Principles of Automatic Heating and Cooling Controls Flashcards
How can we think of the idea of heat?
energy in transit from a high-temperature object to a lower temperature object
Define temperature
a measurement of an object’s thermal intensity
Cold is the _____ _____ of heat
relative lack
The quantity of heat cannot be _____, but must be _____
measured, calculated
What is the metric unit of measurement for energy?
joules
what is the metric unit of measurement for power?
watts
what is the imperial unit of measurement for energy?
btu
what is the imperial unit of measurement for power?
btuh
What does Btu stand for?
british thermal units
How many calories are in one btu?
250
How many joules are in one btu?
1055
What is a British thermal unit?
the quantity of heat required to raise the temperature of one pound of water, one degree Fahrenheit
What are three ways that heat is transfered from one object to another?
conduction, convection, radiation
Define conduction
the method by which heat is transfered through solids
Define convection
the method by which heat is transfered in fluids such as air or water
Define radiation
the method which heat is transfered by electromagnetic wave motion
Does heat always transfer from a high temperature object to a lower temperature object?
yes
What does human comfort depend on?
temperature, humidity, air movement and radiation
What is the average temperature that is comfortable for most people?
20 degrees celsius
What is the percentage of humidity in the air that is comfortable for most people?
40% at 20 degrees Celsius
Name the main reasons for using an automatic control system
comfort, safety, economy and convenience
Describe a basic heating cycle in a room step by step
1) thermostat closes
2) gas valve opens
3) burner ignites
4) plenum warms
5) fan switch closes
6) blower starts
Describe a basic cooling cycle in a room step by step
1) thermostat opens
2) gas valve closes
3) burner stops
5) plenum cools
6) fan switch opens
7) blower stops
What are the basic components of a forced-air heating system?
Main burners, fan or blow, load devices, controlling devices, heat exchanger, flue or stack
What is the purpose of the main burner?
burn the fuel to heat the inside surface of the heat exchanger
What is the purpose of the fan or blower?
to draw clean air from the house return air vents and blow this air over the outside surfaces of the heat exchangers
What is the purpose of load devices such as gas valves, relays, motors, igniters, etc?
They respond according to the feedback they receive from the controlling devices
What do controlling devices do?
They are sensing devices that provide feedback to a load device and tell it what to do
What are some examples of controlling devices?
thermostats, humidistats, limit switches, pressure switches, flame rods, sail switches, etc
What is the purpose of the heat exchanger?
Heat is conducted through the heat exchanger walls to warm the clean air circulated by the blower
What is the purpose of the flue or stack?
it is a pipe that carries gasses from the burning fuel out of the heat exchanger to the chimney
Is there feedback in a closed loop system?
yes
What is a device that reacts to changes in a variable condition?
a controlling device
What does a limit or safety device do?
detects dangerous conditions in the system
Is the control circuit for a residential heating system an example of a closed loop or open loop control system?
closed
In a domestic gas furnace, the fan motor starts when:
the air in the plenum warms up to a pre-set temprature
Can the fan motor keep running after the main burner shuts off?
yes
What do schematic diagrams show?
the logic operation of components
If you wanted to find the relative position of components in a heating system and the wiring between them, what kind of diagram would you look at?
wiring diagram
A schematic diagram is drawn with power _____ and ____ heat
off, no
What kind of device is a relay coil?
load device
A pressure switch is an example of what kind of device?
controlling
