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Troubleshoot flammable vapor on gas water heater
Troubleshoot FV system
Troubleshoot FVIR
Troubleshoot thermal switch on water heater
Whirlpool ECO thermal switch
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Thermal reset
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3) troubleshoot thermal cutoff  ... TCO
Located on front of combustion chamber near the sight glass, with two wires that connect to gas control valve.
Thermal cutoff trips when combustion chamber get too hot. Prevents overheating and is part of the FV system: 
Push inward to reset, or replace part as needed.
Tripped TCO stops all gas from reaching the pilot light or the burner... it causes a complete shut down of gas control valve, and no gas reaches the combustion chamber. If there is a pilot light burning, or flutter of gas on the burner, the TCO is not tripped, then gas control valve is suspect.
Causes of tripped TCO are dirty combustion parts, clogged vent... these things cause overheated combustion chamber.
Solution: Push in button to reset, replace TCO, clean burner parts, check vent is drafting air.

Is the thermal switch bad? For example TCO trips when combustion chamber is not hot. Test the thermal switch. Temporarily jumper a wire across the two terminals and see if water heater functions.
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Thermal reset


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Flammable sensor kit
Back to 10 things for troubleshooting water heater
4) troubleshoot flammable vapor sensor/ variety of systems
Primary issue for FV is exposure to explosive flammable vapors that can be drawn into combustion flame, causing flashover that travels from inside the water heater and accelerates back into surrounding room. This potentially deadly situation has caused fatalities, injury and property loss. Flammable vapors are commonly found in oil paint, shellac, varnish, adhesives, gasoline. Turn off all pilot lights when working with flammable materials. Do not store flammables in same area as water heater.
FV lockout reduces risk of fire either by turning off gas control valve or limiting exit of flame from combustion chamber.
Some water heaters have external vapor sensor (shown in illustration on left), and some have internal FV system (shown below), or both.
External vapor sensor can be seen attached to front of water heater, near the bottom as pictured on left.
With this type FV sensor: If water heater has no gas, then FV is suspect: test and replace part. If water heater has some gas but flame flutters out then internal FV might be suspect, or air intake clogged ... keep reading and then check #6 gas control valve plus #7 #8 #9.
How the external FV sensor operates: Two wires connect the vapor sensor to the gas control valve, the sensor 'shorts out' when exposed to gasoline vapors, and shuts down gas valve during vapor event.
Buy:
Flammable sensor kit
Resource:
How FV sensor works
How to test FV sensor
American FVIR bulletin
FVIR troubleshoot guide
FV sensor If flammables materials are used, then all gas burning appliances in the vicinity must be shut off and pilot lights extinguished. Open doors and windows for ventilation while flammable substances are in use.
The FV sensor is located inside a plastic case to keep it dry and clean... if the sensor is blocked, or has been covered with dust, leaves, water etc, it will not function. Replace part.
The FV sensor must have continuity, so a small current of electricity can pass through the sensor and signal to gas control valve that all is ok.
Failure of part, or FV event will 'short out' the sensor and shut off gas. Dirty environment will cause problems with all modern gas appliances. Replace the part and clean out water heater as described farther down on page.
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Flammable sensor kit
Rheem TRD Rheem thermal device:

Buy:
Rheem TRD/ thermal device

Resource:
Verify activated TRD pdf
How to repair TRD
Raise water heater 18" off floor
Raise water heater 18" off floor to avoid flood damage and flammable vapors
Buy:
Water heater stand
4) Flammable vapor Lockout varies by brand, year and model.
When lighting water heater, if you have NO gas or pilot light but burner goes out when knob turned to ON ... then FV system is suspect.
Some water heaters have FV system that is tripped by variety of different problems including flammable vapors.
a) Push in TCO located on front of combustion chamber   See #3 above for TCO.
b) Keep air intake screen, or flame arrestor screen clean.  See #2 above. Dust and dirt can clog screen. Use feather duster, used compressed air. Install heater in clean environment free from dust and chemicals. Laundry room is not good location for FV water heater.
When arrestor screen is clogged, air flow slows down. Same amount of gas is being burned but heat is retained inside combustion chamber instead of venting up flue. The overheating trips FV system.
Solution for FV lockout event varies by brand and model, and by gas control valve. Try cleaning arrestor screen first. Push in TCO reset on front of combustion chamber.
Gas control valve may need to be replaced. FV sensor may need to be replaced. Part replacement depends on model and brand.
c) Keep combustion parts and chamber clean. Lazy yellow flame, instead of clear blue flame, and sooting on the burner surface indicate incomplete combustion. Explosive gas can accumulate. Can trip FVIR.
d) To prevent FV lockout and combustion problems: Perform maintenance shown in product manual, and keep air intake clean, allow plenty of fresh air, avoid dusty-dirty environments, and avoid flammable vapors.

Help prevent FVIR event by installing gas water heater at least 18" off the floor.
#1) Make sure there is adequate fresh air, and that air is clean and list free, and does not have trace chemicals.
#2) Always turn off gas to water heater when painting with oil-base paint or oil-base varnish etc, and when cleaning with solvent-based products, and when pouring gasoline etc.
For example when storing gasoline near water heater, the water heater should sit minimum 18" off floor, gasoline should be tightly capped, and do not pour gas to fill gas tank in same room as water heater or other gas appliance.
water heater stands Water heater should sit minimum 18" off floor
FV system is not 100% certain to protect from flammable vapor fire.
Gasoline vapors are heavier than air and generally do not rise above 18" in a typical ventilated room. Exceptions and dangers exist with all gas appliances and precautions should be taken.
Buy:
Water heater stand
FV lockout Rheem-Ruud-Richmond-GE
Air intake on side.
Some models use TRD glass vial trigger for overheating event. See image
Arrestor screen located under combustion burner.
Locate arrestor screen by removing burner
Clean arrestor screen with compressed air after removing burner. Look for broken glass.
Company might send glass vial replacement.
Reset TCO thermal reset on front of unit.
Rheem does not post service manuals
Buy:
Rheem water heaters at Amazon
FV sensor kit
Resources/ across all brands:
How to repair Rheem FVIR
FVIR intellivent troubleshoot .pdf
Bradford White service manual .pdf
Bradford White 2 .pdf
General FVIR troubleshoot .pdf
FV lockout AO Smith-American-State- Craftmaster-Reliance-Kenmore-Whirlpool
Air intake and arrestor screen are same and located under combustion burner on bottom of water heater. Best location for ez cleaning. See image No need to remove combustion parts to clean screen. Clean arrestor screen with feather duster/ compressed air.
Arrestor screen stops acceleration of flames into room.
Set 3-legged water heater on bricks for ez access to arrestor screen.
Reset TCO thermal reset on front of unit
Clean screen and reset TCO will usually restore FV lockout
AO Smith posts service manuals.
Service manuals also posted with gas control valves lower on this page
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AO Smith water heaters at Amazon
Reliance gas water heaters
50 gallon high efficiency
Bradford White-Boch-Lochinvar
Air intake on side. ScreenLok flame arrestor screen located inside. Image
Arrestor screen stops acceleration of flames into room.
Arrestor screen located under combustion burner.
Locate arrestor screen by removing burner.
If dust and dirt collects on arrestor screen over timer, clean with wire brush and compressed air after removing burner, and this will usually restore FV lockout.
TCO thermal reset on front of unit will cut off gas to pilot and gas control valve. Reset.
Bradford Posts service manuals.
Service manuals also posted here and also with gas control valves lower on this page
Buy:
Bradford White water heaters at Amazon
Bradford White water heater parts
FVIR Resources
How ScreenLok flame arrestor works
Bradford white FV troubleshoot
Bradford white power vent FV troubleshoot

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

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