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What does a TCO do?
How to test water heater thermopile
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Search using
Waterheatertimer on Twitter |
What
is a TCO? What does a TCO do? How to test water heater thermopile |
 Thermal cutoff (TCO)
is mechanical bi-metal disc thermostat, also called a temperature
sensor,
thermal switch, thermal limit switch ... that operates as a single-pole
switch to turn a circuit on or off depending on design specification.Bi-metal means two different pieces of metal are joined together. Each piece of metal expands and contracts differently when exposed to change in temperature. The difference in expansion of two joined metals, causes them to bend. The bending motion, or deformity of metal can be used to switch a circuit ON and OFF. TCO are used for a variety of applications, responding to high or low temperatures. TCOs are reliable, reasonably accurate, and long lasting. For example TCOs respond to high temperature on a water heater, but a different TCO will respond to low temperature on a freeze switch. Make a freeze switch A typical residential water heater has a TCO located on front of combustion chamber. This TCO is specified to turn off the low voltage thermopile circuit when temperature inside combustion chamber exceeds 160°F to 180°F depending on heater. On commercial heaters, including boilers, booster heaters etc that require a higher range water temperature, then the TCO might be designed to trip off at 210°F. 3 types of TCO 1) Some bi-metal thermostats reset automatically. For example on a power vented heater, there is a thermal switch on the vent pipe that shuts down heater if vent pipe exceeds temperature range, and then automaticzally resets after vent pipe cools sufficiently. 2) A second type, like used for the combustion chamber on a residential water heater, require manual reset if temperatures exceed design. 3) A third type, the single-operation type, trips off and must be replaced. For example, this type TCO is found inside the combustion chamber of residential ultra low NOx heater. This type of thermal switch is also found inside the thermostat tube of mechanical gas valves, where it serves as the single-use high limit or ECO that will trip when temperature of water inside tank exceeds 180-188°F. Keep in mind, all these examples are for high heat TCOs, while there are also thermal switches for low temperature applications, it's just that this page is generally about water heaters. TCO .pdf |
| On
a typical residential gas water heater,
the TCO
is a manually resettable thermal switch located on the front of
combustion
chamber door. The TCO is integral with the thermocouple or thermopile depending on type of gas control valve. The TCO has 3
functions:
1) Respond to flammable vapor ignition 2) Respond to high temperatures caused by inadequate air supply or 3) Respond to high temperatures caused by gas valve malfunction, such as out-of-calibration thermostat etc. Aside from failed gas valve, other common causes of tripped TCO can include exposure to flammable vapors, dirty combustion parts, clogged vent ... these things cause overheated combustion chamber. Solutions: Push in button to reset, replace TCO, clean burner parts, check vent is drafting air. Remove flammable vapors from area, get more fresh air, clean air instke etc. |
 TCO for gas water heater with
thermopile The more recent Honeywell gas valves are powered by a thermopile. The Red and White wires plug into front of gas control valve before dropping down to the TCO and then to the thermopile located inside combustion chamber. Thermopile is heated by pilot flame, causing a small current of electricity that powers circuit board located inside gas valve. TCO trips off when temperatures inside the combustion chamber reach 160°F - 200°F depending on models. When the TCO trips, the circuit board receives no power, and gas valve solenoids shut off and stop release of fuel, and both pilot light and burner flames go out. Resource: Temperature cutoff points for TCO range from 160°F to 200°F depending on model. The pilot has to be relit after the TCO cools down below 120° F |
 The TCO or thermal cut off
is located on front of combustion chamberIf temperature inside combustion chamber exceeds 180°F (160°-200° depending on model), the TCO trips and causes gas control valve to shut off gas to burner. The model shown in picture has electronic Honeywell gas valve with a thermopile. There is no thermocouple on this type heater. The TCO turns off current produced by the thermopile. If the gas control valve is no longer receiving current, it will shut down and turn off gas to pilot and burner. The TCO protects the gas valve thermostat, and prevents water inside tank from exceeding 180° high limit, which would trip the ECO limit resulting in shut down and possible gas valve failure. In this regard, the resettable TCO saves expensive repairs. Resources: How to replace Honeywell gas control valve Honeywell gas control valve blinking 4 times |
 Larger image TCO for gas water heater with thermocouple  The
older mechanical gas valves are not electronic, and have no circuit
board. As a result they operate on a smaller current of electricity
that is produced by heating a thermocouple in the pilot flame.The small current travels from thermocouple through a copper tube that connects on bottom of gas valve. When temperature inside combustion chamber exceeds thermal rating, the TCO turns off power to heater ... but the TCO on a heater with thermocouple cuts off power to water heater in a different way than heaters that have a thermopile. Instead if cutting power to a circuit board, they turn off the electrical current that flows through a second thermal switch called the ECO. Note that on the back of mechanical gas control valves is a thermostat probe that contains a thermister and single-use ECO. The ECO is a type of single-use thermal switch that is designed to respond to a high limit, The ECO thermal switch trips when water inside tank exceeds the 180°F high limit. The ECO is not likely to trip since the TP valve releases water at 210°F, causing fresh cold water to enter bottom of tank next to the gas valve probe.  When the TCO trips, it turns off the ECO wire .. causing the main electromagnet inside gas control to stop receiving electric current, which de-energizes the main electromagnet and shuts off gas to pilot and burner. This will not affect gas valve operationg once problem is resolved, unless the gas valve has come out of calibration or otherwire caused the combustion chamber to overheat. Aside from failed gas valve, other common causes of tripped TCO can include exposure to flammable vapors, dirty combustion parts, clogged vent ... these things cause overheated combustion chamber. Solutions: Push in button to reset, replace TCO, clean burner parts, check vent is drafting air. Remove flammable vapors from area, get more fresh air, clean air instke etc. |
 Larger image |
Image
shows the Honeywell WV8840, WT8840 series gas control valve
... that controls regular-NOx and some low
NOx atmospheric
vented heaters. Atmospheric vent mean no 'blower or motor assisted' venting ... in other words, this type gas valve is not found on flue damper models, power vent, power direct vent models, condensing heaters etc. We know this is not ultra low NOx because the TCO is located on the front of combustion chamber, and not inside the combustion campartment.  For
residential ultra low NOx heaters ... the TCO is located inside
combustion compartment and requires removal of manifold assembly
(burner) from the combustion compartment. See burnerUltra low NOx heaters are designed differently, with different burner that is intended to keep flame temperatures below the threshold of NOx formation. See burner Regular water heaters that are NOT rated for NOx, will release approximately 54 ppm (parts per million) per BTU hour of operation. Low NOx (20 ppm per BTU) burners. Ultra Low NOx (14 ppm per BTU for burners up to 75,000 BTU) All heaters with 75,000+ BTU must be ultra low NOx by federal mandate. |
  How
to test thermopileSource: Bradford White "Open circuit Thermopile testing (Honeywell Gas Control) The following test is performed with pilot flame on. Step 1. Turn knob to pilot position and depress. Step 2. Continue pressing knob and remove red (+) wire from resettable thermal door switch. Step 3. Using a multimeter capable of measuring millivolts, connect the positive side of the multimeter to the terminal of the resettable thermal door switch. Connect the negative side of the multimeter to any earth ground location (jacket base, screw, etc.).  Normal thermopile operation will be between 350mV - 850mV. If reading is less than 350mV, replacement of pilot assembly is recommended Step 4. If thermopile reading is between 350mV - 850mV, remove multimeter and reconnect red wire to positive (+) terminal of Gas Control. Step 5. Release Gas Control knob and turn to desired setting to resume normal operation."  Note:
your water heater jacket may not be grounded ... bring over 3-prong
extension cord and insert multimeter Negative probe into ground prong |
|
How to test TCOIs the thermal switch bad? For example TCO trips when combustion chamber is not hot. Test the TCO thermal switch. Temporarily jumper a wire across the two terminals and see if water heater functions. Buy: Thermal reset Resource: How to test thermopile |
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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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