[D2L] Final Exam Flashcards
What does a pressure switch do in a furnace?
1) opens if refrigerant pressure gets too low turning off the compressor
2) closes based on a negative pressure energizing the gas valve
3) Centrifugal switch closes at 10% of motor speed
4) closes when the blower runs proving air flow across the heat exchanger
closes based on a negative pressure energizing the gas valve
What is the purpose of the inducer in a furnace?
cause fresh air to be brought in for combustion and exhaust the flue gas
What is the switching action of a fan cycling controller on a condenser?
Close on rise of pressure, Direct acting
Where is an averaging element often used?
Across a heating coil in a duct
What is an example of an analog output signal?
a pneumatic tstat
What is a bimetal element?
1) two metals with different coefficients of ductility bonded together
2) two metals with different coefficients of malleability bonded together
3) two metals with different coefficients of thermal expansion bonded together
two metals with different coefficients of thermal expansion bonded together
What state is a high limit controller typically in?
normally closed
What is a low limit controller used for?
interrupt the system operation if the condition drops below the desired value
What is an Oil Failure Pressure control (OFC)?
A differential pressure control
What is the Differential?
The difference in the values of a controllers on and off set points
What is an example of an open loop?
a kitchen faucet
What is a flow switch used to control?
A) Water
B) Air
C) Fluid flow
D) All of the above
All of the above
In a MUA, the modulating gas valve delivers gas to the burner, increasing the discharge air temp. The “flow of natural gas” is also known as the…
Manipulated Variable
In a MUA, the modulating gas valve delivers gas to the burner, increasing the discharge air temp. The “natural gas” is also known as the…
Control Agent
What type of control system are relays and contactors considered?
electromechanical
A compressor rated for 20 Amps will require a relay to control it
T or F
False
On a call for cooling in a furnace, R to Y closes in the thermostat which also sends power to G
T or F
True
The Relays we wired up in class were single pole, single throw
False, single pole-double throw
What part of a contactor is not replaceable?
body
What type of control is used to delay or pause in the activation and deactivation of a circuit?
time delay
What type is a timer?
A) electromechanical
B) electronic
C) thermostat
D) both a and b
both a & b
electromechanical
electronic
What device is used to protect end use devices only?
Overload device
What is the most important factor with regards to piping design in low temperature applications?
compressor oil return
A crankcase heater helps reduce refrigerant migration and oil frothing on start up
True
How is compressor short cycling reduced?
Delay on break timer
What does DTC stand for?
defrost time clock
A timer motor in a time clock is powered at all times
True
What pressure should a pump out cycle shut the compressor off?
Just above 0 psig usually set at 2-5 psig
Refrigerant boiling out of the oil in a compressor crankcase can cause a system to short cycle
True
Why are electrical thermostats preferred over electromechanical?
1) more reliable
2) auto-changeover functions
3) more accurate
4) all of the above
all of the above
What is a high limit control used for in a residential furnace?
shut off gas valve if heat exchanger becomes to hot
If the differential is set too small on a fan cut in cut out switch, what will happen to the device its controlling?
short cycling
An old residential furnace fan limit controller has set points of 120F, 140F, and 180F. If the helix bi-metal heats up to 210F what will be energized?
The blower
What is a manual reset on a high pressure switch?
If the unit trips on high pressure it will need to be physically reset
What temperature (SDT) should a high pressure switch be set to?
135F
Cut–out on a low pressure control is set by?
cut-in minus differential
What does an OFC protect?
Compressor running without oil
What is a heat exchanger an example of?
process plant
What is the actuator in a closed loop furnace system?
gas valve solenoid
If the thermostat is calling for cooling and you read 24v between R and Y which component is running?
Nothing, should read 0v
If you read 0V between R and Y which components should be running?
1) Blower
2) Nothing
3) Cooling
4) Heating
5) Both a and c
Both A & C
Blower
Cooling
Furnace will be heating if 24v is measured across R and W
T or F
False, it should read 0v
What kind of control(s) do most RTU (roof top unit) use?
1) Electronic
2) Electromechanical
3) Both
Both
What position should a heating zone valve be in the event of a power outage?
open, heated water flowing
On a typical high efficient furnace what comes after the thermostat closes R to W?
Inducer motor comes on (pre-purge)
Is an Economizer Analog or Digital?
Analog
Which BAS is the most current system?
Distributed direct digital control system
What is the demand on an analog control receiving a 8vdc from a 2-10vdc control system
75%
Is a Thermostat in heating mode Direct acting or Reverse acting?
Reverse, cooling is direct acting
In order to produce a smaller differential of temperature, this type of control activates earlier than it would otherwise activate
anticipator
What is the purpose of a restrictor in a pneumatic system?
restrict the buildup of pressure in the branch line
One similarity between a 3 pipe controller and a unitary controller is…
They are typically hidden in a cabinet
Which Device would interrupt the current flow on the control circuit side, which would cause a contactor coil to be deenergized, turning off a motor?
Pilot Duty
A very sensitive thermal device which resistance responds proportionally to the temperature. As temp increases, its resistance increases
RTD (Resistive Temperature Device)
The difference between the highest and lowest values the user can set the controller
Controller span
What does the thermostat do in our blue bin system?
energizes the solenoid
