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Final 2 Flashcards

1
Q

Flame rectification is a process by which:

A

An electrical current passes through a flame

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2
Q

What commonly causes the problem of constantly having to relight the pilot and condensation on windows?

A

A cracked heat exchanger

(due to combustion gases leaking into the home and increasing indoor humidity.)

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3
Q

How would you correct the short cycling problem in a furnace with no heat anticipator?

A

Adjust the thermostat cycle rate per the furnace manufacturer’s instructions

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4
Q

What instrument do we use to measure gas pressure with?

A

Manometer

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5
Q

Name 3 types of igniters used in gas furnaces.

A

Spark igniter, hot surface igniter, intermittent pilot

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6
Q

What could be the problem if the furnace stays on too long and the home experiences cold drafts?

A

Thermostat calibration issue or faulty anticipator (fan control set too low)

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7
Q

True or False? When venting a Category I furnace, best results can be obtained if Type B, double wall pipe is used for the vent and vent connector.

A

True

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8
Q

What precaution must be taken during installation of Category IV (condensing) furnaces?

A

Make sure the vent pipe slopes back toward the furnace

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9
Q

What must be done if manifold pressure cannot be adjusted to fire the burners at the correct input?

A

Replace orifices with ones a size larger or smaller

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10
Q

What should you do if the pilot flame is yellow and soot has accumulated around the pilot assembly?

A

Disassemble and clean the pilot assembly

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11
Q

What is the first major event in the ignition sequence of an induced combustion furnace?

A

energizing of the inducer motor to establish proper draft.

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12
Q

True or False? For correct natural gas system operation, burner manifold pressure must be set at 3.5 inches w.c.

A

True

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13
Q

Which troubleshooting step for spark re-ignition is incorrect?

A

Check for spark with screwdriver

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14
Q

True or false. Can a glow coil remain energized continuously without risk of burning out?

A

False

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15
Q

What do most glow coils require in series to increase temperature?

A

current limiting resistor to prevent overheating or burnout
(Glow coils operate at specific current levels; resistors control this.)

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16
Q

Put these steps in the proper sequence for direct burner ignition:

A

Purge combustion chamber with inducer fan for 15 seconds
Energize hot surface igniter for 17 seconds
Energize gas valve for 5 seconds
Check flame sensor for next 2 seconds

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17
Q

In an atmospheric gas burner, what does a yellow or orange flame indicate?

A

Not enough primary air

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18
Q

What does the equation [CFM = Btuh / (1.08 × Temp rise)] determine?

A

System airflow

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19
Q

Where is manifold pressure on a gas furnace measured?

A

Between the gas valve outlet and the orifice

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20
Q

What is the most likely problem if the pilot is lit but the main gas valve will not open?

A

The limit switch contacts are open

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21
Q

What would a heat anticipator set to ‘longer’ most likely cause?

A

Longer furnace run time / longer heat cycles

(“Longer” increases current flow, keeping the burner on longer.)

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22
Q

One cubic foot of LP gas (propane) contains _____ BTU’s of heat energy than one cubic foot of natural gas.

A

More

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23
Q

Where is the secondary or auxiliary limit switch located?

A

In the blower compartment of a downflow furnace

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24
Q

Almost all burner problems are caused by improper ___________.

A

Primary air

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25
What causes yellow-tipping?
Shortage of primary air
26
How do you correct lifting flames?
Reduce primary air or reduce gas pressure (Lifting flames are often caused by excessive primary air or high velocity.)
27
What occurs when burning speed is greater than the port velocity?
Flashback
28
What may wavering flames indicate?
Varying gas pressure
29
What results if the burners do not light as soon as the gas valve opens?
Delayed ignition
30
How can flame rollout usually be corrected?
By making sure the front panel is sealed Check for blocked flue or cracked heat exchanger first; sealing the panel may help but is not the root fix.
31
When does a demand for heat occur?
When the room temperature drops below the setpoint
32
Which factor has the least effect on thermostat performance?
Exposure to air flow from the supply registers
33
True or False. The purpose of the heat anticipator in a thermostat is to provide all the heat necessary to satisfy the thermostat?
False
34
What condition would cause the high limit control to open the circuit on a central gas furnace?
The fan control is open (insufficient airflow, such as from a faulty blower motor, dirty filter, or closed vents, which leads to overheating.)
35
What could cause a situation where the flame continually rolls out the front of the heat exchanger?
A heat exchanger or vent that is plugged
36
What is the problem if an intermittent spark ignition system does not receive a flame recognition signal?
No flame recognition signal is received (system will shutdown the gas valve and not proceed to full ignition)
37
What will happen if the heat anticipator in a thermostat fails to open?
Failure of the heat to turn off
38
What must be provided for proper furnace vent operation?
Adequate combustion air
39
On which type of gas application can a spark ignition system with a continuous trial for pilot ignition be used?
Natural or LP gas application
40
What is air that is pre-mixed with the gas in the burner called?
Primary air
41
What is the most common cause for the limit switch to open on a residential gas furnace?
The blower speed is too low
42
What does a voltage reading of 24 volts across a gas valve solenoid indicate?
The gas valve solenoid coil is good
43
True or False? A thermopile thermostat should never be connected to any source of power other than a millivolt pilot generator.
True
44
Hot air will hold _____ moisture than cold air.
More
45
What should a technician do if the manifold pressure is 3.5” WC and the temperature rise is outside the rating plate range?
Increase the blower speed
46
What should the manifold pressure be on an LP gas fired furnace?
Approximately 11 inches of water column
47
How is the indoor blower controlled in the cooling cycle?
By the blower relay
48
What controls the indoor blower in the heating cycle?
The fan-limit control or the fan-on control
49
What could cause a short burner 'on' cycle followed by a shutdown?
A limit switch opening due to overheating
50
When does the fan control in a gas furnace energize the blower motor?
When the furnace heat exchanger reaches a preset temperature
51
What must be done during venting of high-efficiency furnaces to prevent contamination from entering the gas valve?
Prevent condensate from entering the gas valve
52
When does the fan relay or fan control turn off the indoor blower in a heating cycle?
When the heat exchanger cools below the fan-off temperature
53
What happens if the limit switch opens during furnace operation?
The gas valve closes and shuts off the burners
54
What is the purpose of the rollout switch in a gas furnace?
To shut off gas flow if flames roll out of the combustion chamber
55
How is ignition typically proven when a furnace uses a hot surface igniter?
With a flame sensor
56
What does the flame sensor detect?
Microamps of current through the flame
57
Why must vent piping slope back toward the furnace on condensing units?
To allow proper drainage of condensate
58
What will happen if the inducer motor does not run at the start of the heating cycle?
The pressure switch will not close and the ignition cycle will not proceed
59
What is the function of the pressure switch in an induced draft furnace?
It proves that the inducer is running and the vent is clear
60
What should be checked if the furnace starts but the burners shut off after a few seconds?
Flame sensor cleanliness and signal strength
61
What two functions does a fan-limit control switch perform?
It turns the blower on/off and shuts off the burner if the furnace overheats
62
How many inches of water column does a typical natural gas system operate at?
3.5 inches
63
What can cause lifting flames in a natural gas furnace?
Excess primary air or high gas velocity (Lifting flames = too much primary air or pressure; not a reduction.)
64
What could cause a gas furnace to be turned off by the high limit?
Blower failure or restricted airflow (A transformer failure would stop the furnace from running, but not trigger the high limit.)
65
What is true of a gas furnace with a non-100% shutoff gas valve?
If the pilot flame goes out, pilot gas will still continue to flow
66
What is a possible cause of intermittent burner cycling in a condensing furnace?
Blockage or restriction in the combustion air or vent piping
67
What must be done when installing a non-condensing, induced combustion furnace as a replacement for a natural draft furnace?
Line the masonry chimney with an approved liner
68
Which components provide the same function?
Fusible link and flame rollout switch
69
What does the fan switch regulate on a gas furnace?
Turns blower on/off based on heat exchanger temp (Inducer speed is usually not regulated by the fan switch.)
70
What type of gas system can a spark ignition system with continuous trial for ignition be used on?
Natural or LP gas application
71
What is air that is premixed with the gas in the burner called?
Primary air
72
How many BTUs of heat energy does one cubic foot of natural gas contain?
Approximately 1000
73
What is a direct drive blower?
One in which the blower is attached to the motor shaft
74
How is temperature rise on a furnace defined?
Temperature of supply air minus temperature of return air
75
What device stops the flow of gas to the burners when the pilot flame is extinguished in a standing pilot gas furnace?
The pilot safety solenoid or thermocouple
76
What is likely the cause if the spark ignites the pilot flame but the main gas valve will not open?
Flame rectification current is low or missing
77
What is probably the cause if the gas flame ignites in the venture of the burner?
Too much primary air
78
What is the typical voltage to energize the igniter in a gas furnace with a hot surface igniter?
120 volts
79
What precaution must be taken when handling a hot surface igniter?
The device is fragile and must be handled carefully , as hot surface igniters are fragile and skin oils can cause them to fail prematurely.
80
What is a possible consequence of too low a temperature rise in a gas-fired condensing furnace?
Condensation may form in the secondary heat exchanger

1070 (7 decks)

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