Wireman · Tankless water heater trips or no heat: troubleshooting for electricians · 2023 NEC ·

Tankless water heater trips or no heat

Flow detection, every breaker and element circuit, then elements and connections.

Before you start

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How it works

An electric tankless heater heats water only as it flows, so it needs a lot of current at once. Whole-house units commonly draw over 100 A at 240 V, split across two to four elements, each on its own 2-pole breaker and conductors. A flow sensor turns the elements on once flow passes a minimum rate. When one element or circuit is out, you get lukewarm water rather than none, so check every breaker and every element.

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

What’s the problem?
- No hot water at allOpen a hot tap fully. Does the unit show that it detects flow?
  - No flow detectedFlow too low to turn the heater on
  - Flow detected, no heatAre all of the heater’s breakers on? A whole-house unit may have two, three or four.
    - One was trippedWhen does the breaker trip?
      - Instantly when resetTest: Test each elementExpect: Near R = V² ÷ W from the element’s rating (about 6.4 Ω for 9 kW at 240 V), and OL to ground.
        An element reads OLBurned-out element
        An element reads to groundElement shorted to ground
        All elements goodControl, relay or high limit
      - After running a whileTest: Look for heat damageExpect: Clean, tight connections with no discoloration.
        Discolored, melted or looseOverheated connection
        Looks fineBreaker or conductor sizing
    - All are onTest: Clamp each element circuitExpect: Each circuit draws close to its element’s watts ÷ volts (for example about 37.5 A for a 9 kW element at 240 V).
      - One or more read 0 ATest: Test each elementExpect: Near R = V² ÷ W from the element’s rating (about 6.4 Ω for 9 kW at 240 V), and OL to ground.
        An element reads OLBurned-out element
        An element reads to groundElement shorted to ground
        All elements goodControl, relay or high limit
      - All draw currentWorking, but at its limit
- Lukewarm, or not hot at high flowAre all of the heater’s breakers on? A whole-house unit may have two, three or four.
  - One was trippedWhen does the breaker trip?
    - Instantly when resetTest: Test each elementExpect: Near R = V² ÷ W from the element’s rating (about 6.4 Ω for 9 kW at 240 V), and OL to ground.
      - An element reads OLBurned-out element
      - An element reads to groundElement shorted to ground
      - All elements goodControl, relay or high limit
    - After running a whileTest: Look for heat damageExpect: Clean, tight connections with no discoloration.
      - Discolored, melted or looseOverheated connection
      - Looks fineBreaker or conductor sizing
  - All are onTest: Clamp each element circuitExpect: Each circuit draws close to its element’s watts ÷ volts (for example about 37.5 A for a 9 kW element at 240 V).
    - One or more read 0 ATest: Test each elementExpect: Near R = V² ÷ W from the element’s rating (about 6.4 Ω for 9 kW at 240 V), and OL to ground.
      - An element reads OLBurned-out element
      - An element reads to groundElement shorted to ground
      - All elements goodControl, relay or high limit
    - All draw currentWorking, but at its limit
- A breaker tripsWhen does the breaker trip?
  - Instantly when resetTest: Test each elementExpect: Near R = V² ÷ W from the element’s rating (about 6.4 Ω for 9 kW at 240 V), and OL to ground.
    - An element reads OLBurned-out element
    - An element reads to groundElement shorted to ground
    - All elements goodControl, relay or high limit
  - After running a whileTest: Look for heat damageExpect: Clean, tight connections with no discoloration.
    - Discolored, melted or looseOverheated connection
    - Looks fineBreaker or conductor sizing
- The display shows an errorThe heater has identified a fault