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New Mexico Mm-98 Mechanical
Contractor Exam QUESTIONS AND
CORRECT ANSWERS
Below is a synthesized set of scenario-based multiple-choice questions
drawn from publicly available practice materials covering MM-98
competency areas. Each question includes an answer and italicized
summary rationale.
1. A mechanical contractor is installing a hydronic heating system with
a closed-loop expansion tank initially filled to 12 psi at 60°F. As water
temperature rises substantially, which component is primarily
responsible for accommodating the increase in water volume?
A) Air separator
B) Pressure-reducing valve
C) Expansion tank
D) Differential-pressure bypass valve
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Answer: C. The expansion tank provides a compressible volume that
absorbs thermal expansion of water in a closed hydronic system .
2. A contractor is sizing a duct system for a variable-air-volume
application. If duct velocity is increased substantially while
maintaining the same airflow, what is the most likely consequence?
A) Lower friction losses and lower fan horsepower
B) Higher duct static pressure and generally greater noise
C) Lower velocity pressure and increased duct size
D) Elimination of balancing requirements
Answer: B. Increasing velocity increases velocity pressure and generally
increases friction and pressure losses, as well as turbulence and noise .
3. A refrigeration system has significantly high superheat at the
evaporator outlet while evaporator pressure is lower than expected.
Which condition is most consistent with these symptoms?
A) Excessive refrigerant charge
B) Flooded evaporator
C) Insufficient refrigerant feed to the evaporator
D) Condenser airflow that is too low
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Answer: C. High evaporator superheat combined with low suction
pressure commonly indicates the evaporator is starved of refrigerant
due to restriction, insufficient charge, or improper expansion device
adjustment .
4. A mechanical contractor is installing a pump in a chilled-water
system selected for 500 GPM at a specified head. If system resistance
increases significantly after installation, what will generally happen to
the actual operating point?
A) Flow necessarily remains at exactly 500 GPM
B) Flow increases because pump head increases automatically
C) Flow decreases until the pump curve intersects the new system curve
D) Pump efficiency becomes exactly 100%
Answer: C. A centrifugal pump operates at the intersection of its pump
performance curve and the system resistance curve; increasing system
resistance shifts the operating point toward lower flow .
5. A boiler has a pressure-relief valve set at a specified maximum
pressure. Which practice is most appropriate when installing the relief
valve?
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A) Install a smaller discharge pipe to increase discharge velocity
B) Install a shutoff valve between the boiler and relief valve
C) Plug the discharge opening after testing
D) Provide an unobstructed discharge arrangement sized according to
applicable requirements
Answer: D. A relief valve is a critical safety device; its discharge path
must allow safe pressure relief without restriction. A shutoff valve
between the vessel and relief device creates a dangerous condition .
6. During startup of a newly installed air-handling unit, the supply fan
operates but airflow is substantially below design. Fan speed is
correct, but measured external static pressure is much higher than
expected. Which condition should be investigated first?
A) Refrigerant subcooling
B) Boiler water chemistry
C) Excessive duct or component pressure resistance
D) Thermostat calibration only
Answer: C. High external static pressure with low airflow strongly
suggests excessive system resistance .
New Mexico Mm-98 Mechanical
Contractor Exam QUESTIONS AND
CORRECT ANSWERS
Below is a synthesized set of scenario-based multiple-choice questions
drawn from publicly available practice materials covering MM-98
competency areas. Each question includes an answer and italicized
summary rationale.
1. A mechanical contractor is installing a hydronic heating system with
a closed-loop expansion tank initially filled to 12 psi at 60°F. As water
temperature rises substantially, which component is primarily
responsible for accommodating the increase in water volume?
A) Air separator
B) Pressure-reducing valve
C) Expansion tank
D) Differential-pressure bypass valve
, Page 2 of 112
Answer: C. The expansion tank provides a compressible volume that
absorbs thermal expansion of water in a closed hydronic system .
2. A contractor is sizing a duct system for a variable-air-volume
application. If duct velocity is increased substantially while
maintaining the same airflow, what is the most likely consequence?
A) Lower friction losses and lower fan horsepower
B) Higher duct static pressure and generally greater noise
C) Lower velocity pressure and increased duct size
D) Elimination of balancing requirements
Answer: B. Increasing velocity increases velocity pressure and generally
increases friction and pressure losses, as well as turbulence and noise .
3. A refrigeration system has significantly high superheat at the
evaporator outlet while evaporator pressure is lower than expected.
Which condition is most consistent with these symptoms?
A) Excessive refrigerant charge
B) Flooded evaporator
C) Insufficient refrigerant feed to the evaporator
D) Condenser airflow that is too low
, Page 3 of 112
Answer: C. High evaporator superheat combined with low suction
pressure commonly indicates the evaporator is starved of refrigerant
due to restriction, insufficient charge, or improper expansion device
adjustment .
4. A mechanical contractor is installing a pump in a chilled-water
system selected for 500 GPM at a specified head. If system resistance
increases significantly after installation, what will generally happen to
the actual operating point?
A) Flow necessarily remains at exactly 500 GPM
B) Flow increases because pump head increases automatically
C) Flow decreases until the pump curve intersects the new system curve
D) Pump efficiency becomes exactly 100%
Answer: C. A centrifugal pump operates at the intersection of its pump
performance curve and the system resistance curve; increasing system
resistance shifts the operating point toward lower flow .
5. A boiler has a pressure-relief valve set at a specified maximum
pressure. Which practice is most appropriate when installing the relief
valve?
, Page 4 of 112
A) Install a smaller discharge pipe to increase discharge velocity
B) Install a shutoff valve between the boiler and relief valve
C) Plug the discharge opening after testing
D) Provide an unobstructed discharge arrangement sized according to
applicable requirements
Answer: D. A relief valve is a critical safety device; its discharge path
must allow safe pressure relief without restriction. A shutoff valve
between the vessel and relief device creates a dangerous condition .
6. During startup of a newly installed air-handling unit, the supply fan
operates but airflow is substantially below design. Fan speed is
correct, but measured external static pressure is much higher than
expected. Which condition should be investigated first?
A) Refrigerant subcooling
B) Boiler water chemistry
C) Excessive duct or component pressure resistance
D) Thermostat calibration only
Answer: C. High external static pressure with low airflow strongly
suggests excessive system resistance .
