Thermocouple
Common pilot and ignition part on gas water heaters.
The EcoSmart ECO 18 is a 18kW whole-home electric tankless water heater designed for smaller homes and warmer climates where incoming groundwater temperature is above 60°F. It’s a compact, self-modulating unit — but like all electric tankless heaters, it depends on precise electrical power delivery and clean water flow to operate correctly. When something goes wrong, it displays error codes on the digital panel.
This guide covers every EcoSmart ECO 18 error code, explains the differences from the higher-capacity ECO 27 and ECO 36, and walks through the fixes.
What Does the EcoSmart ECO 18 Error Code Mean?
An ECO 18 error code tells you which basic requirement for heating water is missing: enough water flow, a valid temperature reading, or a healthy heating circuit. Because the ECO 18 is fully electric, most faults come down to flow restriction, bad thermistors, failed heating elements, or power delivery problems.
How the ECO 18 Error System Works
The ECO 18 displays single-letter or letter-number codes on its LED panel. The unit’s self-modulating technology adjusts power output based on incoming water temperature and flow rate — so some apparent “errors” are actually the unit operating at reduced capacity because of inlet conditions. Understanding the difference matters.
Important note on capacity: The ECO 18 is an 18kW unit requiring a 200-amp service panel and is designed for whole-home use in the Southeast and Southwest U.S. where groundwater temperatures are 60–70°F. In northern climates with 40°F groundwater, the ECO 18 cannot raise water temperature adequately at normal flow rates — this isn’t an error code, it’s an undersized unit. The ECO 27 (27kW) or ECO 36 (36kW) are appropriate for colder climates.
EcoSmart ECO 18 Error Codes
E1 — No Water Flow / Low Water Flow
The flow sensor detected insufficient water flow to activate the heating elements. The ECO 18 requires a minimum flow rate of approximately 0.3 GPM to initiate heating.
Causes:
- Flow sensor fouled with sediment or mineral deposits
- Fixture flow restrictor blocking adequate flow
- Partially closed supply valve
- Low water pressure at the unit (under 30 PSI)
How to fix E1:
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Check water supply pressure. The ECO 18 needs minimum 30 PSI at the inlet. Very low pressure (rural well systems, heavily loaded municipal lines) can trigger E1.
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Remove and clean the flow sensor. Locate the flow sensor on the cold water inlet. Turn off the supply valve and disconnect the sensor. Flush debris from the sensor turbine and reinstall. Sediment from older pipes is a very common cause of E1 on ECO series heaters.
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Check inlet screen filter. The ECO 18 has a fine mesh screen filter at the cold water inlet. Mineral buildup and sediment accumulate here over time. Remove, clean, and reinstall.
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Verify the shower or fixture flow rate. Low-flow showerheads (under 1.0 GPM) may not trigger adequate flow. Install a higher-flow fixture or remove the restrictor insert from low-flow showerheads.
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Check for closed valves. Any partially closed valve between the water supply and the heater reduces flow below the activation threshold.
E2 — Inlet Temperature Sensor Fault
The cold water inlet temperature sensor (thermistor) has failed or is reading outside its valid range.
What it means: The ECO 18 can’t determine how much power to apply because it doesn’t know the incoming water temperature. Self-modulation is disabled; the unit may not heat correctly or may refuse to operate.
How to fix E2:
- Power cycle the unit — disconnect power at the breaker for 60 seconds, restore, and retest.
- Locate the inlet thermistor on the cold water pipe inside the unit. Check the connector for security.
- Test thermistor resistance with a multimeter — at 70°F (21°C), a functioning sensor reads approximately 10–15kΩ depending on the specific sensor. An open reading or zero ohms indicates failure.
- Replace the sensor if out of spec. Inlet thermistors for the ECO series are available as replacement parts.
E3 — Outlet Temperature Sensor Fault
The hot water outlet temperature sensor has failed or is reading abnormally.
What it means: The unit cannot verify that hot water temperature is within safe limits. It may refuse to operate or operate in a degraded mode.
How to fix E3: Same diagnostic procedure as E2, applied to the outlet thermistor (located on the hot water outlet pipe). Replace if resistance is out of spec.
E4 — High Temperature Cutoff
The water temperature inside the unit reached an unsafe level (above 160°F at the heating module) and the thermal cutoff activated.
Causes:
- Very low flow rate (extremely low demand)
- Partially restricted inlet causing low flow through the heating chamber
- Temperature setpoint too high (factory max is 140°F on ECO series)
- Failed flow sensor causing the unit to heat without adequate flow
How to fix E4:
- Do not reset immediately. An E4 lockout means dangerously hot water may be present in the lines. Wait 10 minutes.
- After cooling, turn power off at the breaker.
- Check the flow sensor and inlet screen for restrictions.
- Verify the temperature setpoint is at 120°F or below for standard residential use.
- Restore power and test with a fixture that provides at least 0.5 GPM.
- If E4 recurs: The thermal cutoff safety device inside the unit may have triggered. Some ECO units have a manual-reset thermal cutoff button inside the unit access panel. Locate and press the reset button. If it trips again immediately, there’s a persistent low-flow or high-power condition to diagnose.
E5 — Heating Element Fault / Electrical Fault
One of the ECO 18’s heating modules has detected an electrical fault. The ECO 18 uses copper-sheathed immersion heating elements.
Causes:
- Failed heating element (burnout or short)
- Breaker undersized or tripping
- Incorrect voltage supply
- Loose wire connection at the element terminal
How to fix E5:
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Check the circuit breaker. The ECO 18 requires two 40-amp, 240-volt double-pole breakers (or one 80-amp double-pole, depending on the installation configuration). If either breaker has tripped, reset it. If it trips again immediately, there’s an electrical fault in the circuit.
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Verify supply voltage. Measure voltage at the unit terminals — you should see 240V across both legs. Check each leg to neutral/ground — you should see approximately 120V on each. An imbalanced reading (one leg low) indicates a wiring or utility problem.
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Test heating element resistance. With power off and elements disconnected, test each element between its two terminals. A functional element shows 2–8Ω depending on the wattage. An open (OL) reading means the element has failed.
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Check connections. Loose wire connections at the element terminals cause arcing, which can trigger E5 and eventually cause element failure. Check all terminal screws.
Low Flow / Lukewarm Water Without an Error Code
Not an error code, but the most common ECO 18 complaint. The ECO 18 delivers maximum output at approximately 1.6 GPM in warm climates. Above that flow rate, output temperature drops as the self-modulating system divides power across more gallons.
This is not a fault — it’s the unit operating within its physical limits. Solutions:
- Reduce fixture flow rate
- Lower setpoint to 105°F for showers (reduces required temperature rise)
- Upgrade to ECO 27 or ECO 36 for higher simultaneous demand
ECO 18 vs ECO 27 vs ECO 36 — Key Differences
| Model | Power | Max Flow @ 35°F Rise | Breaker Requirement |
|---|---|---|---|
| ECO 18 | 18kW | ~3.6 GPM | 2x 40A (240V) |
| ECO 27 | 27kW | ~5.4 GPM | 3x 40A (240V) |
| ECO 36 | 36kW | ~7.2 GPM | 4x 40A (240V) |
The ECO 27 and ECO 36 use the same error code system as the ECO 18. The difference is that they have more heating modules — so E5 on an ECO 27 may affect one of three modules, while on the ECO 18 it affects one of two. Troubleshooting is otherwise identical across the ECO series.
How to Fix It
- For E1: Start with the flow sensor and inlet filter screen — these are the most common causes.
- For E2/E3: Power cycle first. Test thermistor resistance before ordering a replacement.
- For E4: Check flow restriction and verify the temperature setpoint before attempting resets.
- For E5: Check breakers, verify 240V supply, test element resistance.
- Never operate with a fault present — the ECO 18’s self-modulation depends on all sensors being functional.
Parts You May Need
| Part | Why You Need It | Approx. Cost |
|---|---|---|
| EcoSmart Flow Sensor Replacement | E1 fix — replaces fouled or failed flow sensor turbine | $20–$45 |
| Electric Water Heater Thermistor Sensor | E2/E3 fix — inlet or outlet temperature sensor replacement | $10–$30 |
| 240V Electric Water Heater Heating Element | E5 fix — replaces burned-out immersion heating element | $20–$60 |
| Tankless Water Heater Flush Kit | Annual maintenance — clears mineral scale from elements and chamber | $20–$50 |
| 40 Amp Double Pole Circuit Breaker | Replacement for undersized or failed breakers causing E5 | $15–$40 |
Need replacement parts? Search for parts and tools on Amazon.
When to Call a Pro
Call a licensed electrician or plumber when:
- E5 appears and the breaker keeps tripping — this indicates an electrical fault in the unit or wiring that needs licensed repair
- You need to upgrade to ECO 27 or ECO 36 — higher-capacity units need additional 240V circuits; electrical panel work requires a licensed electrician
- E4 keeps recurring after clearing flow restrictions — the thermal cutoff may have failed in a way that requires unit replacement
- The unit is over 5 years old and experiences multiple faults — EcoSmart ECO series heaters have a limited service life; replacement is often more economical than multiple repairs
Frequently Asked Questions
Q: My ECO 18 shows no error but delivers lukewarm water. Is it broken?
A: Probably not — the ECO 18 self-modulates power based on incoming water temperature and demand. If groundwater temperature is 45°F and you’re running 2.5 GPM, the unit is mathematically incapable of heating to 120°F — it doesn’t have enough watts per gallon. Check incoming water temperature and actual flow rate. If demand exceeds the unit’s capacity, you need the ECO 27 or ECO 36.
Q: How do I reset the ECO 18 after an E4 high-temperature fault?
A: First, wait 10 minutes for the unit to cool. Turn off power at the breaker. Open the front access panel — there is a small reset button near the heating modules on many ECO units. Press it. Restore power and test. If E4 returns immediately, there’s a persistent low-flow condition causing the overtemperature — address that before resetting again.
Q: Can I install an ECO 18 myself?
A: The plumbing connections are DIY-accessible for experienced homeowners. The electrical work requires a licensed electrician in most jurisdictions — two 40-amp 240V circuits are significant electrical work that needs to meet code. Improper wiring is the #1 cause of E5 errors and heating element failures on new installations.