Lochinvar AO Smith Field Guide: Burner Assemblies, Door Switches & Relief Valves on Commercial Water Heaters

When a Lochinvar commercial water heater locks out on a burner-door temperature fault, the problem is almost never the control board — it is a safety device doing exactly what it was built to do in response to a combustion or airflow fault upstream. The three parts that strand a tech in a commercial mechanical room most often are the Lochinvar burner door temperature switch, the Lochinvar final burner assembly, and the Lochinvar ASME pressure relief valve. Each fails in a distinct, recognizable pattern, and confusing one for another is what turns a 30-minute repair into a callback. This guide walks the failure mechanism behind each, what you will see at the unit, and the confirmed OEM part numbers that drop in without field modification.

Why Do Lochinvar Boilers Lock Out on a Burner Door Temperature Fault?

A "Burner Door Temp" fault means the hex door temperature switch sensed heat at the burner interface above its factory-set limit and opened the safety circuit. On Lochinvar Armor and Knight boilers and high-recovery A.O. Smith units, this switch is a high-limit cut-off mounted at the combustion door — when it opens, the call for heat is answered with a lockout rather than ignition.

The trap here is that the switch is the messenger, not the cause. A door switch that trips is usually reporting one of three upstream problems: a partially blocked flue or vent restricting combustion-gas removal, a sooted or misfiring burner raising door-side temperatures, or a degraded door gasket allowing hot flue gas to wash across the switch body. Field observation confirms that techs who simply replace the switch without inspecting the burner and vent see the same lockout return within days, because the new switch is reading the same overheating door.

The confirmed OEM part is 100292715 (supersedes 100292), documented for models with serial index F14 and later. It is a factory-set device — never bypass or attempt to calibrate it. Bypassing a burner-door safety switch on a sealed-combustion unit is how carbon monoxide events happen.

What Causes Soot and Flame-Failure Problems on Lochinvar Burners?

Soot accumulation, pilot instability, and premature thermopile failure on Lochinvar and A.O. Smith natural-gas water heaters almost always trace back to a degraded burner and orifice — not the gas valve. The final burner assembly carries a precision-drilled orifice that meters fuel to the exact air-fuel ratio the FVIR (Flammable Vapor Ignition Resistant) combustion chamber requires. When that orifice clogs, corrodes, or the burner face distorts from thermal cycling, the flame pattern drifts rich, and soot starts coating the chamber and heat exchanger.

The confirmed OEM assembly is 100111113, a NAT FV burner that ships pre-equipped with a #27 orifice, factory-calibrated for the specific BTU input of the compatible Lochinvar and A.O. Smith natural-gas models. It cross-references the legacy A.O. Smith 9005959005.

Field Observation: This is a natural-gas-only assembly. Installing it on a unit that has been converted to LP — or assuming the orifice is interchangeable across fuel types — will produce improper combustion and hazardous carbon monoxide levels. Always confirm the data-plate fuel type before ordering, and never reuse an orifice across NG and LP.

The "Why" matters here: a burner that is metering fuel incorrectly does not just run dirty, it drives a cascade. Soot insulates the heat exchanger, recovery times stretch, the unit short-cycles chasing setpoint, and the added thermal stress at the door is exactly what eventually trips the 100292715 door switch. Techs who chase the lockout without addressing the burner are treating the symptom at the end of the chain.

When Should a Lochinvar Pressure Relief Valve Be Replaced?

A commercial boiler or water-heater relief valve should be replaced the moment it fails to reseat cleanly after a manual test, weeps continuously, or shows mineral buildup or corrosion at the seat. A relief valve is the last line of overpressure protection on a high-output system — there is no second chance if it is stuck. Commercial boiler installations also rely on properly functioning HVAC pressure switches upstream to flag abnormal operating conditions before they reach the relief valve.

The confirmed OEM valve is 100274785, a 1-1/2" x 2" ASME safety relief valve set at 50 PSI with a relief capacity of 4,237,000 BTU/hr. It is the official updated number for the legacy 0383792 and drops in without field modification.

The failure mechanism on relief valves is almost always the seat. Repeated thermal expansion cycles drive the valve open and closed, and over time mineral scale builds at the sealing surface. Once the seat cannot reseat fully, the valve weeps — and a weeping relief valve in hard-water commercial service accelerates its own failure as scale bridges the gap. The misdiagnosis trap: a tech sees the weep, assumes the system is overpressurized, and starts chasing an expansion-tank or fill-valve problem that does not exist. Inspect the relief valve seat first.

Lochinvar Commercial Parts: Symptom-to-Cause-to-Action

How Do These Three Failures Relate to Each Other?

On a commercial Lochinvar or A.O. Smith unit, these parts sit on the same failure chain. A burner metering fuel incorrectly (100111113) runs hot and dirty, which raises burner-door temperatures and trips the door safety switch (100292715). That same elevated thermal stress accelerates seat wear on the relief valve (100274785). A tech who replaces only the part that threw the fault — usually the door switch — restores operation for a few cycles and then gets the callback. Records consistently show that the durable repair addresses the burner and combustion path, not just the safety device that reported the fault.

This is also why "universal" thinking fails on commercial water heaters. The 100111113 orifice is sized to a specific BTU input; the 100274785 valve is rated to a specific 4,237,000 BTU/hr relief capacity. A relief valve that is close but under-rated, or a burner with the wrong orifice, will pass a quick bench check and then fail in service under full commercial load. Match the confirmed OEM specification.

Frequently Asked Questions

How do I know if the Lochinvar burner door temperature switch is tripped? If the boiler displays a "Burner Door Temp" fault, or fails to ignite despite a call for heat, the thermal link in the switch (part 100292715) has likely opened the safety circuit. Confirm the vent, burner condition, and door gasket before replacing — the switch is usually reporting an upstream overheating problem, not failing on its own.

Is the 100292715 door switch field-adjustable? No. It is a factory-set safety device. Bypassing or attempting to recalibrate it can create unsafe operating conditions, including undetected overheating of the combustion chamber. Replace it with the OEM part — do not modify it.

Does the 100111113 burner assembly include the orifice? Yes. It ships pre-equipped with a factory-calibrated #27 orifice sized for the specific natural-gas Lochinvar and A.O. Smith models it serves. It is a natural-gas-only assembly and must not be used on LP units.

What is the difference between part 100274785 and 0383792? Part 100274785 is the official updated OEM number and is a direct replacement for the legacy 0383792 relief valve. Same 1-1/2" x 2" connection, 50 PSI set point, and 4,237,000 BTU/hr capacity.

Can the 100274785 relief valve be used on non-Lochinvar systems? It is primarily used in Lochinvar and A.O. Smith commercial units, but it suits any system requiring a 1-1/2" x 2" ASME relief valve at a 50 PSI set point with matching BTU capacity. Always confirm the relief capacity meets or exceeds the system's output before installing — an under-rated valve is a safety failure waiting to happen.

Stocking for Commercial Water Heater Season

Commercial mechanical-room failures do not wait for a convenient parts run, and these three components are the ones that strand a unit hard: a tripped door switch locks out the system, a fouled burner drives a no-heat callback, and a failed relief valve takes the unit out of code-compliant service entirely. Carrying the confirmed OEM parts — burner assembly 100111113, door temperature switch 100292715, and relief valve 100274785 — turns a multi-day parts wait into a same-visit repair.

Browse the full Lochinvar commercial boiler and water heater parts catalog to stock the burner, safety, and relief components your commercial accounts depend on before the next lockout call comes in.