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How to test water heater thermocouple
Test thermocouple/ pdf
Test thermocouple and thermopile/ pdf
Full service manual/ pdf
How to replace thermocouple
How to troubleshoot pilot light going out
How to troubleshoot gas water heater
Buy:
Thermocouples at Amazon
Thermocouple adapters
Gas water heater thermostats at Amazon
Gas control valves Mechanical gas control has thermocouple
Resources:
How to replace thermocouple
Troubleshoot/ service manual/ by type of gas control
How to replace gas control valve thermostat
Buy:
Thermocouples at Amazon
Thermocouple adapters
Gas water heater thermostats at Amazon
Electronic gas control has thermopile/ no thermocouple
Thermopile is several thermocouples together, so voltage produced by heat of pilot is large enough to power electonics inside gas control.
Resources:
Test thermocouple and thermopile/ pdf
How to replace gas control valve thermostat
Buy:
Gas water heater thermostats at Amazon
Water heater gas control thermostat
Larger image
Identify thermocouple: copper tube below gas control
If gas water heater has standing pilot light, then copper tube will be connected to the gas valve.

Thermocouple cannot have kinks, but copper can be rolled if thermocouple is too long.
Thermocouple is finger-tight plus 1/4 turn.
Overtigntening thermocouple will short electrical signal to gas control valve.

Resources:
How to replace thermocouple
Resources for replacing thermocouple
How to troubleshoot pilot light going out
Buy:
Gas water heater thermostats at Amazon
With the mechanical gas control valve thermostats, the pilot flame heats the end of a thermocouple. A thermocouple is 2 dissimilar metals joined together that produce small current when heated. The small electrical current travels from the thermocouple though hollow tube with a wire inside, and screws into to bottom of the control valve, where it connects to an electromagnet.
If the pilot light is ON, the thermocouple is working and connected correctly, and sitting in the pilot flame, then the electromagnet is energized. The electromagnet holds open the safety valve as long as the pilot flame is heating the thermocouple.
If the pilot light is out, the electromagnet closesa valve, and no gas can enter gas contrtol. When pilot is re-lighted, the magnet makes audible sound that can be detected when troubleshooting.

If the pilot flame is extinguished, it can take up to 180 seconds for the thermocouple to cool sufficiently for the electromagnet to de-energize and close the safety valve.
The safety valve controls whether gas flows into gas control valve. If safety valve is open, then gas valve operates normally, and releases gas continually to the pilot light through the pilot gas regulator valve, and releases gas to burner through the main regulator valve when thermostat probe detects water temperature is below set point as selected on dial located on front of gas valve.

With the electronic gas control valve, the pilot flame heats a thermopile instead of thermocouple. The thermopile works on same principle as thermocouple, except is larger and creates more current than thermocouple, enough current to power a circuit board inside the gas control valve. Unlike thermocouple, the current from thermopile travels to gas control valve through two wires that connect to front of gas control. The circuit board controls a series of self-check routines, error compliations of water heater operation, etc and uses electromagnets to control flow of gas that enters the gas control, and gas going to pilot and burner. See basic gas water heater parts

If no current is produced by thermocouple or thermopile, then gas to the valve is shut off until pilot is re-lit or defective thermocouple or thermopile replaced.

The temperature-reading thermostat on a gas valve is located inside a copper tube that protrudes into tank and reads water temperature. When water temperature drops below selected thermostat setting, then the gas valve releases gas through the manifold tube and into the burner located in combustion chamber.
How to adjust temperature

Gas flows out of the burner where pilot light ignites gas. If burner is dirty, sooted or obstructed, the gas may not light immediately, resulting in unburned gas building up until it reaches the pilot flame causing explosion hazard.
Sooted and dirty burner and yellow flame indicate this problem. After cleaning the burner and the combustion chamber, the gas should burn blue with bits of red and yellow.
Clean burner
Thermocouple: Initial test
1) Light pilot.  Hold down pilot button for 30 to 60 seconds.
2) Relase pilot button.
3) If pilot light goes out when button is released, then thermocouple is most likely at fault.
If pilot light stays on without problems as long as gas valve is in PILOT position, but you then turn gas valve to ON position and main burner flutters and disappears within 30 seconds then 99% chance FVIR system is tripped
Resource:
Read about TRD/ FVIR
With the pilot flame lit, turn OFF the gas supply and start to count seconds (one-one thousand, two-one thousand).
Count for a full twenty (20) seconds. The pilot flame should be out.
Listen carefully for a small clicking noise at the gas inlet side of the valve.
If you count for the full twenty (20) seconds and do not hear a click then the thermocouple is fine.
If you hear a click within the twenty (20) seconds then the thermocouple and/or gas valve are bad.
Replace both the thermocouple and gas valve.
How to test thermocouple Remove thermocouple from gas valve using 7/16" wrench.
Thermocouple is finger tight + 1/4 turn
Some thermocouples are reverse threaded.
Test thermocouple using multimeter:
Thermocouple has 2 ends: one end screws into gas valve, other end is positioned in the pilot flame.
1) Make sure thermocouple is positioned correctly in pilot flame.
2) Disconnect thermocouple from the gas valve thermostat using 7/16” wrench.

3) Using multimeter with alligator clip leads:
Attach red lead to the body (copper part) of the thermocouple.
Attach black lead to the end (silver part) of the thermocouple that connects to thermostat.
4) Follow instruction to light the pilot and watch voltage readings on multimeter.
5) If thermocouple is good, then after 45 seconds the meter should read 12 millivolts or more.
6) Clean end of thermocouple and clean inside gas valve connection before reattaching thermocouple.
7) Do magnet test below. A marginal thermocouple can still produce the millivolts – but only when it’s not under load.   When it’s under load (connected to the valve and trying to operate it) it might not work.
Robert shaw thermocouple MAGNET ASSEMBLY TESTING (Robertshaw Control)
Step 1. Disconnect thermocouple fromcombination thermostat/gas valve.
Step 2. Connect a thermocouple adaptor
(Robertshaw P/N 75036) at the thermocouple location
in the combination thermostat/gas valve.
Step 3. Reconnect thermocouple to adaptor. Make certain all connections
are tight (finger tight plus ¼” turn).
Step 4 Using amultimeter capable of measuringmillivolts, connect one
alligator clip to the set screw of the adaptor and the other alligator
clip to copper portion of the thermocouple.
Step 5. Following the lighting instruction label on the heater, proceed to light the pilot and allow to operate for
three minuets.
Step 6. With ameter reading of 13 millivolts or greater, rotate knob of combination thermostat/gas valve to the
“OFF” position.
Step 7. The magnet should remain closed for a drop of at least 6 millivolts. You will here a “snap” or “click” sound
when the magnet opens, if you hear this sound prior to a drop of 6 millivolts, the magnet is out of
specification and the combination thermostat/gas valve should be replaced.
Test White Rodgers gas control valve
Larger image
MAGNET ASSEMBLY TESTING (White Rodgers Control)

Step 1. Following the lighting instruction label on the heater, proceed to light the pilot and allow to operate for
three minutes. If the pilot will not stay lit, hold the pilot button (located on the combination
thermostat/gas valve) down during this test
Step 2. Using amultimeter capable of measuringmillivolts, connect one lead
using an alligator clip to the copper sheath of the thermocouple, use the second lead of themulti meter to
probe the top terminal located at the back of the combination thermostat/gas valve.
Step 6. With ameter reading of 13 millivolts or greater, rotate knob of combination thermostat/gas valve to the
“OFF” position.
Step 7. The magnet should remain closed for a drop of at least 6 millivolts. You will here a “snap” or “click” sound
when the magnet opens, if you hear this sound prior to a drop of 6 millivolts, the magnet is out of
specification and the combination thermostat/gas valve should be replaced.
Multimeter Typical multimeter will work/ might not have alligator clips.
Without alligator clips you may need extra hand to keep pilot flame lit during test.

Buy:
Multimeters at Amazon
Buy non-contact voltage tester at Amazon
Electric testers at Amazon
Thermocouple
Gas burner without piezo igniter
General information about thermocouple
1) CLEAR BLUE FLAME: Pilot Flame and Burner Flame must be clear blue color.
Other colors such as orange and yellow indicate that burner parts, pilot orifice, and combustion chamber need to be cleaned.
Troubleshoot by type of gas control

2) Water heater needs oxygen for combustion. If pilot is going out, then clean air intake and get more fresh air to water heater to see if problem is solved.

3) Thermocouple must sit in the pilot flame so it will stay hot.
Heated thermocouple sends small electric current to gas control valve.
If gas control is defective, or thermocouple is bad, or pilot blows out, or pilot flame is weak, or thermocouple has fallen loose, then gas control valve turns off pilot.

4) Re-install thermocouple: finger tight + 1/4 turn.
If thermocouple is too tight, the insulator is crushed and the thermocouple shorts out against the gas valve. The gas valve cannot read the current and shuts gas off to pilot.

Resources:
Read step to replace themocouple
Troubleshoot by type of gas control resources
How to replace thermocouple/ Utube
How to replace thermcouple 2/ Utube
How to replace thermocouple pdf

Burner is sometimes removed to replace thermocouple.
In that event, go ahead and clean combustion parts and combustion chamber.
If LP gas is used, then manifold tube has reverse threads.
Be careful of reversed threads.
When re-attaching burner tubes to gas control, use fingers to start the threads.
Always check pilot and manifold tubes for gas leak using soapy water.

Buy:
Thermocouples at Amazon
Thermocouple adapters
Gas water heater ECO ECO located inside thermostat probe on gas control
Gas water heater ECO / Energy Cut OFF
If you can get PILOT or MAIN BURNER to light – even for a couple of seconds, then the ECO if good.
Otherwise replace gas control valve.
Resource
How to replace gas control valve thermostat
Pilot flame troubleshoot Troubleshoot pilot flame
Tip of thermocouple or thermopile is 3/8" to 1/2" into pilot flame

If thermocouple or thermopile is not in the flame, or if pilot goes out, or if thermocouple is bad, then no electrical signal will reach gas control valve, and gas control will shut off gas to pilot and burner.
Each gas water heater has ECO and TCO
Energy Cut Off and Thermal Cut OFF
ECO is single-use and gas control valven thermostat must be replaced if ECO senses water temperatures above 180 degrees.
TCO is located on front of combustion chamber, and trips when heat inside combustion chamber exceeds limit.
Rheem/GE/ Richmond water heaters have TRD instead of TCO. TRD is glass vial located inside combustion chamber that breaks and must be replaced if temperature inside combustion chamber exceeds limit.

Each gas water heater under 60,000 BTU also has FVIR safety system
Resources:
Read about TCO, TRD, FVIR
Adjust temperature on gas water heater
Troubleshoot gas water heater
How to replace gas control valve thermostat
Buy:
Gas water heater thermostats at Amazon
Icon Honeywell gas valve Honeywell gas control: has thermopile
Gas control monitors Thermostat Temperature sensor. After setpoint temperature is satisfied, the gas control is shut down, and power removed from relay. The gas control LED flashes steady flash each 4 seconds to indicate proper operation..
If flame is lost, the pilot and main valves are shut down. The gas control attempts to re-light the pilot 4 times. If unsuccessful, then power is removed from relay, and the gas control proceeds to a 5 minute lockout. The gas control re-attempts to light the pilot at next normal heating sequence.

Pilot assembly is replaced on this type of gas valve.

Resources:
Service manuals at Bradford White
Service manual for the Honeywell Gas Control Valve
Honeyell gas valve parts list/ pdf
How to replace WV8840 Honeywell gas control valve
Troubleshoot by type of gas control
Buy:
Honeywell gas control at Amazon
Honeywell gas control at Amazon
Gas water heater thermostats at Amazon

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