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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


Thermocouples at Amazon
Thermocouple adapters
Gas water heater thermostats at Amazon
Gas control valves

Mechanical gas control has thermocouple

How to replace thermocouple
Troubleshoot/ service manual/ by type of gas control
How to replace gas control valve thermostat
Thermocouples at Amazon
Thermocouple adapters
Gas water heater thermostats at Amazon
Electronic gas control has thermopile
Thermopile is several thermocouples together, so voltage produced by heat of pilot is large enough to power electonics inside gas control.
Test thermocouple and thermopile/ pdf
How to replace gas control valve thermostat
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.

How to replace thermocouple
Resources for replacing thermocouple

How to troubleshoot pilot light going out
Gas water heater thermostats at Amazon

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
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.

Typical multimeter will work/ might not have alligator clips.

Without alligator clips you may need extra hand to keep pilot flame lit during test.

Multimeters at Amazon
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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.

More 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.

Thermocouples at Amazon
Thermocouple adapters
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
Read about TCO, TRD, FVIR
Adjust temperature on gas water heater
Troubleshoot gas water heater

How to replace gas control valve thermostat
Gas water heater thermostats at Amazon
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.
How to replace gas control valve thermostat

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.

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
Honeywell gas control at Amazon
Honeywell gas control at Amazon

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All Costs are energyso total energy-cost equals cost of water heater, plus installation, plus expected repairs-and-parts, plus water softener-and-filter, plus disposal-recycling cost, divided by years of tank life. These numbers are added to monthy energy cost to measure true energy cost & efficiency of a water heater.
Government/industry efficiency numbers consider only energy cost per gallon of heated water.

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