How to Replace a Thermocouple on a Standing Pilot Water Heater
Why this matters
"The pilot won't stay lit" is one of the most common gas water heater calls, and the thermocouple is the most common cause. It is also the part most often replaced when it is not the cause, because it is cheap and fast. The tech swaps it, the pilot holds for a day, and the callback reveals a dirty pilot orifice, a pilot flame that never reached the tip, or a gas control whose magnet has failed. This HowTo replaces the thermocouple properly and, more important, proves before and after that it was the fault and that the new one is doing its job.
Gas work is done only where your plumbing license covers gas appliances, which varies by state and locality; where it does not, it goes to the licensed gas fitter. The appliance shutoff and connection fall under the fuel gas code in force (IFGC or NFPA 54, as adopted).
What the thermocouple does
A thermocouple is two dissimilar metals joined at the tip. Heated by the pilot flame, the junction generates a small voltage, measured in millivolts (mV). That voltage flows down the lead to an electromagnet in the gas control, which holds the pilot safety valve open. When you hold the pilot button down, you are holding that valve open by hand until the tip gets hot enough to take over. If the pilot goes out, the tip cools, the voltage drops, the magnet releases, and the gas to both pilot and burner shuts off. It is the heater's flame safety.
So "pilot won't stay lit after the button is released" has three main causes, in the order of how often they show up: a thermocouple that no longer makes enough voltage, a pilot flame that does not heat the tip properly, and a gas control whose magnet will not hold on good voltage. On a sealed combustion chamber heater there is a fourth: a tripped thermal cutoff or flammable-vapor sensor in the thermocouple circuit, which opened for a reason that has to be found.
The pilot assembly
PILOT FLAME, blue, wraps the
top 3/8 to 1/2 in of the tip
____
PILOT HOOD -> ( )
directs flame ( () )<- THERMOCOUPLE TIP
across tip and (_||_) (hot junction)
toward burner [hood]|
|| |
PILOT BRACKET ===++==+=== on the burner
|| |
|| +--- thermocouple lead to
|| gas control, snug
|| plus a quarter turn
++------ pilot tube from gas
control (gas feed)
The pilot hood throws the flame sideways across the thermocouple tip and on to the main burner, so it lights the burner and heats the tip at the same time. A tip pushed down out of the flame, or a flame lifting off the hood, makes the same symptom as a failed thermocouple.
Before you start
Gas safety first. If you smell more than the faint trace of a recently extinguished pilot, everyone leaves the house, nobody operates a switch, a light or a phone inside, and the call to the gas utility is made from outside. Wear a personal CO monitor for the whole job.
Have on hand: a thermocouple of the length and fitting the gas control calls for (or the maker's burner assembly kit on a sealed-chamber heater), a new inner door gasket if the heater has one, open-end wrenches for the control fittings, leak-check solution or an electronic detector, a digital meter with a millivolt range, a thermocouple adapter for closed-circuit testing if you have one, and the lighting label on the heater, which is your procedure for relighting.
Check before taking it apart: light the pilot per the label and watch it. Is the flame blue and steady, and does it wrap the tip? A yellow, lazy or small flame usually means a dirty pilot orifice or a partly blocked air intake, and a new thermocouple will not fix it. If the flame is good, measure the thermocouple as described under Finish test before replacing anything. A low reading on a good flame condemns the thermocouple; a normal closed-circuit reading on a pilot that still drops out points at the gas control (a normal open-circuit reading alone does not clear the thermocouple).
The procedure
Turn the gas control knob to OFF and close the appliance shutoff. The burner is hot; let the chamber cool so you are not reaching past a red-hot burner and tube. Reason: you are about to break gas fittings at the control. Skip the shutoff and the control is the only thing between you and line pressure.
Disconnect the three lines at the gas control: the main burner manifold tube, the pilot tube and the thermocouple. Use a backup wrench on the control body for the manifold fitting so the control is not twisted. Reason: the burner assembly comes out as one piece on most modern heaters, and pulling it with a line still attached kinks the pilot tube.
Remove the burner assembly. On an older open-chamber heater, slide the outer door off and pull the burner out through the opening. On a sealed-chamber heater (the flammable vapor ignition resistant design listed under ANSI Z21.10.1), remove the inner door screws and pull the door with the assembly. Inspect the burner ports and the pilot hood for rust and lint while it is out. Reason: you cannot set the tip correctly or see the flame path with the burner in the tank.
Swap the thermocouple. Release the old one from the pilot bracket (a clip or a nut) and seat the new one in the same bracket position. Reason: the bracket sets the tip's height in the flame. Some sealed-chamber heaters do not sell the thermocouple separately and require the maker's whole burner or pilot assembly; follow the maker's parts list rather than adapting a universal part, since the maker's instructions are part of the listing.
Set the tip in the flame position. The tip sits where the pilot hood will throw the flame across its top 3/8 to 1/2 in, at the same height the old one sat. Route the lead without sharp kinks and away from the burner face. Reason: too low and the tip heats only at its edge and makes weak voltage; a kinked lead can crack internally.
Reinstall the assembly with the chamber sealed. On a sealed-chamber heater, fit a new inner door gasket if the maker calls for one, and seat the door so it seals all round. Reason: the seal is part of the flammable-vapor protection, and a gap lets the burner draw air from the floor around the heater, which is what the design is meant to prevent.
Reconnect the lines at the gas control. Thread the thermocouple fitting into the control by hand, then snug it with a wrench, typically a quarter turn past finger tight, or as the gas control maker states. Reason: the thermocouple fitting makes an electrical contact, not a gas seal, and overtightening crushes the insulator in the control's power unit, which ruins the control. Reconnect the pilot tube and manifold tube with a backup wrench.
Open the appliance shutoff and relight per the label. The label's sequence (knob to OFF, wait the stated time for gas to clear, knob to PILOT, hold, ignite, hold for the stated time, release) is the procedure; follow it exactly.
Leak check the pilot tube and manifold fittings with solution or a detector, with the pilot lit and then with the main burner firing. Growing bubbles mean a leak: knob to OFF, appliance shutoff closed, remake the fitting, retest. Never use a flame to look for a leak.
Turn the control to the customer's setting and watch the main burner light from the pilot and burn blue.
Finish test
The finish test is the pilot holding after release, backed by a number.
Pilot holds. After the hold time on the label, release the button. The pilot stays lit. Cycle the main burner on and off with the temperature dial at least twice; the pilot stays lit through both.
Measure the output against the maker's figure. Two readings are possible:
- Open circuit. Disconnect the thermocouple at the control, keep the pilot lit by holding the button, and read between the lead's center contact and its copper sheath. Healthy thermocouples commonly read in the 20s of millivolts open circuit, but this reading only shows that the junction works, not that it delivers enough current under load.
- Closed circuit. With a thermocouple adapter installed between the lead and the control, read with the pilot holding itself. This is the reading the gas control maker publishes a minimum for, and it is the one that decides the part is good. Use the figure in the gas control's or heater's service literature, not a remembered number.
Write both the reading and the maker's figure on the ticket.
Then the flame check. Blue, steady, wrapping the top 3/8 to 1/2 in of the tip, no lifting off the hood when the main burner lights.
Worked example: the pilot that dropped every morning
A 40 gallon standing-pilot heater, sealed chamber, about nine years old. The customer relit the pilot twice in a week; each time it held for a day, then went out.
Before taking it apart. The plumber lit the pilot per the label. The flame was blue and wrapped the tip properly, which ruled out a dirty orifice and tip position. With the button held, the open-circuit reading at the disconnected lead was 9 mV (illustrative). The maker's literature for the new part listed a far higher open-circuit figure, and 9 mV is well below the 20s of millivolts a healthy thermocouple commonly makes. The thermocouple was condemned on a reading, not on the complaint.
Thermal cutoff check. Because this is a sealed-chamber heater, the plumber checked the thermal cutoff in the thermocouple circuit for continuity: closed. Had it been open, the question would have been why the chamber overheated (a plugged air intake screen, lint, a flammable vapor event) before resetting or replacing anything.
Repair. Gas control to OFF, appliance shutoff closed, ten minutes to cool. Three lines off at the control. Inner door and burner assembly out; lint brushed from the intake screen. The maker sold the thermocouple separately for this model, so the new one went into the bracket at the old height, tip positioned for the flame to wrap its top. New inner door gasket, door seated. Thermocouple snugged a quarter turn past finger tight; pilot and manifold tubes reconnected with a backup wrench.
Start-up. Lit per the label. Pilot held on release. Solution on the pilot and manifold fittings, pilot only and then with the burner firing: no bubbles. Closed-circuit reading through the adapter was above the gas control maker's published minimum, and the reading and minimum were both written on the ticket. The burner cycled twice from the dial with the pilot holding.
Why the readings mattered. If the old thermocouple had read normally closed circuit with that good flame, the fault would have been the gas control's magnet, and a new thermocouple would have held for a day and failed the same way. The before reading is what made the repair a diagnosis.
When the thermocouple is not the answer
- Good flame, good closed-circuit reading, pilot still drops: the gas control's power unit has failed. That is a gas control replacement, with the water drained below the control opening first.
- Pilot will not light at all: no gas to the pilot (shutoff, a plugged orifice, a kinked tube) or a dead igniter on a piezo-lit pilot. Different repair.
- Pilot holds but the burner will not light: the control's thermostat side, not the flame safety.
- Pilot goes out only when the burner fires or a door opens: a draft or a combustion air problem. Check the room against Combustion Air Requirements Reference and the venting against Atmospheric Water Heater Vent Connector Rules.
References
- Water heater and gas control manufacturers' service instructions and lighting label, which are part of the listing
- ANSI Z21.10.1, the standard for storage water heaters under which flammable vapor ignition resistant designs are listed, as referenced by the adopted fuel gas code
- IFGC 2021 or NFPA 54, as adopted
- See related: Anatomy of a Tank Water Heater and Its Connections
- See related: Combustion Air Requirements Reference