires loss of airflow protection in electric heaters. Each heater must be electrically interlocked with the fan so the heater cannot operate unless the fan circuit is energized and air is therefore flowing.
• The thermostat should be replaced if a room thermostat must be calibrated more than 10 degrees.
• Amperage relays are electromagnetic types.
• An amperage relay too large for a motor may not allow the relay contacts to close.
• Low-pressure controls will stop the compressor motor to protect it from overheating and prevent the compressor from pumping oil out of the crankcase. They are also used as a temperature control on smaller units.
• When checking a low-pressure control, with the compressor running, front-seat the suction service valve and observe the pressure on the compound gauge when the pressure control stops the compressor.
• The best way to check a capacitor is by the use of a capacitor analyzer.
• Solid-state starting relays will start virtually all split-phase 115-volt hermetic compressors up to ⅓ horsepower.
• If the evaporator temperature reaches approximately 50°F (10°C), a temperature-sensing device that terminates the defrost period is a defrost limiter.
• Devices used to detect ice/frost on the outdoor coil during the heating cycle are heat pump defrost controls.
• The purpose of the primary control is to control the operation of the oil burner and the ignition procedure.
• A sufficient amount of frost, or ice, will not be removed from the coil to allow proper refrigeration to occur, if the defrost termination point is too low.
• Potential relays are starting relays.
• A hard start kit is designed for use when conditions are encountered that prevent the PSC motor from starting normally.