140302A - Electronic Ignition Systems Flashcards

1
Q

Why did it take so long for residential appliances to utilize electronic ignition?

A

Older systems were too bulky to fit, newer tech has allowed them to become small enough to fit

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2
Q

What can electronic modules do now, other than just light a pilot? 3 points

A
  • sequence operation
  • start fans
  • open dampers
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3
Q

Name the 4 ways electronic ignition can be configured

A
  • intermittent pilot using spark ignition
  • intermittent pilot using hot surface
  • direct ignition using spark
  • direct ignition using hot surface
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4
Q

Why are interrupted pilot systems not usually configured with electronic ignition?

A

The additional cost of controls makes it prohibitive

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5
Q

How does an interrupted pilot system work? Sequence of operation 7 pts

A
  • call for heat, ignition system lights pilot
  • system proves pilot
  • main burner lit
  • system proves main burner
  • shuts off pilot
  • system continues to monitor main burner
  • call for heat ends and burner shuts off
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6
Q

How does intermittent pilot differ from interrupted pilot ignition?

A

intermittent leaves pilot lit during main burner operation. interrupted shuts pilot off after ignition

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7
Q

Why is intermittent pilot preferable to interrupted?

A

The system costs slightly less as there are fewer components

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8
Q

How are interrupted and intermittent pilot systems usually ignited?

A

direct spark, but a hot surface version using a smart valve does exist

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9
Q

is hot surface usually direct ignition?

A

True

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10
Q

What is the main difference between single and double rod pilot burner systems?

A

single uses a combination spark ignitor and flame rod. dual has two separate

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11
Q

Should you use your meter on a single rod system? Why

A

No it could damage your meter when taking flame current

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12
Q

Should you clean an oxide coated hot surface ignitor? Why

A

No never, they are very brittle

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13
Q

Do newer direct spark ignition systems have fuses in the modules?

A

No they are usually external now

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14
Q

Why should a direct spark ignitor arc to a ground surface instead of the main burner?

A

So it will not blow a hole in the burner casing over time

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15
Q

What is the current path of a flame rod?

A

Current is passed from the module through the flame to the flame rod and back to the module. This current path proves the flame

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16
Q

How should you handle the brittle silicone carbide hot surface ignitor?

A

By the ceramic base only

17
Q

Why shouldn’t you touch a hot surface ignitor with your fingers? Other than it is brittle

A

They often burn out at that point

18
Q

What should you use to clean a flame rod?

A

Scotch brite pads or 000 steel wool

19
Q

What may happen if you use sand cloth to clean a flame rod?

A

It will leave deep scratches in the rod causing a change in the ground to flame rod ratio and the system will not prove the flame

20
Q

Name the 2 main principles in flame rod detection

A
  • flame rectification

- flame conductivity

21
Q

What is a rectifier?

A

A device for converting AC to DC

22
Q

How does rectification apply to flame detection?

A

Ions formed from burning gas will conduct an electrical current

23
Q

How does flame rectification prove a flame?

A
  • two electrodes are used, one flame rod and one ground
  • heat from the gas knocks ions out of the rods
  • ions move to the pos or neg rods
  • AC current is applied through the rods, changing their polarity many times a second
  • ion flow changes direction as the voltage changes polarity
24
Q

Why are flame conductivity systems not common today?

A

They cannot distinguish between a flame and a short circuit

25
Q

How can flame rectification systems identify short circuits?

A

Using different sized rods, ground electrode 4x bigger than flame rod. More current is produced when flame rod is positive

26
Q

How does a flame rectifier change AC to DC current? How is this used to prove the flame

A

Via the different sized electrodes. More current is produced when the flame rod is positive, so you get a pulsating DC current. This is used to pull in the flame relay

27
Q

How does the flame rectification system identify a short circuit?

A

It will see AC current and not the pulsating DC current, thus not pulling in the flame relay and shutting down the system

28
Q

What must the ratio of area of the electrode exposed to the flame be?

A

4:1 ground to flame rod

29
Q

Give a sequence of operation for a spark ignition intermittent pilot fan assisted forced air furnace

A
  • R to W terminals close on call for heat
  • 24VAC sent to relay starting inducer fan
  • fan reaches speed and pressure switch closes
  • power flows to ignition system
  • safe start check completed to verify no flame
  • relays close that open pilot gas and spark generator, pilot is lit
  • flame rectifier senses pilot flame and spark shuts off
  • blower motor comes on after delay or temp reached
  • main gas valve energized as long as pilot flame is sensed
  • call for heat ends and inducer fan stops, opening pressure switch and shutting down burner and pilot
30
Q

Describe lockout and retry

A

a lockout timer starts with the ignition cycle. pilot must be proved in the time frame or gas and spark are shut off. retry allows multiple trys, becoming a recycling control

31
Q

When can you use continuous trial for ignition?

A

For pilot ignition natural gas systems only, never on propane or direct ignition type systems

32
Q

Describe the sequence of operation of a direct spark fan assist forced air furnace

A
  • R to W closes on call for heat
  • 24VAC reaches inducer relay which powers inducer fan
  • inducer fan draft closes pressure switch
  • power flows to ignition module
  • safe start check performed to confirm no flame present
  • ignition sequence begins
  • gas valve opens, spark begins, and flame sensor circuit powered
  • main burner lights and flame is proven, spark shuts off
  • call for heat ends turning off inducer and opening pressure switch
  • main valve closes
33
Q

Describe sequence of operation on a hot surface direct ignition fan assist forced air furnace

A
  • call for heat R to W
  • inducer starts and closes pressure switch
  • safe start check confirm no flame
  • timer started and hot surface warms up
  • 17-34 seconds later main gas valve opens and flame circuit powered
  • once flame is proven hot surface turns off
  • call for heat ends, inducer shuts off and pressure switch opens
34
Q

What temp can a hot surface ignitor reach?

A

2600F or 1427C

35
Q

What is the sequence for a direct hot surface ignitor trial for ignition?

A

Time it takes for the ignitor to warm up, gas valve to open, and CPU monitoring for a flame signal

36
Q

How long does the blower off delay setting last?

A

90 to 180 seconds usually