No hot water from a tankless heater? Navien, Rheem and A. O. Smith checks

Which model and manual apply?

Start with the rating plate, not the brand name on the cabinet. Photograph the full model, series, serial number, fuel and conversion label, then identify the indoor or outdoor arrangement, controller and any linked heaters. Preserve model suffixes when selecting instructions; a conversion label is relevant only where that manufacturer permits conversion.

This comparison covers gas condensing Navien NPE-180S2, Rheem IKONIC RTGH-S11i and RTGH-SR11i, and A. O. Smith ATI-540H-N, Series 100. Navien NPE-180A2 appears separately because its internal pump, buffer and recirculation configuration change low-flow delivery. Electric tankless heaters, combi boilers and other generations need their own manuals.

A. O. Smith's ATI-540H Series 100 support listing marks that model discontinued and links the operating manual used here.

What belongs in the manual register?

Record the document title, identifier, revision, printed page and direct URL alongside the asset. Navien's download listing calls its installation document an installation and operation manual, while the served PDF is titled Installation Manual, Version 2.0, January 2026, 20049624B; the separate user's manual is Version 1.0, 2020, 20049624A.

The installation manual governs installation and supply limits, the user's manual supports normal observations, and the technical manual supplies service diagnostic paths. For selection rather than fault diagnosis, compare water heater types.

When do you stop checking and leave?

Gas odor ends the diagnostic sequence. The front-cover warnings in Navien NPE-2 Installation Manual v2.0, Rheem AP22745 Rev 04 and A. O. Smith 2000534710 Rev. C direct occupants not to attempt ignition, operate electrical switches or use a telephone inside; leave and call the gas supplier from outside, then the fire department if the supplier cannot be reached. Follow the supplier's instructions about reentry.

The U.S. Consumer Product Safety Commission's CO guidance directs occupants to move outside immediately when a carbon monoxide alarm sounds, call emergency services and wait for responder permission before reentering.

A damaged or disconnected vent, soot, flooding, or water contacting electrical equipment also ends firing trials. Navien NPE-2 User's Information Manual v1.0, p. 5, requires discontinued operation for damaged venting; Rheem AP22745 Rev 04, p. 3, prohibits operation without proper venting. Use the model's shutdown instructions only from a safe location; stay clear of wet electrical equipment. The water-heater venting and combustion air guide explains the installation questions a technician must resolve.

A. O. Smith 2000534710 Rev. C, pp. 6 and 31, requires a flooded heater to be replaced by a qualified installer or service agency. Drying it out and resetting it is not the prescribed remedy. Navien's v1.0 user manual, p. 5, instead requires inspection before reuse after flood damage; preserve the installed manufacturer's disposition.

Safe observations are the accessible display, literal code, normal heating indicator, affected fixtures and visible external conditions. Measure discharge temperature with a suitable thermometer, and collect water only where the stream is safe to handle. Gas-pressure measurement, combustion analysis, internal wiring tests, component replacement and vent alteration belong to a qualified service technician; never defeat scald protection to force activation. The temperature warnings in Rheem AP22745 Rev 04, p. 4, and A. O. Smith 2000534710 Rev. C, p. 46, apply during diagnostic draws as well as normal use.

Is the fault at the fixture, heater or supply?

Describe what fails before choosing a test. No water at the hot tap, cold water flowing normally, delayed hot water, a cold interruption and lukewarm water during heavy use are different complaints.

Compare one affected fixture with a second accessible fixture while recording heater flow and heating state. If the second fixture gets hot, the heater has demonstrated heat production under that draw condition. The first fixture still needs its own flow and temperature comparison: a restricted cartridge or mixer can reduce hot-side demand enough to prevent activation.

A. O. Smith's 2000534710 Rev. C says the controller display goes dark after five minutes without operation; Easy-Link child controllers also do not display the set temperature. Check the STAND BY indicator and normal demand response before treating a blank screen as a power failure. Standby power alone does not establish burner operation.

These gas heaters still require electricity for controls and operation. The ATI-540H no-hot-water checklist specifies 120 VAC, 60 Hz, so record a recent outage and whether the controller is enabled before attributing the symptom to a failed board. When both hot and cold flow are weak throughout the building, follow water-pressure and PRV diagnosis alongside the heater checks.

The following triage is Anvilfield's diagnostic interpretation of the manufacturer troubleshooting paths. Note recent plumbing work, accessible shutoff positions and previous resets without operating unfamiliar gas valves.

What flow starts each heater?

Navien's January 2026 NPE-2 Installation Manual lists 0.5 GPM as the NPE-S2 minimum flow, with a separate less-than-0.01 GPM option for A2 models. The A2 option belongs to its documented ComfortFlow recirculation configuration; it is not an S2 ignition adjustment. See the installation specification, p. 7 and the specification footnote before applying it.

Rheem's IKONIC Technical Training and Service Manual AP23209 Rev 03 specifies 0.4 GPM minimum activation flow and 0.26 GPM extinction flow for RTGH-S11i and RTGH-SR11i.

A. O. Smith's ATI-540H Owner's Guide 2000534710 Rev. C requires 0.5 GPM to activate at the default temperature and 0.4 GPM to continue firing after ignition.

Compare those limits with flow through the heater. A shower's mixed discharge contains cold water that never passed through the heater, while other fixtures and recirculation can add heater flow. Startup, continuation, sensor resolution and maximum thermal output describe separate boundaries; a rounded controller reading exactly at a threshold does not prove a stable operating margin.

Why can a second tap make the hot water work?

A second hot-side draw can lift total heater flow above startup. Record the flow and heating response with each fixture alone and with both open; that supports a flow-related branch without identifying the restriction or sensor at fault. For the scoped models, Anvilfield's arithmetic places 0.45 GPM between ATI-540H continuation and startup, and 0.30 GPM between IKONIC extinction and activation.

How do you measure flow and separate it from mixing?

Divide collected gallons by collection seconds and multiply by 60 to obtain gallons per minute. Use a marked container and a timer, keep the draw steady, and repeat the collection before comparing a near-threshold result. Mark the test mixed or hot-only and note other open fixtures, because total fixture discharge and heater flow answer different questions. A single-handle fixture set toward hot can still have a temperature limiter or mixing path; verify its arrangement before treating its entire discharge as heater flow.

For ATI-540H, the normal INFO display cycles through inlet temperature, outlet temperature and flow in that order, according to 2000534710 Rev. C, p. 48. Record the three readings during the same steady draw. Reconcile mixing, other taps, recirculation and instrument resolution before interpreting disagreement with a bucket test as a flow-sensor failure.

Worked example: a low-flow draw at an ATI-540H

Use a selected mixed draw of 0.70 GPM at 105°F, supplied by 60°F cold water. Compare heater outlet conditions of 140°F and 120°F; the first is a configured calculation case, and the second is the factory default in the ATI-540H guide. The building's approved temperature target and scald protection govern any actual setting change.

Multiply mixed flow by the fraction of the temperature rise supplied by hot water. The mixing equation is Qhot = Qmixed × (Tmixed − Tcold) / (Thot − Tcold).

Anvilfield estimates the hot-side demand needed to deliver 0.70 GPM at 105°F as 0.39 GPM with a 140°F source and 0.53 GPM with a 120°F source. Compared with the published flow values, the first nominal demand is below startup and just below continuation; the second exceeds startup. Use the calculated demand to choose the next measurement, then confirm actual flow and burner response at the installed setting.

Collection uncertainty matters near the boundary. With a selected volume uncertainty of ±0.01 gallon on the 30-second collection, mixed flow spans 0.68 to 0.72 GPM. Holding temperatures fixed, Anvilfield estimates hot-side demand of 0.38 to 0.41 GPM at 140°F, crossing the continuation boundary, versus 0.51 to 0.54 GPM at 120°F. Record thermometer resolution and repeated readings on the actual job as well.

Method: Anvilfield estimates use the selected flow and temperature inputs, a steady mixing balance, equal water heat capacity, negligible pipe losses and no recirculation or crossover contribution. Operating references are A. O. Smith 2000534710 Rev. C, pp. 46-48 and 64, and AOSRG46835, April 2023, p. 2, checked November 5, 2026. Calculations retain full precision and display flows to two decimals; compare predicted demand with actual controller flow and outlet temperature. A technician controls any setting change, verifies scald protection and preserves the facility's water-management temperatures.

Which Navien codes apply to NPE-2?

The NPE-2 User's Information Manual, Version 1.0, 2020, pp. 16-18, assigns E003 to ignition failure, E012 to flame loss, E110 to abnormal air pressure and E439 to abnormal flow-sensor operation. The Navien user manual gives the complete list and basic response.

Read each code as a detected condition. E003 directs ignition and supply diagnosis; it does not establish that an igniter needs replacement. E012 requires the sequence showing whether heat started before flame was lost. E110 directs the air and vent branch, whose installation limits come from the actual vent configuration.

For cold-water complaints, the user troubleshooting path includes through-heater draw, the inlet filter, displayed errors and the return check valve where external recirculation exists. When the findings lead to fuel delivery, use the exact manual limits with gas supply and piping sizing.

Why is the A2 exception separate?

Navien's January 2026 installation manual describes A2 recirculation advantages including lower minimum delivery flow and cold-sandwich control. Its specification sheet conditions the low-flow option on ComfortFlow and notes increased energy use with recirculation. Record whether that configuration is installed and enabled before applying its delivery claims; an NPE-180S2 cannot inherit them from the family name.

Which Rheem codes apply to IKONIC?

Rheem AP23209 Rev 03 identifies A011 as failure to detect flame and A012 as flame loss after ignition. Record whether heat ever began, how long it lasted and which other gas appliances operated during the event.

A029 directs the condensate-trap and drainage branch: AP23209 Rev 03, p. 45, describes a full trap or failure to detect its water level. The service path includes drainage, connections and sensing, so the code alone does not prove a blocked drain. A090 identifies detected venting blockage and leads the technician through vent, air-filter and inlet-pressure checks. Observers retain visible routing, leakage, exterior obstruction and weather conditions; drain disassembly and combustion work remain service tasks. The IKONIC technical manual, pp. 45 and 60, governs those paths.

Rheem IKONIC T111 is a maintenance reminder under AP23209 Rev 03, p. 64, while A. O. Smith 111 is an ignition-failure code under 2000534710 Rev. C. Rheem calls for draining and flushing and warns of eventual protective shutdown; complete that service before clearing T111.

AP23209 Rev 03 sets the service alert at 500 combustion hours by default, offers 750- and 1,000-hour selections, and discourages changing the default. Combustion hours accumulate during firing; they are not calendar years or a universal descaling interval. Record service hours and the prior maintenance date together.

With no code and no heat, technical-manual p. 37 begins with demand, enabled state, low or absent flow, inlet debris and sensing. Establish those conditions before selecting a repair. Keep codes from other Rheem generations, including bare 11, 12 or P1, tied to those generations' manuals.

Which A. O. Smith codes apply to ATI-540H?

The ATI-540H troubleshooting list in 2000534710 Rev. C assigns 111 to ignition failure, 121 to loss of flame and 991 to imperfect combustion. For 991, stop repeated firing trials and arrange the gas, air, vent and combustion service branch.

A. O. Smith's 2000534710 Rev. C assigns code 311 to the heat-exchanger thermistor on the 540 model but to the outlet thermistor on 240 and 340 models. The same guide assigns 331 to the 540 outlet thermistor. Its 441 flow-sensor and 651 flow-adjustment entries carry an Easy-Link-system-only qualifier, so a single-unit flow complaint does not justify assigning either code.

Can warm incoming water explain no ignition?

A. O. Smith 2000534710 Rev. C says the heater will not activate when incoming water is too close to the set temperature; its no-ignition checklist gives no numerical temperature difference for that condition. Record inlet, outlet and setpoint together, especially with preheated supply or a return loop. If inlet and outlet are already near target but the fixture stays cold, compare the distribution path and mixing arrangement.

The same no-ignition checks include low flow, a dirty inlet filter, reversed connections and cross-plumbing. A complaint beginning immediately after installation or plumbing work warrants a connection and configuration review before part replacement. The following selected matrix retains the model qualifiers from the three linked model manuals. Each code identifies a diagnostic branch, not a part to order; use its linked manual for the complete fault list and test sequence.

Is it a cold-water sandwich, crossover or recirculation fault?

A restart sandwich is hot-cool-hot delivery after a stopped draw resumes: warm water remains downstream, cooler water passes during restart, then heated water arrives. An initial wait while a cooled pipe clears is a different sequence. Repeated cold interruptions during an uninterrupted draw require simultaneous heater-flow, heating-state and fault observations.

Navien describes the A2 buffer and pump arrangement as its response to cold-water sandwich and delivery delay in the condensing tankless practical guide.

A hot heater outlet with a cold fixture directs attention to distribution and mixing. A failed cartridge can admit cold water downstream, while a return check-valve problem can change the flow path. Compare near-heater outlet, affected fixture and return temperature under the same draw condition, then use mixing-valve and scald-control diagnosis.

Which return arrangement is installed?

Identify whether the recirculation loop uses a dedicated return or an intentional thermostatic crossover that returns water through the cold line. The crossover's intended function differs from leakage through a defective mixer; temperature alone cannot identify the failed device. Record a piping sketch and follow recirculation return, balancing and controls for the design questions.

Check the recirculation schedule and demand-control state against the complaint time. A pump that is intentionally off and a failed pump can leave the same cooled branch; record the commanded state before assigning a hardware fault.

An active uncirculated fixture branch explains a delivery wait. An unused or abandoned dead leg creates a separate stagnant-branch issue.

Anvilfield's crossover test interpretation requires a technician to account for returns, crossover devices, multiple heaters and residual pipe pressure before interpreting hot-side flow with the heater's cold supply isolated. Short residual drainage differs from a sustained alternative supply path. Save valve positions and the piping sketch with the result so another technician can reconstruct the test.

Why do winter weather and large draws change the answer?

A. O. Smith's 540 output chart in 2000534710 Rev. C, p. 64, gives 4.7 GPM at 40°F inlet and 120°F outlet, and 6.3 GPM at 60°F inlet and 120°F outlet. These chart values assume a properly sized gas line. The corresponding temperature rises are 80°F and 60°F. Colder incoming water therefore reduces available flow at the same delivery target; use the actual inlet temperature to size hot-water capacity at the required temperature rise.

Record the simultaneous fixture load, outlet target and altitude configuration before calling a unit undersized. A heater that starts reliably at a small draw but loses delivery under a large draw needs an output and supply comparison.

Natural-gas inlet pressure is 3.5 to 10.5 in. WC under Navien NPE-2 Installation Manual v2.0, January 2026, versus 4.0 to 10.5 in. WC under Rheem IKONIC AP23209 Rev 03 and A. O. Smith ATI-540H 2000534710 Rev. C.

Propane inlet pressure is 8.0 to 13.0 in. WC under Navien NPE-2 Installation Manual v2.0, January 2026, and Rheem IKONIC AP23209 Rev 03, versus 8.0 to 14.0 in. WC for the factory-propane models covered by A. O. Smith 2000534710 Rev. C, p. 29. The ATI-540H-N in this comparison is natural gas; that manual warns against converting it to propane. A qualified technician measures static and loaded inlet pressure at the heater with simultaneous gas loads recorded, using the installed fuel's limits rather than manifold-pressure values.

A. O. Smith 2000534710 Rev. C, p. 50, requires electrical power for freeze protection and limits that protection to internal heater piping. Navien NPE-2 User's Information Manual v1.0, p. 15, also requires electrical supply and identifies gas supply as additional freeze protection for A2 models. Record an outage or exposed piping and follow external-pipe freeze protection; suspected freezing or leakage ends firing trials. Follow the model's thawing and leak-inspection instructions before restoring operation.

When do maintenance and resets belong in diagnosis?

Navien's NPE-2 user's manual calls for annual intake and vent inspection; A. O. Smith 2000534710 Rev. C recommends an annual qualified check and annual vent and burner inspection. Rheem AP22745 Rev 04 calls for flushing annually or as professionally recommended, with water conditions and average usage driving frequency.

Record hardness, inlet-filter condition, prior flushing, usage and any service-hour reminder. Debris or scale becomes a supported service finding when inspection and operating evidence point to it. A cold fixture, interrupted electrical supply or insufficient hot-side demand needs its own diagnostic path even when maintenance is overdue.

Keep the cleaning procedure in water heater maintenance and tankless descaling and the supply-water response in water treatment and hardness control.

Photograph and transcribe the code before resetting. Navien's user's manual provides a Back-button reset for minor issues, while recurring faults continue through the relevant diagnostic path. Use only the installed model's documented reset when no stop condition exists; clearing an IKONIC T111 reminder follows its prescribed maintenance. Save previous reset history because an intermittent fault that disappears during the visit still needs a defined next test.

What should the service quote include?

Separate diagnosis, repair authorization and verification in the service quote. List travel, parts, indicated descaling, vent or condensate corrections, gas-piping work and permits as distinct scope items. Access, intermittent operation, parts availability and the required outage window determine the restoration schedule.

Confirm exact part applicability and manufacturer claim authorization before applying warranty credit. Navien's January 2026 Installation Manual, final service page, requires technical-support preauthorization for repairs; preserve the claim reference with the work order.

How do you write a public-facility scope?

A public-facility scope should identify every heater model, the outage window, the diagnostic tests and the records required at closeout. Estimators building a public-facility water-heater service pipeline find agency plumbing bids and past awards through PorchRocket Bid Control. Include access arrangements and temporary hot-water requirements in the work package.

Use equipment counts and plumbing takeoff to organize assets and tasks, and the service labor and dispatch framework to separate diagnosis from approved repairs.

Does no hot water mean the heater needs replacement?

Repair a confirmed fault when the unit can meet operating requirements and the applicable part is available. Compare replacement scope when a failed heat exchanger, unavailable parts or unresolved capacity requirements drive the decision. A discontinued support listing alone does not establish that a serviceable heater needs replacement; flooded equipment follows the manufacturer's replacement or inspection instructions.

Keep intermittent diagnosis and repair authorization separate. When delivery remains unresolved, record the temporary hot-water arrangement, next test, responsible party and return date.

What should you record before handing the job over?

Record identity, symptom sequence, observations, units and disposition before the evidence changes. Stop conditions end observation; below-threshold draw enters the flow branch; adequate confirmed flow without heat enters model diagnostics; a hot outlet with a cold fixture enters distribution and mixing diagnosis.

Safe observation checklist

• ☐ Stop for gas odor or a CO alarm; make calls and record observations after reaching safety.
• ☐ Photograph the complete model, series, serial, fuel or conversion label and literal display code; preserve every prefix and suffix.
• ☐ Record one-fixture versus building-wide failure and whether it occurs at startup, during continuous use, after restart or with simultaneous demand.
• ☐ Capture accessible enabled state, heating indicator, safe fixture temperature, collection volume and duration; identify mixed versus hot-only flow and other open taps.
• ☐ Note outage, freeze exposure, visible vent or condensate conditions, maintenance history and return configuration; hand over without opening the cabinet or clearing evidence.

Technician handoff record

Blank operating readings mean “not measured,” never zero. Attach the governing manual revision and printed page, instrument resolution, repeated readings and before/after test conditions.

Anvilfield's closeout method repeats the complaint-producing draw and comparison fixture with flow, temperatures, heating state and code status recorded. Repeat stop/restart timing for a sandwich complaint and the recorded simultaneous load for a high-demand complaint. Accept the work when delivery meets the recorded job target throughout that sequence; write down any departure and the next action.

A vanished error message alone does not establish restored delivery. An unresolved intermittent fault needs its remaining test, responsible party and next service date in the handoff, so the next visit begins with evidence rather than another reset.

Review and citation

Reviewed by Anvilfield on November 5, 2026. Update record, 2026-11-05: Rechecked the twelve primary sources; clarified flooded-heater disposition, fuel applicability and condensate fault interpretation; added service-scope evidence and measurement checks.

When citing a comparison, include its evidence-unit fragment and governing manual revision.

How to cite this page: Anvilfield, No hot water from a tankless heater? Navien, Rheem and A. O. Smith checks, 2026, https://anvilfield.com/field-guides/plumbing/tankless-water-heater-no-hot-water-model-matrix/

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Sources checked

• S1. Navien, NPE-180S2 downloads. Manufacturer model and document register; checked 2026-11-05.
• S2. Navien, NPE-2 Installation Manual, Version 2.0, January 2026, 20049624B. Printed pp. 7, 24-27 and final service page: flow, fuel pressures, recirculation and repair authorization. Checked 2026-11-05.
• S3. Navien, NPE-2 User's Information Manual, Version 1.0, 2020, 20049624A. Printed pp. 5, 15-18: inspection, freeze protection, troubleshooting and codes. Checked 2026-11-05.
• S4. Navien, NPE-2 Specification Sheet, 1-NPE2-SS-001-1.1. Flow footnote and A2 ComfortFlow configuration. No calendar edition printed. Checked 2026-11-05.
• S5. Rheem, IKONIC Technical Training and Service Manual, AP23209 Rev 03. Printed pp. 7, 35, 37-45, 60, 64: flow, pressures, service hours and selected fault paths. Checked 2026-11-05.
• S6. Rheem, Condensing Tankless Water Heater Use and Care Manual, AP22745 Rev 04. Printed pp. 84, 87, 92-93 and safety sections: condition-based maintenance and owner service response. Checked 2026-11-05.
• S7. A. O. Smith, ATI-540H 100 model support page. ATI-540H-N, Series 100 installed-equipment support; discontinued status and governing manual link. Checked 2026-11-05.
• S8. A. O. Smith, On-Demand Water Heater Installation Manual and Owner's Guide, 2000534710 Rev. C, 68A161-7. Printed pp. 6, 29, 31, 46-48, 50-56 and 64: flooded-heater replacement, fuel applicability, supply, flow, display, diagnostics and capacity. Checked 2026-11-05.
• S9. A. O. Smith, Commercial-Grade Residential/Commercial Gas Water Heaters, AOSRG46835, April 2023. Printed p. 2: model scope, startup and continuous flow, gas-pressure limits. Checked 2026-11-05.
• S10. Navien, Condensing Tankless Water Heaters: A Practical Guide. Undated live manufacturer guidance: A2 buffer, pump and cold-sandwich rationale. Checked 2026-11-05.
• S11. U.S. Consumer Product Safety Commission, Carbon-Monoxide-Questions-and-Answers. Undated live agency guidance: CO alarm exit, emergency call and reentry response. Checked 2026-11-05.
• S12. A. O. Smith, legacy gas-tankless UEF specification library. Manufacturer provenance for AOSRG46835 and the installed ATI/ATO family. Checked 2026-11-05.