HVAC Gas Heat Exam (Latest 2024 / 2025 Update) 100% Correct Questions and Verified Answers | Grade A
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Course
HVAC
Institution
HVAC
HVAC Gas Heat Exam (Latest 2024 / 2025 Update) 100% Correct Questions and Verified Answers | Grade A
Question:
How does a thermocouple flame-prove a pilot flame.
Answer:
A thermocouple produces a small voltage when heated by the pilot flame. The thermocouple is connected to a shutoff valv...
HVAC Gas Heat Exam (Latest 2024 /
2025 Update) 100% Correct
Questions and Verified Answers |
Grade A
Question:
How does a thermocouple flame-prove a pilot flame.
Answer:
A thermocouple produces a small voltage when heated by the pilot flame. The
thermocouple is connected to a shutoff valve. As long as the electrical current
energizes the coil in the valve, the gas can flow. If the flame goes out, no
current will flow and the gas valve closes, shutting off the gas.
Question:
What are the three categories of ignition systems?
Answer:
Intermittent pilot (IP)
Direct-spark ignition (DSI)
Hot surface ignition (HSI)
,Question:
Describe IP ignition.
Answer:
A spark from an electronic module ignites the pilot which in turn ignites the
main gas burners. The pilot only burns when the t-stat calls for heat. The pilot
turns off to conserve fuel. It uses flame rectification to verify that the pilot has
fired.
Question:
What is the fastest and surest way of proving there is a flame in an IP system?
Answer:
Flame rectification.
Question:
How does a DSI system work?
Answer:
A DSI is designed with a spark ignition direct to the main burner. No pilot is
used. It uses flame rectification to verify that the furnace has fired. The system
will go into safety lockout if the flame is not established within a set time
(usually 4-11 seconds) due to large amount of fuel coming through the burners
compared to an IP system.
,Question:
What is standard gap for electronic ignition systems?
Answer:
1/8" gap.
Question:
How does a HSI system work?
Answer:
Instead of a flame, a HSI uses a material that is highly resistant to electrical
current. Using line voltage, the material can heat up to 1100°. The HSI igniter
can light a pilot or the main furnace burner. The system must flame prove that
it was lit or it will go into safety lockout.
Question:
Explain flame rectification.
Answer:
A flame can conduct a small electrical current due to the ionized combustion
gases. An AC current is fed to electrodes in the flames. Due to different sizes
of the electrodes, current will flow better in one direction than the other. A
small DC current (1-3 microamps) will flow as long as the flame is lit. This DC
signal s what the furnace recognizes as proof of a flame. If for some reason, it
received the AC signal back, it would prevent the main gas valve from
opening.
, Question:
Describe the two types of flame rectification systems.
Answer:
Single-rod (or local) sensing consists of an igniter and a sensor all in one rod.
There is only one wire running from the ignition module to the pilot
assembly.
Dual-rod uses a separate igniter and flame-sensing rod. There are two wires
running from the pilot assembly to the ignition module.
Question:
How do you numerically write 1 microamp
Answer:
.000001 amps
Question:
How can you measure the flame current?
Answer:
Place the microammeter in series with the wire coming from the burner
ground terminal to the module.
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