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Missouri (St. Louis County) Commercial HVAC
Servicer-Installer - 588-MO-SL Contractor EXAM
QUESTIONS WITH CORRECT SOLUTIONS ALL WITH
DETAILED RATIONALES JUST RELEASED THIS YEAR
Randomized MCQs (Answers + Italics Rationales)
1. A commercial rooftop unit fails to start. The technician measures 240V across L1 and L2 on
the line side of the contactor, but after the contactor engages, reads 0V across T1 and T2. What
is the most likely cause?
A) The thermostat is set incorrectly
B) The contactor coil is burnt out
C) The contacts are stuck open or severely pitted
D) The compressor is seized
Answer: C Voltage present on line side but absent on load side with contactor engaged indicates
the electrical path is broken through the contacts themselves.
2. Which agency in St. Louis County is responsible for issuing the Mechanical Contractor
License?
A) Missouri Department of Labor
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B) St. Louis County Department of Public Works
C) Board of Mechanical Examiners
D) OSHA
Answer: C The Board of Mechanical Examiners reviews applications and issues the license;
Public Works enforces compliance.
3. A commercial kitchen exhaust system is located 6 feet from the building’s fresh air intake.
According to local amendments, what device is required?
A) Grease filter
B) Backdraft damper
C) Fire damper
D) Volume damper
Answer: B *An 8-foot separation triggers the need for a backdraft damper; at 6 feet, it is
required to prevent exhaust re-entering the intake.*
4. How many hours of continuing education are required every two years to maintain a St. Louis
County mechanical contractor license?
A) 8 hours
B) 12 hours
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C) 16 hours
D) 24 hours
Answer: B County ordinance mandates 12 hours of approved CEUs for license renewal.
5. A technician pulls a vacuum on a refrigeration system and the gauge stalls at 2000 microns.
What does this most likely indicate?
A) The system is properly dehydrated
B) There is a leak or moisture still present in the system
C) The vacuum pump is working perfectly
D) The system is overcharged
Answer: B A deep vacuum should reach below 500 microns; stalling at 2000 microns indicates
moisture boiling off or a small leak.
6. Which document must be submitted with a permit application for a new commercial HVAC
installation in St. Louis County?
A) Manufacturer’s warranty
B) Contractor’s liability insurance certificate
C) OSHA 10-hour card
D) Energy Star rating report
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Answer: B Proof of liability insurance is required to demonstrate financial responsibility before a
permit is issued.
7. Under the International Mechanical Code, what is required for condensate disposal from an
air handling unit located above a finished ceiling?
A) Discharge directly onto the roof
B) Discharge to a sanitary sewer or outside grade via an approved indirect waste pipe
C) Discharge to the building’s storm drain
D) No disposal required if a drip pan is present
Answer: B IMC Section 307 requires condensate disposal to a sanitary sewer or outside grade to
prevent damage and health hazards.
8. A technician measures superheat at the evaporator outlet and finds it is 30°F on a system
with a fixed orifice. The outdoor ambient is 85°F. This indicates:
A) The system is overcharged
B) The system is undercharged or has a restricted metering device
C) The evaporator fan is running too fast
D) The compressor valves are leaking
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Missouri (St. Louis County) Commercial HVAC
Servicer-Installer - 588-MO-SL Contractor EXAM
QUESTIONS WITH CORRECT SOLUTIONS ALL WITH
DETAILED RATIONALES JUST RELEASED THIS YEAR
Randomized MCQs (Answers + Italics Rationales)
1. A commercial rooftop unit fails to start. The technician measures 240V across L1 and L2 on
the line side of the contactor, but after the contactor engages, reads 0V across T1 and T2. What
is the most likely cause?
A) The thermostat is set incorrectly
B) The contactor coil is burnt out
C) The contacts are stuck open or severely pitted
D) The compressor is seized
Answer: C Voltage present on line side but absent on load side with contactor engaged indicates
the electrical path is broken through the contacts themselves.
2. Which agency in St. Louis County is responsible for issuing the Mechanical Contractor
License?
A) Missouri Department of Labor
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B) St. Louis County Department of Public Works
C) Board of Mechanical Examiners
D) OSHA
Answer: C The Board of Mechanical Examiners reviews applications and issues the license;
Public Works enforces compliance.
3. A commercial kitchen exhaust system is located 6 feet from the building’s fresh air intake.
According to local amendments, what device is required?
A) Grease filter
B) Backdraft damper
C) Fire damper
D) Volume damper
Answer: B *An 8-foot separation triggers the need for a backdraft damper; at 6 feet, it is
required to prevent exhaust re-entering the intake.*
4. How many hours of continuing education are required every two years to maintain a St. Louis
County mechanical contractor license?
A) 8 hours
B) 12 hours
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C) 16 hours
D) 24 hours
Answer: B County ordinance mandates 12 hours of approved CEUs for license renewal.
5. A technician pulls a vacuum on a refrigeration system and the gauge stalls at 2000 microns.
What does this most likely indicate?
A) The system is properly dehydrated
B) There is a leak or moisture still present in the system
C) The vacuum pump is working perfectly
D) The system is overcharged
Answer: B A deep vacuum should reach below 500 microns; stalling at 2000 microns indicates
moisture boiling off or a small leak.
6. Which document must be submitted with a permit application for a new commercial HVAC
installation in St. Louis County?
A) Manufacturer’s warranty
B) Contractor’s liability insurance certificate
C) OSHA 10-hour card
D) Energy Star rating report
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Answer: B Proof of liability insurance is required to demonstrate financial responsibility before a
permit is issued.
7. Under the International Mechanical Code, what is required for condensate disposal from an
air handling unit located above a finished ceiling?
A) Discharge directly onto the roof
B) Discharge to a sanitary sewer or outside grade via an approved indirect waste pipe
C) Discharge to the building’s storm drain
D) No disposal required if a drip pan is present
Answer: B IMC Section 307 requires condensate disposal to a sanitary sewer or outside grade to
prevent damage and health hazards.
8. A technician measures superheat at the evaporator outlet and finds it is 30°F on a system
with a fixed orifice. The outdoor ambient is 85°F. This indicates:
A) The system is overcharged
B) The system is undercharged or has a restricted metering device
C) The evaporator fan is running too fast
D) The compressor valves are leaking
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