Troubleshooting Final Review
An R-22 system operating with a liquid pressure of 226 psig (110° saturation temperature). The specified system subcooling is 5°F. The liquid line temperature should be
105 degrees F
Which of the following measures is the standard airflow across an air-conditioning coil?
400 cfm per ton
An air conditioning system charged with R-410A is operating with a suction pressure of 130 psig which equates to a 45°F saturation temperature. If the manufacturer calls for a 15°F superheat, what should the suction line temperature be?
45°F + 15°F = 60°F suction line temperature
The compressor in an existing split system is being replaced. The system has a 35 foot line set. The factory charge for the condensing unit is 80 ounces. The installation instructions state that the factory charge assumes a line length of 15 feet and gives the per foot adjustment as 0.6 ounces per foot. How much refrigerant should be added to the system after the system has been evacuated?
80 ounces + ((35-15) x 0.6) = 92 ounces
The standard AHRI "A" design condition used for rating unitary air conditioning equipment is
95°F outdoor ambient temperature, 80°F dry bulb, 68°F wet bulb indoor temperature
For air conditioners, a rise in the return air wet bulb temperature causes
A decrease in the temperature difference across the evaporator coil
A system that is overcharged will operate with
A high subcooling and a low superheat
In general, a system that is overcharged will have
A high subcooling and a low superheat
A system with an inefficient compressor will operate with
A high suction pressure and a low head pressure
A fixed restriction system operating with a refrigerant undercharge will have
A high system superheat
A system that is undercharged will operate with
A low subcooling and a high superheat
In general, a system that is undercharged will have
A low subcooling and a high superheat
Organizations that support the TAB industry by developing standards and offering certification include
AABC, TABB, and NEBB
An air conditioner with a thermostatic expansion valve is operating with a condenser saturation temperature of 95°F and a liquid line temperature of 92°F. The factory specified subcooling for the current operating condition is 10°F. The technician should
Add charge because the system is undercharged
How does the refrigerant charge affect a system's subcooling?
Adding refrigerant increases the system subcooling
The amount of moisture in the air can be measured with
An electronic hygrometer
Why is testing and balancing an important part of HVACR residential systems?
Because HVACR systems are the largest energy appliances in a residence and energy costs have increased
What color should the flames be in a correctly adjusted gas burner?
Bright blue inner cone with darker outer cone
The velocity pressure of air moving through ductwork is measured
By subtracting static pressure from total pressure
The formula for measuring airflow using temperature rise is
CFM = Btuh ÷ (1.08 x Temperature Rise)
How can a technician check to see if the coil and condensing unit on an AC split system are matched?
Call the equipment supplier and provide the system model numbers
When changing a low voltage transformer, service technician should
Check all low voltage controls for shorts and grounds
When troubleshooting an induced draft gas furnace, what should be checked if the induced draft fan comes on but the igniter is never energized?
Check the draft pressure switch to see if it is closed
Before starting the unit:
Check the power supply with the unit off.
After replacing a blown fuse in a unit disconnect, the technician should
Check the system amp draw and compare it to the data plate rating
The first step in performing a TAB service on a system is
Collecting system specifications and equipment information
The reversing valve on most air source residential heat pumps is energized
During the cooling and the defrost cycles
The air filter should be checked
During the preliminary inspection
An anemometer is used to measure
Feet of air for air velocity calculations
When performing a test and balance procedures on a zoned system, correct airflow should first be established
For each zone before balancing the individual branches
Why should caution be exercised when replacing a low efficiency air filter with a higher efficiency air filter?
Higher efficiency filters have a higher pressure drop that can cause poor airflow.
The most important thing to know about a system before an operational check is:
How it should function
Instruments that can read the pressure in a duct system include
Inclined manometer, magnehelic gauge, and digital manometer
In general when a blower motor moves more air its amp draw will
Increase
How does scheduling maintenance save time and money?
It prevents small problems from becoming big ones.
How will an extremely dirty air filter affect the amp draw of a PSC centrifugal blower motor?
It will cause the motor current to decrease because the fan will be moving less air.
What should you do to isolate the root cause of the malfunction?
Perform tests to isolate the cause.
A common cause of low system superheat is:
Poor airflow across the evaporator
How is a temperature-pressure relief valve on a hot-water boiler tested?
Pull lever and look for water at outlet of discharge pipe.
The refrigerant condensing temperature for an air cooled system having a 20°F split is
RCT = 95°F + 20°F = 115°F
After any repair or part replacement, the most important thing you must do is:
Recheck the system to be sure the concern has been resolved.
An air conditioner with a fixed restriction metering device is operating with an evaporator saturation temperature of 40°F and a suction line temperature of 42°F. The factory specified superheat for the current operating condition is 12°F. After checking the airflow and determining that it is within specifications, the technician should
Recover charge because the system is overcharged
An air conditioner with a thermostatic expansion valve is operating with a condenser saturation temperature of 110°F and a liquid line temperature of 85°F. The factory specified subcooling for the current operating condition is 10°F. After verifying the condenser coil and airflow are good, the technician should
Recover charge because the system is overcharged
An air conditioner is operating with low system pressures, an evaporator saturation temperature of 20°F and a suction line temperature of 24°F. The suction line is frosting. The most likely cause of these symptoms is
Reduced airflow across the evaporator
How does performing planned maintenance save the customer money?
Reduced operational cost and longer equipment life
Adding a little extra refrigerant to a fixed metering system over the amount recommended by the manufacturer
Reduces both system capacity and efficiency
The phases of a planned maintenance service call are:
Routine maintenance, pre-start inspection, operational checks, and troubleshooting.
Which statement most accurately describes TAB?
TAB is used to conduct measurements and adjustments that are specific to testing and balancing, which are not normally performed in regular HVACR work.
What is the free area of a register or grille?
The amount of the grille face area that is open
How is a velometer used to determine the CFM at a supply register?
The average air velocity is multiplied by the free area of the register
The contactor on most residential split system condensing units controls
The compressor and the condenser fan motor
With the system operating, a technician gets a reading of 20 volts between the L1 and T1 terminals at the contacts of the contactor. This tells the technician that:
The contacts are defective because they have a measurable voltage drop.
A sling psychrometer can be used to determine the relative humidity by comparing
The dry bulb and wet bulb readings
The most reliable source of information on how to charge any particular piece of equipment is
The equipment manufacturer
When searching for the break in a circuit by hop scotching, the technician checks
The voltage at each switch in the circuit
A TAB technician determines that the total airflow entering the return grilles is much lower than the amount of air the blower is moving. This indicates that
There are leaks in the return side ductwork
A TAB technician determines that the total airflow leaving the supply registers is much lower than the amount of air the blower is moving. This indicates that
There are leaks in the supply side ductwork
The first step in troubleshooting a system is
To understand the system operational sequence
Before starting to change a belt on a belt drive blower, the technician should:
Turn the power to the unit off and lock it out.
A pitot tube is used to measure
Velocity pressure in ducts
The desired end result of performing a TAB service is to
Verify that the system is operating as designed
Can zeotropes be metered in the low side with the unit operating?
Yes, by slightly cracking open the low-side manifold valve and then closing it.
What type of refrigerant must leave the cylinder as a liquid to prevent the separation of the different components in refrigerant?
Zeotropic mixtures
A water source heat pump that trips the high pressure switch during the cooling cycle and the low pressure switch during the heating cycle may have
a fouled water coil
A fixed restriction system operating with a refrigerant overcharge will have
a low system superheat
A system operating with a high superheat and a high subcooling most likely has
a refrigerate restriction
How does system commisioning improve energy efficiency?
by insuring that the system is performing as designed
Most vapor charging is done with the compressor operating while charging is taking place; however, liquid charging
can be done either with the system off or operating.
The main advantage of measuring airflow using the temperature rise method is that temperature rise measurements
can be taken easily using a common thermometer
As an operational safety check, the equipment rooms for combustion appliances should be checked for high:
carbon monoxide levels
As an operational safety check, what should be checked in the combustion gases for combustion appliances?
carbon monoxide levels
As the outdoor ambient temperature increases with fixed metering, the required superheat
decreases
If some components are not functioning, and the system has line and control voltage, you should next
determine if the components are receiving voltage.
The most common times to perform a seasonal check on heating equipment is in the:
fall
The volume of air flowing out a supply register can be directly measured with a
flow hood
As the indoor wet-bulb temperature increases on fixed metering systems, the required superheat
increases
A problem that can look like refrigerant overcharge if a service technician is only looking at system pressures is
low condenser airflow
A problem that can look like low refrigerant charge if a service technician is only looking at system pressures is
low evaporator airflow
A dirty air filter would likely cause which of the following symptoms ?
low suction pressure
Why is planned maintenance often performed during the off-season?
most equipment fails during the off season
Technicians that use a systems approach to troubleshooting
must understand the inter-relationship between different system components
What kind of chart is required when calculating the liquid-line subcooling?
pressure-temperature chart
Two variables that have a great impact on the system performance of air conditioning systems are
return-air wet-bulb temperature and the airflow across the evaporator.
A tachometer is used to measure
rotational speed
When the liquid line is restricted, the supply of refrigerant to the metering device is reduced. What is the effect on suction pressure and superheat?
suction pressure goes down and superheat increases
TAB stands for
testing and balancing
When an evaporator float switch is used in the drain pan of an air conditioner or heat pump, if the drain pan fills up the float will interrupt the low voltage circuit to
the compressor contactor
The low pressure switch on a residential split system condensing unit normally interrupts the circuit to
the contactor coil
Before checking a system's operating superheat, the technician should first check
the evaporator airflow
A technician notes that an operating air conditioning system has ice formed on the suction line at the condensing unit. The technician can reasonably assume that:
the evaporator coil is also frozen over
Which manufacturer-supplied reference would probably be the most helpful on a no heat furnace in letting you know the pressure switch is open or failed to open?
the fault code reference list
In an induced draft gas furnace, the first component to energize on a call for heat is
the induced draft blower
Which manufacturer-supplied reference uses pass/fail tests to guide the technician to the possible cause of system malfunction?
the logic flowchart
What side of an air conditioning coil generally collects dirt?
the side where air enters the coil
The purpose of TAB is to document that
the system is performing as designed
The dampers on registers are normally not used for balancing the system airflow because
they will produce excessive noise in the space
Why must zeotropic refrigerant leave the cylinder as a liquid?
to avoid fractionation
Of the four phases listed below, which one is only performed on an "as needed" basis?
troubleshooting
Which of the following types of charge is usually employed when a partial charge is required?
vapor
Hopscotching is the technique that requires you to start at a point with correct voltage and then move one lead at a time until
voltage is lost.
