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NATE_AIRDIST_SERVICE NEC 2023

NATE Air Distribution Service Practice Test 8

This test covers blower motor types and how they affect airflow: PSC (permanent split capacitor) fixed-speed motors versus ECM (electronically commutated) variable-speed motors, and how each responds to changing static pressure.

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NATE Air Distribution Service

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Frequently Asked Questions

Yes — completely free with no signup required. Every question includes a full explanation of the correct answer.

Two: the NATE Core exam plus at least one specialty exam (for example AC Service, Heat Pump Service, or Air Distribution Service). You can take them on the same day or separately.

The Core exam has 50 questions and each installation or service specialty exam has 100 questions. You need 70% on each to pass — 35 of 50 on Core and 70 of 100 on a specialty. Check natex.org for the current format before you book.

NATE certification lasts two years. To renew, you submit 16 hours of continuing education or retake the specialty exam before your certification expires.

No. These are independent practice tests written to match the published NATE exam topics. They are not official NATE exam questions.

Exam Tip: An ECM blower can adjust its speed to maintain constant CFM as static pressure rises, but only up to a point — it cannot overcome every restriction forever.

30

Questions

30 min

Time

70%

Pass

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NATE Air Distribution Service Practice Test 8 — Question List

Question 1: What does PSC stand for in the context of blower motors?

Permanent split capacitor
Programmable speed controller
Pressure sensing compressor
Positive suction cycle

Question 2: What does ECM stand for in the context of blower motors?

External compressor motor
Electronically commutated motor
Emergency circuit module
Evaporator control mechanism

Question 3: A PSC blower motor generally runs at what kind of speed?

A speed that changes continuously based on refrigerant charge
A speed that changes continuously based on outdoor temperature only
A fixed speed, sometimes with multiple selectable taps
No fixed speed of any kind; it runs at random

Question 4: What is a key advantage of an ECM blower motor related to airflow?

It never needs a filter of any kind
It eliminates the need for return ductwork
It always uses less refrigerant than a PSC-equipped system
It can automatically adjust its speed to help maintain constant CFM even as static pressure changes, up to a point

Question 5: True or False: An ECM blower motor can compensate for any amount of static pressure increase and still maintain full design CFM.

False
True
True, but only for PSC motors
False, ECM motors have no relationship to static pressure

Question 6: A system with a PSC blower motor has a filter that gradually gets dirtier over several months. What is the most likely effect on CFM, since a PSC motor does not automatically compensate?

CFM increases automatically to compensate
CFM gradually decreases as static pressure rises, since the motor speed does not automatically adjust to compensate
CFM stays perfectly constant no matter what
The motor automatically switches from PSC to ECM

Question 7: A system with an ECM blower has the same dirty filter scenario. What is a likely difference compared to the PSC system?

There would be no difference between the two systems at all
The ECM system would show a faster CFM drop than the PSC system
CFM may stay closer to design longer, since the ECM motor increases speed to compensate for the added resistance, at least until the filter gets very dirty
The ECM motor would shut off immediately when the filter gets dirty

Question 8: Why might a technician be misled about filter condition on a system with an ECM blower motor, compared to a PSC system?

ECM motors are always louder than PSC motors, making problems obvious
ECM motors cannot be affected by a dirty filter under any circumstances
ECM motors are only used in commercial buildings, not homes
Because the ECM motor may maintain fairly normal-sounding airflow even with a moderately dirty filter, masking the early signs a PSC system might show sooner

Question 9: A technician checking an ECM-equipped system notices the blower is running at a noticeably higher speed/wattage than usual for that time of year. What could this suggest?

The blower may be working harder to compensate for increased resistance somewhere in the system, such as a dirty filter or coil
This always means the refrigerant charge is too high
This is completely normal and never worth investigating
This means the motor has switched from ECM to PSC automatically

Question 10: Which motor type is more likely to be paired with a zoning system to help manage varying static pressure as zones open and close?

PSC (fixed-speed) motor
ECM (variable-speed) motor
Neither motor type can be used with zoning systems
Both motor types behave identically in zoned systems

Question 11: A PSC motor has multiple speed taps (low, medium, high). What determines which tap is used in a given installation?

The tap changes automatically every few minutes
The refrigerant charge determines the tap automatically
The tap is selected during installation or service based on the airflow (CFM) needed for that specific system and application
The outdoor temperature selects the tap automatically on a PSC motor

Question 12: True or False: Both PSC and ECM motors can be affected by airflow restrictions, but they respond to those restrictions differently.

False, only PSC motors are affected by restrictions
False, only ECM motors are affected by restrictions
False, neither motor type is affected by restrictions
True

Question 13: A technician wants to confirm whether a blower motor is a PSC or ECM type. Where would this information most reliably be found?

The motor's nameplate or the equipment's installation/service documentation
The homeowner's personal guess
The number of speed tap wires alone
The thermostat display screen

Question 14: A homeowner with an ECM-equipped system says their electric bill went up even though comfort seems normal. What could static pressure have to do with this?

Static pressure has no possible connection to electric bills
If static pressure has risen due to a restriction, the ECM motor may be working harder (using more electricity) to maintain the same CFM, even though comfort feels normal
This can only be caused by a refrigerant leak
ECM motors always use more electricity than PSC motors regardless of conditions

Question 15: Which statement correctly compares how PSC and ECM motors respond to a gradual increase in static pressure, such as from a slowly clogging filter?

Both motor types behave identically in this situation
A PSC motor will increase speed automatically while an ECM motor's CFM will drop
A PSC motor's CFM will drop as static pressure rises, while an ECM motor will try to increase speed to hold CFM steady, up to its capability limit
Neither motor type is affected by static pressure changes

Question 16: What is one reason a technician should still measure static pressure directly on an ECM-equipped system, rather than assuming everything is fine because airflow feels normal?

Static pressure only matters on PSC systems, never ECM systems
ECM systems cannot be measured with a standard manometer
Static pressure measurement is optional and provides no useful information on ECM systems
Because the ECM motor's compensation can hide a developing restriction until it becomes severe, so a direct measurement reveals the true system condition

Question 17: A furnace is being replaced and the new one comes with an ECM blower, while the old one had a PSC blower. What should a technician expect regarding CFM behavior as the filter gets dirty over time?

The new ECM-equipped furnace may hold CFM steadier for longer than the old PSC furnace did, before airflow noticeably drops
There will be no difference at all between the two furnaces in this regard
The new ECM furnace will lose airflow much faster than the old PSC furnace
ECM motors cannot be installed in furnaces, only air handlers

Question 18: True or False: An ECM motor's ability to compensate for resistance means a technician never needs to worry about static pressure on ECM-equipped systems.

True
False
True, but only for PSC-equipped systems
False, but only for systems less than one year old

Question 19: A technician wants to increase airflow on a system with a PSC motor set to its lowest speed tap, without addressing any duct restrictions. What is a straightforward option?

Add more refrigerant to the system
Install a smaller filter with a lower MERV rating without checking anything else
Switch the motor to a higher speed tap, if the system's static pressure and design allow for it
Disconnect the return duct entirely

Question 20: Which best describes why manufacturers program some ECM motors to maintain constant airflow (CFM) rather than constant speed?

Constant CFM has no benefit over constant speed
Constant CFM eliminates the need for a filter entirely
Constant CFM automatically prevents all duct leakage
Constant CFM helps maintain consistent heating and cooling performance and comfort, even as conditions like filter cleanliness change over time

Question 21: A technician is diagnosing low airflow on a PSC system and confirms the correct speed tap is selected, the filter is clean, and the coil is clean. What should be checked next?

The duct system itself, for restrictions such as closed dampers, kinked flex duct, or undersized runs
The refrigerant charge
The outdoor fan motor capacitor
The thermostat's cycle rate setting

Question 22: What is the difference between a constant-torque ECM blower and a constant-airflow (variable-speed) ECM blower?

They are the same motor; the two names are used interchangeably by manufacturers
Constant-torque holds CFM exactly at any static; constant-airflow holds a fixed torque
Constant-torque holds torque, so airflow still falls as static rises; constant-airflow changes speed to hold CFM
Constant-airflow runs at one fixed speed like a PSC; constant-torque varies its speed

Question 23: A homeowner asks why their new ECM-equipped system seems quieter at start-up than their old PSC system. What is a reasonable explanation?

This always indicates a failing capacitor
ECM motors often ramp up gradually to their target speed, while some PSC motors start closer to full speed immediately, which can sound more abrupt
This means the new system has no blower motor at all
This is caused entirely by the refrigerant charge

Question 24: Which scenario best illustrates the practical benefit of an ECM motor's constant-CFM behavior?

A homeowner replaces their filter every month with no change in airflow noticed
A homeowner adds a zoning system with no blower changes
A homeowner upgrades their thermostat to a smart model
A homeowner forgets to change their filter for several months, but the system continues delivering close to its design airflow until the filter becomes quite dirty

Question 25: What should a technician do if they are unsure whether a blower motor is compensating for excess static pressure or operating normally?

Measure static pressure and compare it to the equipment's rated value, rather than relying on the motor's sound or apparent effort alone
Assume the motor is fine since it is still running
Replace the motor immediately without further diagnosis
Ignore the concern since motor compensation cannot be measured

Question 26: A technician is reviewing service history and sees a PSC blower motor was set to "high" speed at installation, but the home now has additional balancing dampers partially closed. What might this combination suggest about current airflow?

Airflow is guaranteed to be identical to the original installation
CFM may now be different than when the "high" tap was originally selected, since the added dampers change the system's resistance
The motor has automatically become an ECM motor because of the dampers
Balancing dampers cannot affect a PSC-equipped system in any way

Question 27: Which of these is a valid reason a system designer might choose an ECM motor over a PSC motor for a particular installation?

ECM motors are required by law in every residential installation
PSC motors cannot be used with any duct system
The system may benefit from more consistent airflow performance over time and better compatibility with variable airflow needs, such as zoning
ECM motors eliminate the need for static pressure measurement entirely

Question 28: On a PSC blower, what usually happens to motor amp draw when a supply duct is blocked and airflow drops?

Amp draw decreases, because the blower is moving less air
Amp draw increases sharply because the motor works harder
Amp draw stays exactly the same
The motor reverses direction

Question 29: A system with an ECM motor is measured at 0.80 in. w.c. TESP against a rated static pressure of 0.50 in. w.c., and the motor is running at a much higher percentage of its maximum speed than typical for that system. What does this suggest?

The motor is compensating for a significant restriction, and the underlying cause of the high static pressure should still be found and corrected
This is completely normal and no further action is needed
This means the rated static pressure value on the nameplate is incorrect
This means the motor should be switched to a PSC type immediately

Question 30: How can a technician estimate airflow using a manufacturer's blower performance table?

Read the rated CFM printed on the filter frame and adjust for the filter's age
Divide the blower motor horsepower by the duct's cross-sectional area
Measure total external static pressure and read the CFM listed at that pressure for the speed tap in use
Measure supply air temperature and look up the matching CFM on the P-T chart

EXAM CARD NATE_AIRDIST_SERVICE

Questions30

Time limit30 min

Pass score70%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

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Source: NEC 2026 (NFPA 70)