Thermocouple and Standing Pilot Service

Purpose

This standing instruction makes a pilot-outage call end with two millivolt readings and a corrected pilot flame, rather than with a thermocouple in the bin and the same call in three weeks.

A millivolt pilot safety is one of the few circuits in this trade you can measure end to end with a meter and nothing else, because the thermocouple generates its own power. That makes the diagnosis decidable: a thermocouple that produces a healthy open-circuit voltage and collapses under load has a resistance problem in the connection or the magnet, not a worn element. Swap the part without those two readings and you have replaced the one component you had evidence was working.

Safety actions that gate the work

• If the space smells of gas, nobody relights anything. Everyone leaves the building immediately, touching no switches, no lights and no phone inside, and calls the gas utility emergency number and 911 from outside. Nobody re-enters until the utility clears it.
• Follow the lighting instructions on the appliance label, including its wait time before relighting, so unburned gas has cleared. Propane pools in a pan, a pit or a crawl, so on LP give it longer and check low with a combustible gas detector before an ignition source goes near the pilot.
• Relighting puts your hand and face at an open gas orifice with an ignition source. Light from the side rather than over the top of the burner box, keep the space ventilated, and never hold the pilot button down while reaching past a burner already lit.
• Personal carbon monoxide monitor on before you enter the mechanical space, with the ambient reading recorded with the appliance off, and prove the control circuit dead before opening any line-voltage compartment, proved on a known live source before and after per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2).
• Cleaning the pilot assembly is the disturbance step. Blowing it out with compressed air puts soot and scale into your breathing zone; use a HEPA-filtered vacuum at the deposit and a soft pilot cleaner. Never drill or ream a pilot orifice, because enlarging it changes the pilot rate the appliance was listed with.

Scope

Covers standing pilot ignition on residential and light-commercial gas heating equipment: thermocouple-and-magnet safeties, thermopile-powered millivolt systems, and standing pilots feeding a 24 V control, from the outage through the verified dropout test. Does not cover intermittent and hot surface ignition, owned by the ignitor and flame sensor replacement SOP, or the gas valve itself, owned by the gas valve replacement and verification SOP. Does not cover venting, owned by the flue and venting inspection SOP.

Roles and handoffs

Procedure

1. Identify what kind of millivolt system you are on before you touch a meter. Acceptance: written as thermocouple-and-magnet with a manual gas cock, thermopile-powered with a millivolt thermostat, or standing pilot proving a 24 V control, plus the valve model recorded. Wrong looks like reading a thermopile's expected output against a thermocouple's, which are different circuits with different numbers. Stop rule: a valve whose literature you cannot obtain is measured against the ranges commonly published for its type and flagged as such on the ticket. Hazard: none at this step, the appliance is off and nothing is opened.

2. Observe the pilot before you correct anything, and write what you saw. Light per the appliance label and watch. Acceptance: flame recorded as soft blue and steady, or as lazy, yellow-tipped, lifting or wavering, plus where it sits on the thermocouple; the geometry commonly specified is a soft blue flame enveloping roughly the top 3/8 to 1/2 inch of the tip, so read the appliance's own instructions where they state one. Wrong looks like correcting the flame first and losing the evidence. Stop rule: a pilot that will not stay lit long enough to observe goes to step 4 with that noted. Hazard: this is a relight, so light from the side, space ventilated, label wait time observed, monitor in your breathing zone.

3. Name the failure mode, because the three point at different causes. Acceptance: recorded as will not light at all, lights but drops out when the button is released, or holds pilot but drops out when the main burner fires. The third points at draft or gas pressure sagging under main burner load, not at the safety circuit. Wrong looks like treating all three as one and going straight for the thermocouple. Stop rule: a pilot that drops only when the main burner lights sends you to inlet gas pressure and the venting SOP before any part is ordered. Hazard: made with the appliance firing, so stand clear of the burner face and keep the monitor on.

4. Measure open-circuit output with the pilot lit and the thermocouple disconnected from the valve. Meter in DC millivolts across the thermocouple lead and its sheath, pilot burning steadily for at least 30 seconds first. Acceptance: a stable reading inside the range commonly published for a healthy thermocouple, roughly 25 to 35 mV, held without decay. Wrong looks like reading before the tip is up to temperature and condemning a good part. Stop rule: an output well under that range with correct flame geometry means the thermocouple is genuinely worn and is replaced. Hazard: the connection is inches from a lit pilot inside a hot burner box, so use gloves rated for the temperature and keep the lead clear of the flame.

5. Measure closed-circuit output with the thermocouple connected and the valve holding. Acceptance: a stable reading at or above the minimum hold the valve's own literature states, read off that document; as a sanity check, a healthy circuit under load typically holds roughly half its open-circuit value. Wrong looks like accepting an open-circuit reading as proof the circuit works, which misses every high-resistance connection. Stop rule: a healthy open-circuit reading with a collapsed closed-circuit reading is a connection or magnet fault, so clean and torque the connection at the valve and re-measure before any part is ordered. Hazard: same hot burner box with a lit pilot, and the meter leads stay clear of the burner ports.

6. Correct the upstream cause, then replace parts only where a measurement condemns them. Acceptance: pilot orifice and filter cleared with a soft cleaner and the HEPA vacuum, the connection at the valve clean and torqued, inlet gas pressure at or above the plate minimum with the main burner firing, and the pilot flame back to the geometry the instructions state. Wrong looks like a new thermocouple on a pilot still starved by a partly blocked orifice. Stop rule: never drill the orifice to fix a small flame; a blocked orifice is cleaned or the assembly replaced. Hazard: the assembly is in the burner box, so confirm the cock is closed before reaching in and clean with the vacuum, not compressed air.

7. Restore, then prove the safety still drops out, and write the file. Relight per the label, run one full call, then close the pilot gas and time the drop-out. Acceptance: the main valve closes within the maximum drop-out time the valve's own literature states, closed-circuit millivolts re-read at or above the stated minimum, flue carbon monoxide air-free against the 400 ppm air-free limit the ANSI Z21 series sets for listed vented appliances or the manufacturer's lower limit, and ambient CO with the appliance firing. Wrong looks like relighting and leaving without proving the safety you just worked on. Stop rule: a valve that will not drop out inside its stated time is a failed safety, so the appliance goes off and is tagged. Hazard: fuel and flame go back with the customer in the building, so stand to the side of the burner face while it lights and keep the monitor on until you leave.

The record this produces

One pilot-system record per visit, attached to the equipment:

• System type and valve model, and the source used for its stated values
• Pilot flame description as found, and the geometry it was corrected to
• The failure mode named as one of the three
• Open-circuit millivolts, and the range judged against
• Closed-circuit millivolts before and after any connection work, and the valve's stated minimum hold
• Inlet gas pressure with the main burner firing, against the plate minimum
• Drop-out time measured, against the valve's stated maximum, and post-repair flue CO air-free with its limit

The next tech reads the closed-circuit history, because a reading drifting down over two visits on a clean connection is a magnet on its way out. The manager reads how often a unit comes back, because a third outage in a season is a conversation about replacement, not a fourth thermocouple.

Worked pass: standing pilot furnace in a small retail back room, pilot will not hold

Step 1: thermocouple-and-magnet safety with a manual gas cock, valve model recorded, valve literature obtained.

Step 2: relit per the label after its stated wait. Pilot burns lazy with a yellow tip, and only the very end of the flame reaches the thermocouple tip rather than enveloping the top 3/8 to 1/2 inch the instructions state.

Step 3: failure mode recorded as lights but drops out when the button is released, which is the safety circuit rather than the draft-under-load case.

Step 4: pilot burning 45 seconds, thermocouple disconnected, open circuit reads 26 mV, stable, inside the commonly published 25 to 35 mV range for a healthy thermocouple. So the element itself is generating.

Step 5 FAILS its acceptance and takes its stop rule. Connected and with the valve held in, closed circuit reads 5 mV against the 9 mV minimum hold read off this valve's own literature, 4 mV short. It is also 5 divided by 26, about 19 percent of the open-circuit value, against the roughly half the step names as the healthy check. Under the stop rule that is a connection or magnet fault, not a worn thermocouple, so no part is ordered. The connection at the valve is found finger-tight with a dull grey film on both faces. Faces cleaned, connection torqued, closed circuit re-measured at 14 mV, above the 9 mV minimum and 14 divided by 26, about 54 percent of open circuit.

Step 6: pilot orifice found partly blocked, cleared with a soft cleaner and the HEPA vacuum at the deposit, no drilling. Inlet gas pressure with the main burner firing reads 6.4 in w.c. against a plate minimum of 4.5 in w.c. Pilot flame now soft blue and steady, enveloping the top of the tip as the instructions describe. The thermocouple is not replaced.

Step 7: relit per the label, one full call run. Pilot gas closed with the main burner off: the valve drops out at 38 seconds against the maximum of 90 seconds read off the valve's literature. Closed circuit re-read at 14 mV. Flue CO reads 58 ppm air-free against the 400 ppm air-free ANSI Z21 series limit. Ambient reads 0 ppm with the appliance firing.

Self-audit, criterion by criterion: step 4 demanded a stable open-circuit reading against a published range, and the run prints 26 mV against 25 to 35 mV, labelled as the range commonly published for the type rather than as a manufacturer figure. Step 5 demanded a value against the valve's own stated minimum, and the run prints 5 mV then 14 mV against a 9 mV minimum read off that literature, with the ratios derived inline, 5 over 26 and 14 over 26. Both readings are taken at the same points on the same circuit. Step 7 demanded a drop-out time against the valve's stated maximum, and the run prints 38 seconds against 90, from the same document as the 9 mV.

References

• The gas valve's own literature for the minimum hold millivolts and the maximum drop-out time, and the appliance rating plate for minimum inlet pressure, which are the only correct sources for those figures
• The appliance's lighting instructions label for the relight sequence and its wait time before relighting
• ANSI Z21 series listing limits for carbon monoxide in the flue products of listed vented appliances, at 400 ppm air-free, superseded by a lower manufacturer limit where stated
• 29 CFR 1910.333(b)(2) with NFPA 70E-2021, 120.5, for proving dead before opening a line-voltage compartment
• See related: the ignitor and flame sensor replacement SOP for electronic ignition systems, and the flue and venting inspection SOP where the pilot drops only under main burner load