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Gas practice questions

26 questions from the hvac service and troubleshooting practice test.

Gas practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

On a call for heat the inducer never starts and the board flashes a code for a pressure switch that is closed when it should be open. What does the board check before starting the inducer?

  1. That the flame sensor reads at least 1 µA DC
  2. That the gas valve has 24 V across it
  3. That the pressure switch contacts are open
  4. That the blower has reached full speed
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C. That the pressure switch contacts are open

The control confirms the pressure switch is open with the inducer off; otherwise a welded or jumpered switch could falsely prove draft later. A switch that is already closed, from stuck contacts or a jumper left in place, stops the sequence at the first step.

Reference: Gas furnace sequence of operation; pressure switch check

A pressure switch is rated to close at -0.60 in. w.c. With the inducer running a manometer teed into its hose reads -1.10 in. w.c., yet a voltmeter still reads 24 V across the switch terminals. What does this show?

  1. The pressure switch has failed
  2. The vent pipe is restricted
  3. The inducer motor is weak
  4. The control board relay has failed
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A. The pressure switch has failed

The measured pressure is well beyond the switch's closing point, so the contacts should be closed and read about 0 V across them. Reading the full 24 V means they are still open: the switch itself is defective. A blocked vent would show too little pressure at the hose.

Reference: Induced-draft furnace diagnosis; pressure switch testing

A condensing furnace runs for a few minutes, then the burners drop out and a gurgling sound comes from the cabinet. The board logs the pressure switch opening during the cycle. What is the most likely cause?

  1. A dirty flame sensor rod
  2. A cracked hot surface igniter
  3. A blocked condensate trap or drain line
  4. A high-limit switch that is out of calibration
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C. A blocked condensate trap or drain line

If condensate cannot drain, water backs up into the collector box or inducer housing, changes the pressure the switch sees and opens it. The gurgle is the inducer pulling air through standing water. A dirty flame sensor drops the flame within seconds, not minutes.

Reference: Condensing furnace condensate drainage

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