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Ignition of fuel supply
Troubleshoot hot surface igniter
Back to 14 things for troubleshooting gas water heater
11) Ignition of fuel supply 
by pilot light or hot surface igniter
ThermopilePilot light: typical for atmospheric and direct vent heaters/ ignites gas with flame does not use igniter
Image from burner assembly, or manifold assembly salvaged off old water heater shows the fixed-positioned bracket that holds pilot, thermopile and igniter.

The piezo igniter creates spark that ignites pilot light. The pilot flame burns clear blue when water heater is receiving adequate air supply and gas pressure. The pilot light performs three jobs: 1) Heat the thermocouple or thermopile that creates small electric current that powers operation of gas control that also 2) signals gas control valve that pilot is lit, and 3) ignites the gas when it is released down the manifold tube and comes out of burner.

A thermopile is basically several thermocouples put together. The thermopile produces more current than thermocouple.
Thermopile produces enough current to power circuit board inside the electronic gas valves so it can compile status of heater operation, issue error codes via LED status light, and control operation of heater.
A thermocouple produces a small current for a mechanical gas valve, but not enough current for the circuit board on electronic gas control.
Hot surface igniter
burner assembly-Hot surface igniter is typical for power vented heaters
-Look thru viewport at the flame rod or hot surface igniter, should be glowing hot when burner is on.
-Igniter wears out over time. Measure ohms  If multimeter shows 18.8 ohm or greater resistance at room temperature, then replace ignitor.
-If igniter or ceramic is cracked or damaged, replace igniter
-If igniter resistance is than less than 15.5, replace igniter
-If resistance is within range, and igniter is not damaged, then gently clean with very fine steel wool or very fine sandpaper (like 400) to polish it a bit.
-Do not touch igniter. Oil from fingers can cause damage when igniter is hot.

-Misaligned igniter.
-Just as pilot light keep burner going, the igniter must remain hot throughout the heating cycle to ensure gas flame burns continually.
-If igniter is not sitting in the flow of gas, the heater can 'misfire' and start short cycling.
-Igniter must sit directly in front of a gas port on the burner and be correct distance away.
 
Flame sensor
-When gas is ignited ... if the flame sensor is correctly positioned, and not dirty (If  the flame sensor's metal is off color then change it out, or lightly clean with an abrasive cloth), then flame sensor confirms that flame is present and signals the gas valve to continue releasing gas.
Flame sensor for Honeywell power vent
Flame rod sensor
Location of flame sensor and igniter vary by model and brand of water heater.
Sand carbon deposits off igniter.
Avoid touching igniter
Larger image
Hot surface igniter troubleshoot
Hot surface ignition basics
Troubleshoot hot surface ignition
Hot surface igniter troubleshoot
Power vent hot surface ignition

Tip of flame sensor must be directly in front of burner port.
Use a nickel to set 7/64 gap.
Natural gas flame sensor will be bent, while propane will be straight.
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Intellivent troubleshoot Manual
Power vent surface ignition
Flame sensor for Intellivent power vent

The lower lip or edge of low NOx burner extends out further than standard burner. Flame sensor gap and elevation are measured from lower lip on low NOx burners .... and from top lip on standard burners.
Specifications here are for standard recovery residential power vent and power direct vent 40 and 50 gallon models.
Flame sensor chart

Gas burner without piezo igniter
Larger image
Burner flame is yellow

Flame should burn clear blue/yellow flame indicates:
Low gas pressure.
Dirty combustion parts.
Poor incoming air supply.
Inadequate venting.
Bad gas control valve.
Loose connection between manifold tube and burner
Resource:
How to test gas pressure on water heater

14 things needed for gas water heater & .pdf resources
What you need for operation of gas water heater.
General troubleshoot
1) Basic parts and operation
2) Adequate incoming air supply ... issues: closed doors, tightly insulated house, very hot attic.
3) Adequate air supply getting to burner... issues: dirty vent screen, dirty environment, maintenance.
4) Thermal cutoff  ... TCO ... issues: failed part, maintenance, vent.

5) Flammable vapor lockout .... issues: exposure to vapors, bad FV sensor, failed FV system,
6) Fuel supply to gas valve thermostat ... natural gas and propane are different ... issues: wrong gas, low pressure, moisture in pipe, undersized gas meter, supply line too small for BTU rating.
7) Gas valve thermostat regulates fuel supply ... issues: not enough gas, failed thermostat, failed gas control valve.

8) Fuel supply from thermostat to burner ... issues: maintenance, thermocouple-thermopile, gas control valve.
9) Clean and replace burner ... issue: periodic maintenance requires burner inspection, cleaning, and possible replacement

10) Functioning thermopile-thermocouple-pilot light ... issues: misalignment, failure of part.
11) Ignition of fuel supply by pilot light or hot surface igniter ... issues: pilot light, piezo igniter, electrode.

12) Power vent water heater troubleshoot ... issues: pressure switch, blower, blower sequence from gas control, outlet polarity
13) Venting combustion byproduct .... issues: vent in disrepair, other vents in house drawing air

14) Misc troubleshoot manuals
15) How to install gas water heater
16) How to maintain water heater
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Troubleshoot gas water heater
Troubleshoot electric water heater
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