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Basic parts and operation of gas water heater
Gas water heater parts 
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Energy factor EF .675 - (.0015 x V volume in gallons)
40 gallon tank: .675 - (.0015 x 40) = 61.5% efficiency
38.5% of heat goes up flue pipe into atmosphere.
Basic parts
Gas water heaters are NOT 100% standardized. Models vary.  Manufacturers offer different designs.
Ilustrations show atmospheric vent where natural updraft of air pulls toxic combustion by-product up flue pipe and into atmosphere.
Parts not shown on illustration include: Electronic gas control valve, power vent, flue damper vent, FV sensor, flexible gas line, air intake, thermocouple, thermopile, pilot etc.

Resources
TP valve code
Replace TP valve
Repair water heater valves
Remove short water heater nipple
Broken dip tube
Replace drain valve
How much does it cost to run gas water heater
Index of all plumbing links
Install gas water heater
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Honeywell gas control
Electronic controls have thermopile instead of thermocouple
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Basic gas valve thermostat parts
Gas water heaters have a control valve with a probe that extends into water heater to measure temperature.
Replacement
Troubleshoot gas control valve thermostat
How to replace water heater gas valve
How to light gas water heater

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Gas water heater thermostats Amazon
AO Smith gas valves at Amazon

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AO Smith gas water heater at Amazon
Bradford White gas water heater
Rheem gas water heater at Amazon

Mechanical control thermostats have thermocouple
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How gas water heaters work
Gas water heater partsLarger image
How atmospheric vent water heater works
This type heater vents straight up through the roof and requires no electric outlet for operation.

-The gas valve thermostat senses that water inside tank is below set point.
-If the pilot light is ON, it heats the thermocouple or thermopile. The thermocouple or thermopile creates a small current that travels to gas control valve.
-If the pilot light and thermocouple are working correctly, the gas control valve releases gas to the burner.
-The pilot light ignites the fuel.
-When temperature inside tank reaches the set point, for example 120F, the gas control valve thermostat turns off all power, and the water heater goes into standby mode until water temperature drops below thermostat set point again.

-If temperature of water inside tank exceeds high limit set point of thermostat, then gas control valve turns off all gas.
->Once repairs have been made to correct cause of overheating, the gas control valve may require replacement.
-If the thermostat comes out of calibration, and is no longer reading correct temperature, it requires replacement of gas control valve<
Resources:
How to troubleshoot gas control valve
How to troubleshoot thermocouple and thermopile
burner assemblyHow power vent water heater works

This type heater can vent combustion gas through a side wall and requires 120 volt electric outlet for operation. Improper grounding or reverse Hot-Neutral polarity can cause ignition failure and/or short cycling (repeat on-off attempts).
-The gas valve thermostat senses that water inside tank is below set point and closes a circuit that turns on the 120 volt blower located on top of water heater.
-The blower pulls air up through the vent pipe and begins a cycle of releasing toxic combustion by-products, including NOx SO CO CO2 and acidic water vapor, into the atmosphere, where it creates an exciting new future for the planet.
Alternative for some: Connect solar panels directly to electric water heater
-The vacuum safety switch is connected to blower via rubber tube. The rush of air through the vent pipe creates vacuum pressure. 
-The vacuum or pressure switch measures the vacuum created by the blower and verifies the blower is operating at speed. This ensures the blower can evacuate combustion gasses and support complete burning of fuel. Complete burning prevents explosion hazard caused by ignitable fuel gathering insinde combustion chamber or vent pipe. The efficiency of gas water heater combustion is typically 60-61%, with the remaining 39-40% of heat going up the flue pipe. Calculate efficiency
-Once the blower operation is confirmed by the vacuum switch, it closes (turns on) a small circuit that signals the gas valve thermostat that the blower is working.
-If the 120 volt outlet has correct voltage, polarity and grounding, the gas valve electronic control board activates the hot surface igniter, igniter glows red with heat, and gas is released down the burner tube, through the orifice into the burner head.
-If the gas pressure good, and the orifice is clean and correctly sized, and hot surface igniter and burner head are clean and in good working order, and the igniter is correct distance away and positioned correctly in front of a gas jet, the gas will ignite and burn blue. Flame sensor confirms that flame is present and signals the gas valve to continue releasing gas. Water temperature inside tank begins to heat.
-If the water heater has pilot light instead of hot surface igniter, the burner assembly has a pilot tube in addition to the burner tube that connects to gas control valve. The control valve releases gas into the pilot tube continually as long as thermopile is functioning and correctly positioned in the pilot flame so it sends small electric current to gas control valve, signaling that pilot flame is working. If the pilot flame and thermopile system are working, the ignition sequence is ok to release gas once the blower operation is confirmed.
-If the blower is not operating, or running at lower speed because of a brownout or lack of voltage or malfunction, the vacuum switch does not receive enough vacuum pressure and will not close, and the ignition sequence will not take place.
-The blower has a resettable thermal safety switch that monitors vent temperatures. If temperature on vent pipe at the blower exceeds 180, the thermal switch opens (shuts off) the circuit between gas valve and pressure switch. The vacuum switch resets to open position and gas valve thermostat turns off gas supply until vent pipe cools, after which the heat cycle begins again. Repeated failures in the ingnition and heating sequence are recorded by the gas control valve. When errors exceed parameter, the gas control valve issues error code.

-If temperature of water inside tank exceeds high limit set point of thermostat, then gas control valve turns off all gas. Older style gas control valve may require replacement. Newer gas control valves can be reset (with exceptions) once repairs have been made to correct cause of overheating. If the thermostat comes out of calibration, and is no longer performing, it requires replacement of gas control valve.

-When temperature inside tank reaches the set point, for example 120F, the gas control valve thermostat turns off gas to burner, and a moment later turns off blower. The water heater goes into standby mode until water temperature drops below thermostat set point again.
Resources:
Intellivent wiring
How to troubleshoot gas water heater
Troubleshoot power vent water heater
Troubleshoot power vent gas valves
14 things needed for gas water heater & .pdf resources
What you need for operation of gas water heater.
Troubleshoot gas water heater
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
Additional links
gas water heater timer Gas water heater timer
Fits Honeywell, White Rodgers, Robert Shaw
Both Propane and Natural gas models

Resource:
Gas water heater timer
Gas and electric water heaters Support the economy:
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50 gallon Gas water heaters at Amazon
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How to install gas water heater
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Troubleshoot gas water heater
Troubleshoot electric water heater
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