Gas Furnace Ignition Quiz

Ten questions on the furnace ignition sequence and its safeties. Pick an answer to see whether you are right and why. Related reading: The Code: gas heat.

1. In a typical induced-draft furnace, what starts first after a call for heat?

   1. The gas valve
   2. The igniter
   3. The inducer motor
   4. The blower

   The inducer starts first to purge the heat exchanger and establish draft. The pressure switch, igniter and gas valve follow in order.

2. The pressure switch proves:

   1. Gas pressure
   2. That the inducer has created enough draft through the vent
   3. Refrigerant pressure
   4. Water pressure in the condensate trap

   The pressure switch closes on the negative pressure the inducer creates. If the vent or intake is blocked, or the inducer fails, the switch stays open and ignition stops.

3. The hot surface igniter glows, but the gas valve never opens. A likely cause is:

   1. A failed gas valve, or no 24 volts reaching it from the board
   2. A dirty air filter
   3. A dirty evaporator coil
   4. Too much excess air

   Check for 24 volts at the gas valve during the trial for ignition. If the voltage is there and the valve does not open, the valve or the gas supply is the problem.

4. The furnace lights, runs for a few seconds, then shuts off and tries again. A common cause is:

   1. A dirty or poorly grounded flame sensor
   2. A bad thermostat
   3. A blocked supply register
   4. An open limit switch at startup

   The board must prove flame within a few seconds. If the flame sensor signal is weak, the board closes the gas valve even though the burner lit.

5. Flame rectification works because:

   1. The flame acts like a diode, letting a small DC current flow from the sensor to ground
   2. The sensor measures flame temperature
   3. The board detects the flame's light
   4. The gas valve measures flow

   The board applies AC voltage to the sensor, and the flame rectifies it into a tiny DC current. No flame, no current.

6. Flame sensor current is measured in:

   1. Amps
   2. Milliamps
   3. Microamps
   4. Kilovolts

   Flame signals are tiny, typically a few microamps. Check the manufacturer's minimum value for the furnace you are working on.

7. What does a flame rollout switch protect against?

   1. Flames escaping the burner area, for example because of a blocked heat exchanger or vent
   2. Low gas pressure
   3. A dirty filter
   4. An overcharged air conditioner

   Rollout switches sit near the burners and open if flame rolls out of the combustion area. A tripped rollout switch is a warning to investigate, not just to reset.

8. The high limit switch opens when:

   1. The heat exchanger area gets too hot, often because of low airflow
   2. Gas pressure is too high
   3. The thermostat is satisfied
   4. The inducer starts

   Limits protect the heat exchanger. Dirty filters, closed registers and blower problems are common causes of limit trips.

9. Why should you never touch the element of a hot surface igniter?

   1. It is magnetic
   2. The elements are brittle, and oil from your skin can create hot spots that crack them
   3. It runs on 24 volts
   4. It is always cold

   Silicon carbide and silicon nitride igniters are fragile. Handle them by the base to avoid early failure.

10. The furnace control board is flashing a fault code. The best first step is to:

    1. Replace the board
    2. Read the code against the chart on the furnace door or in the manual
    3. Reset the power repeatedly until it runs
    4. Jump the safety switches

    The code tells you which step of the sequence failed. Start there, and never bypass a safety to get the furnace running.