Lochinvar Water Heater Not Heating: Step-by-Step Diagnosis and Fix

Common Symptoms

• Completely cold water from all hot water taps — no heat at all
• Lochinvar gas model: pilot light is out or will not stay lit after relighting
• Lochinvar gas: status LED on gas control valve blinking 1, 2, 3, 4, or 7 times
• Lochinvar Knight/Crest: control board displaying fault code or lockout LED
• Lochinvar electric: 240V circuit breaker tripped or won't stay on
• Lochinvar electric: ECO reset button on upper thermostat is raised or tripped
• Pilot lights but main burner never fires
• Unit was working normally — suddenly producing only cold water

Most Likely Causes

1. 1

   Lochinvar Gas: Thermopile or Thermocouple Failure (Most Common)

   The most frequent cause of complete no-heat failure on Lochinvar gas tank water heaters (Copper-Fin series and standard residential models using the Honeywell WV8840 gas valve) is a failed thermopile or thermocouple. The thermopile is a thermoelectric generator mounted in the pilot flame — it produces a DC millivolt signal (650–850mV when healthy) that powers the WV8840 electronics and holds the main gas valve open. When the thermopile degrades below approximately 325–350mV, the WV8840 refuses to open the main valve and the unit produces no heat. The WV8840 LED blinks 1 or 2 times to signal low thermopile voltage. Replacement thermopile: AO Smith 9005502 (compatible with Lochinvar models using the WV8840 gas valve). On older Lochinvar models with standing pilot thermocouple: test the TC terminal on the valve — below 17mV minimum, replace with AO Smith 9003972.

2. 2

   Lochinvar Gas: 4-Blink ECO / High-Limit Trip

   Lochinvar gas water heaters with the Honeywell WV8840 gas valve include an Energy Cut-Off (ECO) high-temperature safety switch built into the valve. If tank temperature exceeds approximately 190–200°F — caused by a blocked flue, restricted combustion air, or a partially stuck-open gas valve — the ECO trips and shuts off all gas flow. The WV8840 LED blinks 4 times to signal this condition. The unit will not heat until the root cause of the overtemperature event is corrected and the ECO is reset. Lochinvar Knight condensing models have a high-limit aquastat or safety sensor connected to the control board — the board will display a high-limit fault code. Never reset the ECO or high-limit without investigating why the unit overheated.

3. 3

   Lochinvar Gas: 3-Blink or 7-Blink Gas Valve Fault

   A 3-blink WV8840 code indicates an internal gas valve fault, usually a failed solenoid, damaged electronics, or a corroded wiring harness connector. A 7-blink code is the most serious: thermal switch open (due to severe overtemperature) or an internal WV8840 failure. Both conditions cause complete no-heat failure. For 3-blink: first reseat all push-on wiring connectors at the WV8840 — corrosion is a common cause. If 3 blinks persist after clean, tight connections, the WV8840 must be replaced. A 7-blink code with no known overtemperature event almost always indicates gas valve replacement is required — do not attempt to bypass the lockout.

4. 4

   Lochinvar Knight/Crest: Control Board Lockout or Ignition Failure

   Lochinvar Knight condensing water heaters and Crest commercial storage models use Lochinvar's proprietary control board (or a Sage or similar OEM board), which controls electronic ignition, flame sensing, gas valve modulation, and safety interlocks. A lockout condition is displayed on the board's LED or LCD as a fault code (e.g., Ign Fail, SHt, LoN, or E1–E9 type codes). Common causes: failed spark igniter, failed flame sensor (flame rod) coated with oxide, gas supply interruption, or a control board that has locked out after repeated failed ignition attempts. A power cycle (disconnect 120V power for 30 seconds) often clears a lockout on Lochinvar Knight models and allows a fresh ignition attempt.

5. 5

   Lochinvar Electric: Tripped ECO or Failed Heating Elements

   Lochinvar electric tank water heaters use two 4500W heating elements — upper and lower — each controlled by a thermostat with a manual-reset ECO (Energy Cut-Off) button. If the upper thermostat ECO trips, the entire heating circuit loses power including the lower element, producing complete cold-water failure identical to a tripped breaker. Press the red ECO reset button on the upper thermostat behind the upper access panel. If the ECO trips repeatedly, an element has shorted to ground: test each element terminal-to-tank-shell with a multimeter — any reading other than OL (open) indicates a ground fault. Replace the shorted element.

6. 6

   Lochinvar Electric: Tripped 240V Circuit Breaker

   Lochinvar electric water heaters operate on a dedicated 240V double-pole circuit breaker (typically 30A). A heating element with an internal ground fault draws excess current and trips the breaker. A breaker that resets once but then trips again within minutes has a persistent fault — almost always a ground-faulted element. With the breaker off: test each element terminal to the tank shell with a multimeter on the ohms range. Any measurable resistance other than OL = ground fault — replace that element before restoring power.

Quick DIY Checks

1. 1Step 1 — Lochinvar gas (WV8840 models): read the LED blink code before touching anything: locate the Honeywell WV8840 gas control valve on the front of the unit — it has a temperature dial, a PILOT/ON/HOT/VERY HOT knob, and a small green LED window on the valve face. Observe the LED for 15 seconds and count blinks per repeating cycle: 1 blink = standby (normal if no demand) OR thermopile voltage low; 2 blinks = thermopile voltage confirmed low (<325mV); 3 blinks = gas valve internal fault — check connectors first; 4 blinks = ECO/high-limit trip — check flue before resetting; 7 blinks = thermal switch open or internal gas valve failure. LED completely dark with gas on and pilot out = no voltage to valve. Steady green with no blinks = normal. Write down the blink count.
2. 2Step 2 — Lochinvar gas: relight pilot and test thermopile voltage for 1-blink or 2-blink codes: turn the gas control knob to PILOT. Press and hold the knob in while clicking the igniter — hold for 60 full seconds after the pilot lights (thermopile must warm up). Slowly release the knob — the pilot should stay lit. If the pilot goes out on release, the thermopile is not generating sufficient voltage. With the pilot lit and warmed for 2 full minutes: set a digital multimeter to DC millivolts (mVDC). Disconnect the two thermopile lead wires from the TH/TP terminals on the WV8840. Measure across the two leads. Lochinvar thermopile targets: ≥350mV minimum to hold valve open; 600mV ideal; 650–850mV is healthy full output. Below 350mV = replace thermopile with AO Smith 9005502. If the model has a standing pilot thermocouple (older units): test at the TH terminal on the valve — ≥17mV minimum, ≥25mV ideal. Below 17mV = replace with AO Smith 9003972.
3. 3Step 3 — Lochinvar gas: investigate and reset a 4-blink ECO trip: a 4-blink code means the ECO high-limit inside the WV8840 has tripped due to overtemperature. Before any reset: turn gas control to OFF. Inspect the flue vent pipe from the draft hood at the top of the heater all the way to the exterior vent termination — look for bird nests, debris, ice blockage (winter), or a collapsed vent cap. Confirm the unit has adequate clearance and combustion air (not in a fully sealed closet). After clearing any blockage: restore gas, relight the pilot (60-second hold), turn the knob to HOT, and open a hot tap to create demand. Monitor the LED — if 4 blinks do not return after one full heating cycle, the root cause was the vent obstruction. If 4 blinks return within minutes, call a licensed gas technician — overfiring is likely.

Repair vs Replace

Recommended Tools & Parts

• AO Smith 9005502 Thermopile (Lochinvar WV8840 Compatible)

  Replacement thermopile for Lochinvar gas water heaters using the Honeywell WV8840 gas valve (Copper-Fin and standard residential series). Generates 650–850mV to power the WV8840 electronics. Replace when thermopile output drops below 350mV (1-blink or 2-blink code with pilot lit). Push-on spade terminals at gas valve.

  $25–$45

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• AO Smith 9003972 Thermocouple (Lochinvar Compatible)

  Replacement thermocouple for older Lochinvar gas water heaters with standing pilot thermocouple-type gas valve. 36-inch lead. Replace when thermocouple output drops below 17mV at the TH terminal on the gas valve.

  $12–$25

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• Honeywell WV8840 Gas Control Valve

  Replacement gas control valve for Lochinvar gas water heaters showing 3-blink or 7-blink fault codes. Match BTU rating from the existing valve or Lochinvar data label. Gas valve replacement requires a licensed plumber in most jurisdictions.

  $80–$150

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• Camco 02162 4500W Screw-In Heating Element

  Universal 4500W 240V screw-in element for Lochinvar electric tank water heaters. Replace when element tests OL (open) or shorted to ground. Kit includes replacement rubber gasket. Full tank drain required before removal.

  $18–$35

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• Digital Multimeter (DC Millivolt Mode)

  Essential for testing thermopile voltage at the WV8840 gas valve and heating element resistance on electric models. Look for a meter with a mVDC (DC millivolt) range for accurate thermopile measurement.

  $15–$35

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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