Water Heater Thermocouple and Thermopile by Type: Which of the Three Pilot Systems You Have, Which Length Fits and When the Maker Requires Its Own Assembly
Standing-pilot tanks take a 30 mV Honeywell CQ100A thermocouple, 18 to 36 in. Status-light tanks run a 750 mV thermopile; FVIR makers sell the pilot as one assembly.
- Verified
- Updated
- 6 min read
- 14 sources
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Short answer
A gas water heater uses one of three pilot-safety parts, and the maker’s manual decides which one you may buy. A standing-pilot tank with a 30 millivolt gas control takes a universal thermocouple: Honeywell’s CQ100A series comes in 18 in (CQ100A1021/U), 24 in (CQ100A1013/U), 30 in (CQ100A1039/U) and 36 in (CQ100A1005/U), each described by Honeywell as a 30 millivolt part with a universal adapter for push-in, clamp-in and screw-in pilot burners. A tank with an electronic combination gas control and a status light runs a 750 millivolt thermopile instead: Rheem’s FVIR Use & Care manual flashes 2 red blinks for Thermopile Voltage Low, and a thermocouple does not drive that valve. Many FVIR heaters sell the pilot, its sensor and the igniter as one assembly: A.O. Smith’s GCV manual says Replace pilot assembly, part 183824-190 for natural gas on the GCV-40, never the thermocouple alone. Match the length to the old part and the system to the manual before ordering anything.
The picks
Standing-pilot tank, old thermocouple measures 30 in or less
Honeywell Home CQ100A1039 30-inch replacement thermocouple
Honeywell Store page for CQ100A1039/U: 30 in, designed for 30 millivolt systems, universal adapter for push-in, clamp-in and screw-in pilot burners; gas furnaces, boilers and water heaters
Standing-pilot tank where the 30 in part comes up short
Resideo (Honeywell Home) 36-inch replacement thermocouple
Honeywell Store page for CQ100A1005/U: 36 in, 30 millivolt systems, furnaces, boilers and water heaters, universal adapter; the longest length in the CQ100A series short of the 48 in CQ100A1047/U
If your gas control has a status light, both parts in the grid above are wrong for your tank: read the system table first, then decide whether you are buying anything at all.
Which of the three systems do you have?
The part is decided by the gas control, not by the brand on the tank. The same maker ships both systems, and Rheem is the proof: its FVIR article says the Guardian System “incorporates a standard gas valve and thermocouple”, while the Use & Care manual for its 29 to 60 gallon Residential Gas FVIR Certified line wires a thermopile into a combination gas control.
| System | How the maker describes it | What you buy |
|---|---|---|
| Standing pilot, 30 mV thermocouple | A gas control with a pilot knob and no status light. Honeywell lists its CQ100A thermocouples for “30 millivolt systems” on gas furnaces, boilers and water heaters | A universal thermocouple, matched by length, if the maker’s manual does not call for an assembly |
| Electronic combination gas control, 750 mV thermopile | Rheem’s FVIR manual: check “the thermopile wire connectors at the Combination Gas Control”, status light 2 red blinks for Thermopile Voltage Low, 8 blinks for a false pilot flame signal. A.O. Smith’s GUC-40 manual: the thermopile “determines whether a pilot flame is present” and flashes 2 times for thermopile voltage low | Honeywell’s CQ200A1004/U is a 750 millivolt thermopile with a 35 in lead for “most millivolt output gas valves”, but both makers above replace theirs as an assembly or through a technician |
| FVIR pilot assembly sold as one part | A.O. Smith GCV/XCV manual: “Thermocouple malfunction” and “Thermocouple TCO malfunction”, remedy “Replace pilot assembly”. GUC-40 manual: “Replacing the Pilot/Thermopile Assembly”, pilot orifice stays in the pilot | The maker’s assembly by model and gas type: A.O. Smith GCV/XCV natural gas 183824-190 (30, 40 and 50 gal), propane 183824-182 (30 gal), 183824-172 (40 gal) or 183824-162 (50 gal) |
A thermocouple is a single probe that makes a small voltage in the pilot flame and holds the pilot valve open. A thermopile is a stack of them making a larger voltage, enough to run an electronic gas control with no house power. Honeywell’s two store pages put the figures side by side: 30 millivolt for the CQ100A thermocouple, 750 millivolt for the CQ200A thermopile.
Which thermocouple length fits?
Length is the only spec a universal thermocouple asks you to match, and the old part is the measurement. Honeywell sells five lengths under one design.
| Honeywell part | Length | Rated for | Pilot burner fit |
|---|---|---|---|
| CQ100A1021/U | 18 in | 30 millivolt systems | Universal adapter: push-in, clamp-in, screw-in |
| CQ100A1013/U | 24 in | 30 millivolt systems | Universal adapter: push-in, clamp-in, screw-in |
| CQ100A1039/U (30 in listing ) | 30 in | 30 millivolt systems | Universal adapter: push-in, clamp-in, screw-in |
| CQ100A1005/U (36 in listing ) | 36 in | 30 millivolt systems | Universal adapter: push-in, clamp-in, screw-in |
| CQ100A1047/U | 48 in | Listed in the Honeywell Store sitemap only; the page was not opened | Not read |
The Honeywell Store marks all of these pages discontinued, so they prove the part number, length and rating, not where to buy. The Amazon listing titles carry the same CQ100A numbers, which is how the two picks were matched. No clean Honeywell 24 in listing was in stock when this page was checked, so the 18 and 24 in rows have no link.
Does your maker allow a universal part?
Only one answer here is clean, and it is “replace the assembly”. Nobody printed “a universal thermocouple is fine”, and nobody printed “it is forbidden”.
| Maker | What the document says | Universal thermocouple |
|---|---|---|
| A.O. Smith (GCV/XCV, GUC FVIR) | Troubleshooting remedy for thermocouple faults is “Replace pilot assembly”; the parts list sells the pilot assembly with grommet and door insulation as one part, the gas control valve (natural 182791-004, propane 182792-000) and piezo igniter 183798-000 separately; orders need model number, gas type and serial number | Not documented. The manual’s remedy is the assembly |
| Rheem (Residential Gas FVIR Certified, 29 to 60 gal) | A defective thermopile “should be replaced by a qualified service technician”; “DO NOT attempt repair of gas piping, Combination Gas Control (Thermostat), burners, vent connectors or other safety devices”; parts orders need model, serial number and gas type | Not applicable: this line uses a thermopile. Rheem’s FVIR article describes a thermocouple on the Guardian System, so other Rheem models differ |
| Bradford White (Defender Safety System) | Parts “only available through a professional service provider”, sold “through wholesale distribution”; the pilot “should only have to be lit once, during installation and initial set up” | Not published on any page reached. No part number or length is claimed here |
Rheem’s FVIR article dates the standard to 2003 and ties it to vapours from spilled volatile liquids igniting near a gas appliance.
How often does a thermocouple need replacing?
No maker document opened for this page prints an interval. Bradford White’s FAQ sets the expectation instead: under normal operation the pilot is lit once, at installation, and a pilot that keeps going out is a reason to call a plumbing professional.
If the pilot lights but will not stay lit, the pilot light guide walks the symptom to the part. If there is no pilot and no hot water at all, start with the no hot water guide, because the gas control and the supply come before the sensor.
Measure before you buy
The Honeywell CQ100A1039 is a 30 millivolt standing-pilot part on the Honeywell Store page, and nothing else. It will not fix a status-light tank, and A.O. Smith’s FVIR manuals want the whole pilot assembly, by model and gas type, not this probe. If your old thermocouple runs past 30 in, the 36 in Resideo is the same series one size up.
Frequently asked questions
What is the difference between a thermocouple and a thermopile on a water heater?
Both sit in the pilot flame and both hold the gas valve open only while the flame is there. Honeywell describes its CQ100A thermocouples as parts for 30 millivolt systems and its CQ200A1004 thermopile as a 750 millivolt generator for self-generating pilot systems. A.O. Smith's GUC-40 manual says the thermopile determines whether a pilot flame is present and shuts off gas to the burner and pilot if it is absent.
How do I know whether my water heater has a thermocouple or a thermopile?
No maker page opened for this article prints a visual rule, so the model's manual decides. Rheem's FVIR Use & Care manual troubleshoots a pilot that will not stay lit by checking the thermopile wire connectors at the combination gas control, and its status light flashes 2 red blinks for thermopile voltage low. A standing-pilot control with no status light is the thermocouple case.
Can I put a universal thermocouple on an FVIR water heater?
Check the manual for your model rather than assuming. A.O. Smith's GCV/XCV manual lists thermocouple malfunction and thermocouple TCO malfunction and gives one remedy for both: replace pilot assembly, which the parts list sells as 183824-190 for natural gas on the GCV-40. Its GUC-40 manual replaces the pilot and thermopile as one assembly. Rheem's manual refers thermopile replacement to a qualified service technician.
What length thermocouple does a water heater take?
Measure the old one. Honeywell's CQ100A series is sold by length: CQ100A1021/U at 18 in, CQ100A1013/U at 24 in, CQ100A1039/U at 30 in, CQ100A1005/U at 36 in and CQ100A1047/U at 48 in, all described as 30 millivolt parts with a universal adapter for push-in, clamp-in and screw-in pilot burners. A longer lead can be coiled; a short one cannot be stretched.
Where do I buy a Bradford White thermocouple or thermopile?
Bradford White's FAQ says it sells through wholesale distribution rather than to consumers, that warranty parts are only available through a professional service provider, and that its site has a contractor locator. The same FAQ says the pilot should only have to be lit once, during installation and initial set up, so a pilot that keeps going out is a call to a plumbing professional.
Fact sheet and how to cite this page
- Answer
- A standing-pilot gas water heater with a 30 millivolt gas control takes a universal thermocouple: Honeywell’s CQ100A series in 18, 24, 30 and 36 in. A tank with an electronic combination gas control and a status light runs a 750 millivolt thermopile, which Rheem’s FVIR manual flags with 2 red blinks, and A.O. Smith’s GCV manual replaces the pilot assembly as one part, 183824-190 for natural gas.
- Applies to
- System: standing pilot, 30 mV thermocouple
- System: electronic gas control with status light, 750 mV thermopile
- System: FVIR pilot assembly sold as one part
- Compatibility basis
- Honeywell Store product pages for CQ100A1021/U, CQ100A1013/U, CQ100A1039/U, CQ100A1005/U and CQ200A1004/U; Rheem Use & Care Manual for Residential Gas FVIR Certified water heaters (document 367839); A.O. Smith GCV/XCV instruction manual and parts list and GUC-40 Ultra Low NOx manual; Bradford White FAQ pages on parts, pilot lighting and the Defender Safety System
- Assumptions
- The Honeywell Store pages are marked discontinued there; they are used only for the part number, length and 30 or 750 millivolt wording, not for availability. The Amazon listing titles print the same CQ100A numbers.
- No maker page opened this session tells an owner how to identify a thermocouple against a thermopile by looking at it. The table uses each maker’s own manual wording for the model it covers, and the model’s manual decides.
- Bradford White publishes no thermocouple or thermopile part number, length or universal-part policy on the pages reached this session, so its row states only what its FAQ says.
- Honeywell Q-series, Robertshaw and BASO part numbers were not opened on any primary page this session and do not appear here.
- Method
- Part numbers, lengths, millivolt ratings, status-light codes and replace-as-assembly instructions copied from each maker’s manual, parts list or product page as printed. OwnerSpec removed no part, lit no pilot and measured nothing.
- Primary source
- A.O. Smith, GCV/XCV 30, 40, 50 gallon FVIR gas water heater parts list (PDF) (manufacturer or vendor document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Water Heater Thermocouple and Thermopile by Type: Which of the Three Pilot Systems You Have, Which Length Fits and When the Maker Requires Its Own Assembly", https://ownerspec.com/plumbing/parts/water-heater-thermocouple-thermopile-by-type/, facts verified 2 October 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Honeywell Store, CQ100A1021/U 18-inch replacement thermocouple (manufacturer or vendor document)
- Honeywell Store, CQ100A1013/U 24-inch replacement thermocouple (manufacturer or vendor document)
- Honeywell Store, CQ100A1039/U 30-inch replacement thermocouple (manufacturer or vendor document)
- Honeywell Store, CQ100A1005/U 36-inch replacement thermocouple (manufacturer or vendor document)
- Honeywell Store, CQ200A1004/U 750 millivolt replacement thermopile generator (manufacturer or vendor document)
- Honeywell Store sitemap (lists the 48-inch CQ100A1047/U page) (manufacturer or vendor document)
- Rheem, Use & Care Manual, Residential Gas FVIR Certified water heaters, document 367839 (PDF) (manufacturer or vendor document)
- Rheem, What is FVIR (manufacturer or vendor document)
- A.O. Smith, GCV/XCV 30, 40, 50 gallon FVIR gas water heater instruction manual (PDF) (manufacturer or vendor document)
- A.O. Smith, GCV/XCV 30, 40, 50 gallon FVIR gas water heater parts list (PDF) (manufacturer or vendor document)
- A.O. Smith, GUC-40 Ultra Low NOx FVIR gas water heater instruction manual (PDF) (manufacturer or vendor document)
- Bradford White FAQ, Where can I get parts for my Bradford White water heater (manufacturer or vendor document)
- Bradford White FAQ, How often do I have to light the pilot (manufacturer or vendor document)
- Bradford White FAQ, What is the Defender Safety System (FVIR) (manufacturer or vendor document)