CSLB C20 COMPREHENSIVE EXAM 2026 QUESTIONS
AND ANSWERS GRADED A+
✔✔How do you check for overcharge?
with a high-side glass
with a low-side glass
with a manifold gauge
with a sling psychrometer - ✔✔with a manifold gauge
✔✔For an A/C unit, H = 250 psi, L = 68 psi. The compressor runs constantly. Which of
the following is the most likely cause?
low line voltage
fused contacts
open pressure switch
open power switch - ✔✔fused contacts
✔✔The horizontal run of a furnace flue pipe should be pitched toward the chimney with
a rise of at least _____ inch per running foot. - ✔✔1/4
✔✔Why is a cap installed on a chimney?
to keep out rain
to prevent downdrafts
for decoration only
to contain heat - ✔✔to prevent downdrafts
✔✔If the potential-start relay is open, the compressor:
will start and then shut off on overload
will hum but won't start
won't do anything
will run normally - ✔✔will hum but won't start
✔✔What voltage is commonly used in thermostat circuits? - ✔✔24V
✔✔What kind of hazardous gas(es) come(s) from a gas furnace flue?
carbon monoxide
carbon monoxide and methane
nitrogen
hydrogen - ✔✔carbon monoxide
, ✔✔Sequence controllers are commonly used to operate two or more electric switches in
a predetermined sequence. The reason for using this type of controller is:
to prevent overload
to provide on/off control of humidifiers, air cleaners, and other equipment energized
when the fan is operating
to provide minimum air-flow
to provide fan-interlock control - ✔✔to prevent overload
✔✔Which of the following refrigerants can you mix?
R12 and R502
R500 and R502
R500 and R22
none of the above - ✔✔none of the above
✔✔When designing the temperature for heating, with which of the following should you
most concern yourself?
the coldest outside temperature on record
the hottest outside temperature on record
the coldest inside temperature on record
the hottest inside temperature on record - ✔✔the coldest outside temperature on record
✔✔Which of the following can not activate an unloader?
solenoid valve
oil pressure
low pressure
high pressure - ✔✔oil pressure
✔✔When power tools are used outside, they must be:
made of plastic
plugged into an adjacent connector
double-insulated
made of stainless steel - ✔✔double-insulated
✔✔What is the minimum size of a condensate drain?'
¾"
1"
1-½"
2" - ✔✔3/4"
AND ANSWERS GRADED A+
✔✔How do you check for overcharge?
with a high-side glass
with a low-side glass
with a manifold gauge
with a sling psychrometer - ✔✔with a manifold gauge
✔✔For an A/C unit, H = 250 psi, L = 68 psi. The compressor runs constantly. Which of
the following is the most likely cause?
low line voltage
fused contacts
open pressure switch
open power switch - ✔✔fused contacts
✔✔The horizontal run of a furnace flue pipe should be pitched toward the chimney with
a rise of at least _____ inch per running foot. - ✔✔1/4
✔✔Why is a cap installed on a chimney?
to keep out rain
to prevent downdrafts
for decoration only
to contain heat - ✔✔to prevent downdrafts
✔✔If the potential-start relay is open, the compressor:
will start and then shut off on overload
will hum but won't start
won't do anything
will run normally - ✔✔will hum but won't start
✔✔What voltage is commonly used in thermostat circuits? - ✔✔24V
✔✔What kind of hazardous gas(es) come(s) from a gas furnace flue?
carbon monoxide
carbon monoxide and methane
nitrogen
hydrogen - ✔✔carbon monoxide
, ✔✔Sequence controllers are commonly used to operate two or more electric switches in
a predetermined sequence. The reason for using this type of controller is:
to prevent overload
to provide on/off control of humidifiers, air cleaners, and other equipment energized
when the fan is operating
to provide minimum air-flow
to provide fan-interlock control - ✔✔to prevent overload
✔✔Which of the following refrigerants can you mix?
R12 and R502
R500 and R502
R500 and R22
none of the above - ✔✔none of the above
✔✔When designing the temperature for heating, with which of the following should you
most concern yourself?
the coldest outside temperature on record
the hottest outside temperature on record
the coldest inside temperature on record
the hottest inside temperature on record - ✔✔the coldest outside temperature on record
✔✔Which of the following can not activate an unloader?
solenoid valve
oil pressure
low pressure
high pressure - ✔✔oil pressure
✔✔When power tools are used outside, they must be:
made of plastic
plugged into an adjacent connector
double-insulated
made of stainless steel - ✔✔double-insulated
✔✔What is the minimum size of a condensate drain?'
¾"
1"
1-½"
2" - ✔✔3/4"
