OBT3 EXAM PRACTICE SOLUTION
PACK 2026 COMPLETE TESTED
QUESTIONS AND CORRECT
ANSWERS GRADED A+
◉ Name the two most common methods of sensing flame used in oil
primary controls.
a) Bi-metallic heat sensing switch (stack relay)
b) Optical sensor (Cad Cell). Answer: bruh
◉ A cadmium cell:
a) Reflects light into itself, opening a circuit
b) Has a lower resistance when exposed to light, allowing a circuit to
close
c) Has a higher resistance when exposed to light, allowing a circuit to
close
d) Has a higher resistance when exposed to light, allowing the motor
run. Answer: b) Has a lower resistance when exposed to light, allowing
a circuit to close
◉ A stack mounted protector relay:
a) Shuts off delayed oil valve to stop oil flow
b) Opens delayed oil valve to start oil flow
c) Shuts off burner if no heat is detected
,d) Shuts off oil pump if no heat is detected. Answer: c) Shuts off burner
if no heat is detected
◉ When are the cold contacts closed on a stack mounted primary
control?
a) When the control is not sensing flame.
b) When the control is sensing flame.
c) When the thermostat contacts are closed.
d) When the heat anticipator is not energized.. Answer: a) When the
control is not sensing flame.
◉ If a resistance reading 100,000 ohms is measured across a cad cell
that is exposed to light, a technician would conclude that:
a) The cad cell is defective
b) The cad cell is good
c) The primary control is defective
d) The ignition transformer is defective. Answer: a) The cad cell is
defective
◉ A typical primary control used on oil furnaces is:
a) Stack mounted, sensing temperature
b) Burner mounted, sensing light
c) Stack mounted, sensing light
d) Either A or B. Answer: d) Either A or B
,◉ A stack mounted primary control:
a) Uses a cad cell which "sees" the flame
b) Uses a thermal sensor which opens its contacts to stop heating a
safety switch
c) Uses a thermal sensor which closes its contacts to start heating a
safety switch
d) Uses a device to sense the velocity of the flue products to close a sail
switch. Answer: b) Uses a thermal sensor which opens its contacts to
stop heating a safety switch
◉ To test a stack mounted primary control safety:
a) Hold the helix with a pair of pliers
b) Disconnect the ignition leads
c) Remove the unit from the vent pipe
d) Put a jumper between T-T. Answer: c) Remove the unit from the vent
pipe
◉ A stack mounted primary control must be mounted:
a) In the vent pipe upstream of the barometric damper
b) In the vent pipe downstream of the barometric damper
c) At the base of the chimney
d) At the top of the primary heat exchanger. Answer: a) In the vent pipe
upstream of the barometric damper
, ◉ What is the best way to test a cad cell relay?
a) Disconnect a wire from T-T
b) Disconnect a wire from F-F
c) Disconnect the orange wire from the relay
d) Shine a light on the cad cell. Answer: b) Disconnect a wire from F-F
◉ When a particular cad cell is exposed to light its resistance is
1,500ohms. One would conclude that:
a) The cad cell is defective
b) The cad cell is good
c) The primary control is operating properly
d) The combustion safety control is defective. Answer: b) The cad cell is
good
◉ A furnace has a stack mounted primary control relay. When the unit is
shutdown, the contact positions are:
a) Cold contact open, hot contacts closed
b) Cold contacts closed, hot contacts open
c) Both closed
d) Both open. Answer: b) Cold contacts closed, hot contacts open
◉ Connecting 120 volts to T & T terminals of the primary control
(relay)would have what affect?
a) No affect
PACK 2026 COMPLETE TESTED
QUESTIONS AND CORRECT
ANSWERS GRADED A+
◉ Name the two most common methods of sensing flame used in oil
primary controls.
a) Bi-metallic heat sensing switch (stack relay)
b) Optical sensor (Cad Cell). Answer: bruh
◉ A cadmium cell:
a) Reflects light into itself, opening a circuit
b) Has a lower resistance when exposed to light, allowing a circuit to
close
c) Has a higher resistance when exposed to light, allowing a circuit to
close
d) Has a higher resistance when exposed to light, allowing the motor
run. Answer: b) Has a lower resistance when exposed to light, allowing
a circuit to close
◉ A stack mounted protector relay:
a) Shuts off delayed oil valve to stop oil flow
b) Opens delayed oil valve to start oil flow
c) Shuts off burner if no heat is detected
,d) Shuts off oil pump if no heat is detected. Answer: c) Shuts off burner
if no heat is detected
◉ When are the cold contacts closed on a stack mounted primary
control?
a) When the control is not sensing flame.
b) When the control is sensing flame.
c) When the thermostat contacts are closed.
d) When the heat anticipator is not energized.. Answer: a) When the
control is not sensing flame.
◉ If a resistance reading 100,000 ohms is measured across a cad cell
that is exposed to light, a technician would conclude that:
a) The cad cell is defective
b) The cad cell is good
c) The primary control is defective
d) The ignition transformer is defective. Answer: a) The cad cell is
defective
◉ A typical primary control used on oil furnaces is:
a) Stack mounted, sensing temperature
b) Burner mounted, sensing light
c) Stack mounted, sensing light
d) Either A or B. Answer: d) Either A or B
,◉ A stack mounted primary control:
a) Uses a cad cell which "sees" the flame
b) Uses a thermal sensor which opens its contacts to stop heating a
safety switch
c) Uses a thermal sensor which closes its contacts to start heating a
safety switch
d) Uses a device to sense the velocity of the flue products to close a sail
switch. Answer: b) Uses a thermal sensor which opens its contacts to
stop heating a safety switch
◉ To test a stack mounted primary control safety:
a) Hold the helix with a pair of pliers
b) Disconnect the ignition leads
c) Remove the unit from the vent pipe
d) Put a jumper between T-T. Answer: c) Remove the unit from the vent
pipe
◉ A stack mounted primary control must be mounted:
a) In the vent pipe upstream of the barometric damper
b) In the vent pipe downstream of the barometric damper
c) At the base of the chimney
d) At the top of the primary heat exchanger. Answer: a) In the vent pipe
upstream of the barometric damper
, ◉ What is the best way to test a cad cell relay?
a) Disconnect a wire from T-T
b) Disconnect a wire from F-F
c) Disconnect the orange wire from the relay
d) Shine a light on the cad cell. Answer: b) Disconnect a wire from F-F
◉ When a particular cad cell is exposed to light its resistance is
1,500ohms. One would conclude that:
a) The cad cell is defective
b) The cad cell is good
c) The primary control is operating properly
d) The combustion safety control is defective. Answer: b) The cad cell is
good
◉ A furnace has a stack mounted primary control relay. When the unit is
shutdown, the contact positions are:
a) Cold contact open, hot contacts closed
b) Cold contacts closed, hot contacts open
c) Both closed
d) Both open. Answer: b) Cold contacts closed, hot contacts open
◉ Connecting 120 volts to T & T terminals of the primary control
(relay)would have what affect?
a) No affect
