Page 1 of 147
Missouri (St. Louis County) Commercial HVAC
Servicer/Installer - 588-MO-SL Contractor Exam
ACTUAL QUESTIONS AND ANSWERS LATEST
UPDATE THIS YEAR
Missouri (St. Louis County) Commercial HVAC Servicer/Installer - 588-MO-SL Contractor Exam
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
Rationale: Voltage present on the line side but absent on the load side with the 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
Rationale: The Board of Mechanical Examiners reviews applications and issues the license; the
Department of 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
Rationale: An 8-foot separation triggers the need for a backdraft damper; at 6 feet, it is required to
prevent exhaust re-entering the intake .
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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
C) 16 hours
D) 24 hours
Answer: B
Rationale: County ordinance mandates 12 hours of approved continuing education for license renewal .
5. When pulling a vacuum on a refrigeration system after a major repair, what is the generally accepted
industry standard minimum vacuum level?
A) 500 microns
B) 1000 microns
C) 1500 microns
D) 2000 microns
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Answer: A
Rationale: A vacuum of 500 microns or lower ensures that non-condensables and moisture have been
removed to prevent system contamination and acid formation.
6. Which document must be submitted with a permit application for a new commercial HVAC
installation in St. Louis County?
A) Contractor's liability insurance certificate
B) OSHA 10-hour card
C) Manufacturer's warranty
D) Energy Star rating report
Answer: A
Rationale: 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
Missouri (St. Louis County) Commercial HVAC
Servicer/Installer - 588-MO-SL Contractor Exam
ACTUAL QUESTIONS AND ANSWERS LATEST
UPDATE THIS YEAR
Missouri (St. Louis County) Commercial HVAC Servicer/Installer - 588-MO-SL Contractor Exam
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
Rationale: Voltage present on the line side but absent on the load side with the 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
, Page 2 of 147
B) St. Louis County Department of Public Works
C) Board of Mechanical Examiners
D) OSHA
Answer: C
Rationale: The Board of Mechanical Examiners reviews applications and issues the license; the
Department of 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
Rationale: An 8-foot separation triggers the need for a backdraft damper; at 6 feet, it is required to
prevent exhaust re-entering the intake .
, Page 3 of 147
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
C) 16 hours
D) 24 hours
Answer: B
Rationale: County ordinance mandates 12 hours of approved continuing education for license renewal .
5. When pulling a vacuum on a refrigeration system after a major repair, what is the generally accepted
industry standard minimum vacuum level?
A) 500 microns
B) 1000 microns
C) 1500 microns
D) 2000 microns
, Page 4 of 147
Answer: A
Rationale: A vacuum of 500 microns or lower ensures that non-condensables and moisture have been
removed to prevent system contamination and acid formation.
6. Which document must be submitted with a permit application for a new commercial HVAC
installation in St. Louis County?
A) Contractor's liability insurance certificate
B) OSHA 10-hour card
C) Manufacturer's warranty
D) Energy Star rating report
Answer: A
Rationale: 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
