Unit 39
Match the letter with the correct standard heat-cool system function.
24 V power: R Cooling: Y Heating: W Fan: G
The most common residential control voltage is:
24 V.
A system that monitors the return air temperature to control room temperature is:
A closed-loop control system.
At its most basic, a control requires:
A sensor, an actuator, and a linkage.
Pneumatic controls operate by:
Air pressure.
A system that monitors the outdoor temperature to control some aspect of system operation is:
An open-loop control system.
A switch controls flow of electricity by:
Breaking the circuit when it is turned off.
A modulating control:
Can position a damper or valve to positions other than full open or closed.
In most 24V AC low-voltage systems the red wire is hot, the yellow wire is the cooling circuit, and the green wire controls the:
Fan circuit.
A closed-loop system:
Monitors conditions that it can control.
An open-loop system:
Monitors conditions that it cannot control.
A control malfunction that results from a system heating or cooling past its setpoint is called:
Overshoot
Types of control systems include:
Pneumatic, electromechanical, electronic, and DDC.
Direct digital controls (DDC) operate by:
Processing electronic signals with a microprocessor.
The newest HVACR control systems operate by:
Processing electronic signals with a microprocessor.
What are proportional controls?
Proportional controls increase or decrease the output of a controlled device in proportion to the level of the measured condition.
What type of control system has the ability to measure the controlled variable and the system output and dynamically adjust the setpoint to achieve more precise control?
Proportional integral derivative (PID) controls
Compared to electro-mechanical controls, pneumatic controls can more easily:
Provide proportional or modulating control.
Electronic control systems operate by:
Providing a varying DC voltage in response to monitored conditions.
The purpose of controls in an HVACR system is to:
Regulate HVACR equipment in response to monitored conditions.
Match the letter designations with the heat pump system function.
Reversing Valve: O Compressor: Y Supplemental heat: W Fan: G
A heat pump condenser contains this unique control device, which switches the flow of refrigerant depending on heating or cooling mode.
Reversing valve
A heat pump defrost control must do three things: Turn on auxiliary heat, turn off the condenser fan motor, and:
Shift the reversing valve from heat to cool.
Pneumatic controls are most commonly used:
To control dampers and valves in large commercial systems.
Electro-mechanical control systems operate by:
Turning relay coils on or off to complete circuits.
What are two-position controls?
Two-position controls simply stop and start devices to maintain the desired condition.
When is the O terminal energized on most low-voltage thermostats?
Whenever the system switch is moved to "Cool"
When is the B terminal energized on most low-voltage thermostats?
Whenever the system switch is moved to "Heat"
How should a thermostat be connected to a system that has two transformers: one in the furnace and one in the air conditioner?
Wire the furnace to RH and the air conditioner to RC.
A difference between a straight heat-cool thermostat and a heat pump thermostat is that:
Y is energized in both heating and cooling.
