Heating And Cooling Systems Flashcards
Define HEAT
A form of energy arising from the random motion and collision of molecules.
Define COLD
The relative lack of heat.
Heating systems are rated according to the:
Rate of energy input.(aka power input)
The ratings of cooling systems are based on the:
Output rather than the input.
What are the 3 ways heat energy is transferred?
Conduction, convection, radiation.
Define CONDUCTION
Conduction is the method by which heat is transferred through solids.
Heating one portion of a solid increases the activity of the molecules in that portion
Define CONVECTION
Convection is the method by which heat is transferred through fluids such as air or water.
Based on the principle that fluids expand when heated and rise due to reduced density.
Define RADIATION
Radiation is heat transfer by electromagnetic wave motion.
This is the method by which we receive heat from the sun.
Human comfort is dependent on which four variables?
Humidity
Air movement
Radiation
Temperature
Temperatures of approximately ___C are considered comfortable for most people.
20C
Relative humidity of about __% at 20C is considered comfortable for most people.
40%
If any of the four variables affecting comfort is not at optimum level, this leads to the condition known as:
COLD20
True or false :
Automatic controls must be able to shut down all or a part of a system in the event of a dangerous or hazard situation.
TRUE
What are the main reasons for using an automatic control system?
Comfort
Safety
Economy
Convenience
What is the definition of 1 BTU?
One BTU( British thermal unit) is the imperial unit for energy that was originally defined as the approximate quantity of heat required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.
What are the most used type of temperature sensing devices?
Bimetal temperature sensing elements.
How do strip type bimetal temperature sensing devices work?
Two types of metal expand and contract at different rates as they warm up or cool down which causes the strip to deflect.
TRUE OR FALSE
IRON expands at a greater rate than COPPER
FALSE. Copper expands at a greater rate.
What are some characteristics of a disc type temperature sensing device?
2-3cm in diameter.
Low in cost
Suitable for controlling larger ampacity loads.
Describe bellows type temperature sensing devices
Use a small bellow shaped like a little accordion.
The fluid inside can be either liquid or Vapor.
Most used fluid is ETHER.
Good for line voltage thermostats for unit heaters or electric heating.
Describe a liquid bulb type temperature sensing device.
A rigid metal bulb attached to one end of a thin metal tube and a flexible diaphragm attached to the other end
Especially suitable for monitoring the temperature of water and air in a pipe or tank.
Copper is used for the bulb.
What are thermocouples?
Thermocouples are temperature sensors that generate a small voltage in proportion to temperature.
Dissimilar metals are connected in series.
When heated a small voltage is generated (typically 30mv dc)
What are thermopiles?
Can be temperature sensing devices as well as power sources for some heating systems.
Has many thermocouples connected in series so that the generated voltage can reach as high as 800mV DC.
Can operate some light electric loads such as a solenoid type gas valve.
What is a thermistor?
A thermistor is a solid state device in which the resistance of the semiconductor material changes in response to changes in temperature.
Have a negative temperature coefficient.
What is a resistance temperature detector?
An RTD has the same physical appearance as a thermistor.
Uses metals such as copper, nickel and platinum.
Can sense temperature with greater accuracy than thermistors.
Why is platinum the most common metal used in RTDs?
Because it is a chemically stable metal that resists corrosion and oxidation in hostile environments.
What is a pyrometer?
A pyrometer measures radiation by using a focusing lens that directs and concentrates the radiation on a sensitive thermocouple.
Can measure temperatures as high as 4000C
Can measure objects at a distance
The common application of a thermostat is in room air temperature sensing and system operation control by:
Comparing the sensed temperature with the set point.
What are the 3 types of thermostat contacts?
Mercury contacts
Spring activated, snap action exposed contacts
Glass enclosed magnet and armature snap acting contacts.
What are the most popular types of contacts used in thermostats?
Mercury contacts
Are mercury contacts suitable for high current rating?
NO
Mercury contacts are not suitable for high current rating.
Are spring activated, snap action exposed contacts suitable for high current switching?
Yes spring activated, snap action exposed contacts are suitable for high current switching
They require more maintenance.
What limits the size of the contacts in a glass enclosed magnet and armature snap acting contacts?
The strength of the magnet.
The tube of a glass enclosed magnet and armature snap acting contacts is filled with _______ gas to reduce ______ _____
The tube of a glass enclosed magnet magnet and armature snap acting contacts is filled with inert gas to reduce contact arcing.
What are the two main differences between heating and cooling thermostat?
For a heating application, the thermostat contacts must CLOSE when the room temperature decreases
For a cooling application, the thermostat contacts must OPEN when the room temperature decreases.
A heat/cool thermostat usually includes a _______ switch for either heating or cooling operation and prevents the operation of the heating system and cooling system at the same time.
A selector switch.
What types of thermostats are usually used in heat/cool thermostats operations?
Usually bimetal spiral type with mercury type contacts.
How do Anticipators work?
Anticipatory are a heating element in a thermostat that generates a small amount of heat to help the thermostat anticipate the delay.
An anticipator is connected in ____________ with the thermostat contacts.
Connected in parallel.
What is the only way to reduce the heat generated by the anticipator?
By reducing the resistance of the anticipator.
How do two stage thermostats work?
Typically uses two burners controlled by a thermostat that has two sets of contacts, each with a different set point.
To operate a two stage heating system properly you need a ______ wire thermostat.
Three wire thermostat.
True or false:
Thermostats that use mercury switches must be absolutely level to work properly
TRUE
What are the common voltage ratings of thermostats?
120v, 24v, and mV.
The label for heating in heat/cool thermostat wiring is:
W
The label for cooling in heat/cool thermostat wiring is
Y
The label for a fan(if used) in heat/cool thermostat wiring
G
The label for source in heat/cool thermostat wiring is:
R, RH, Rc
What are the most common types of electrical heating system?
Resistive heating
(Electric baseboard heating, in floor radiant heat and portable electric heaters)
Where are forced air systems commonly used?
In single family homes.
How does a forced air system work?
A furnace heats the air and forces the warmed air through the space.
How do resistive heating systems work?
Utilizes passive air currents to transfer heat.
Where is hydronic heating commonly used?
To warm larger buildings such as multi family buildings and commercial towers.
How does hydronic heating work?
It utilizes heated water or steam to move heat through pipes from a boiler to the space to be heated.
How does a heat pump work?
Utilizes the process of changing pressure and state of a refrigerant to transfer heat from one area to another.
What is the most common version of cooling?
A simple fan
How does an air conditioner work?
Exact same premise as the heat pump
Change the pressure and state of a refrigerant so heat energy can be moved from one space to another.
What is the main disadvantage of using a thermocouple flame proving circuit?
It’s relatively slow response time.
When testing existing thermocouples or installing a new thermocouple, you should carry out what 5 tests?
An open circuit test
A closed circuit test
A response time test
A dropout point test
A turn down test.
What should the voltage that a thermocouple produces be?
26-32 mV DC
What should the voltage of a closed circuit test of a thermocouple be?
25-50% of the open circuit millivolt output.
When should you use the open circuit test?
To determine the output from the thermocouple when it is not connected to the power unit.
When should you use the closed circuit test?
To determine the output from the thermocouple when the power unit is connected.
When should you use the response time test?
To prove that the thermocouple will respond to a flame failure in the required time limit.
When should you use the dropout point test?
To determine the closed-circuit millivolt reading at the instant the armature of the safety pilot device is released by the electromagnet.
Can be performed at the same time as the response time test.
The turn down test ensures that the pilot flame will satisfactorily ignite the main burner when the pilot is reduced to minimum size.
This test is performed by a _______
GASFITTER (in accordance with the provincial gasfitter’s certification regulation )
What is a solid state temperature sensing device that has a negative temperature coefficient
THERMISTOR
