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Fix a tankless water heater that will not heat

We'll confirm the symptom, rule out power, gas, and flow issues, then isolate the cause—flow sensor, scale in the heat exchanger, heating elements for electric, or igniter for gas—or tell you when to call a pro.

Troubleshooting · Home appliances
15–45 min
What you'll need
  • Multimeter (for continuity tests on electric units)
  • Screwdriver set (Phillips and flathead)
  • Replacement flow sensor, heating elements, or igniter (if tests show a fault)
  • Descaling kit (optional; follow manufacturer procedure)
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Steps

Goal: Confirm the symptom, rule out power, gas, and flow, then isolate the heating fault.

  • Turn on a hot-water faucet and let it run 30–60 seconds.
  • Good: The water stays cold—heating system has failed. Proceed to Check power, gas, and flow.
  • Bad: Water warms slowly or only at one faucet—different problem (flow, plumbing). Check other faucets or call a plumber.

Check power, gas, and flow

Goal: Rule out power loss, gas supply, and low flow before opening the unit.

  • Check power: verify the unit is plugged in or hardwired and the circuit breaker has not tripped. Electric tankless units often need 240 volts; both legs must be on.
  • If gas: confirm the gas supply valve is open and other gas appliances work.
  • Check water flow: tankless units need a minimum flow rate (often 0.5–1 GPM) to fire. A clogged inlet filter or low pressure can prevent this. Locate the inlet filter at the cold-water connection, turn off water, remove and clean it, then reinstall.
  • Good: Power on, gas on (if gas), and strong flow at the faucet. Proceed to Electric path or Gas path based on your unit type.
  • Bad: Breaker keeps tripping, no gas, or flow stays low—fix those first, or call a pro.

Electric path

Goal: Test and replace the flow sensor or heating elements on an electric tankless unit.

  • Check the flow sensor. It detects water flow and signals the unit to heat. If it fails, the unit will not fire. Test per manufacturer procedure or replace if an error code indicates failure.
  • If the flow sensor is good, check the heating elements. Unplug the unit and shut off water. Open the service panel per your model. Locate the heating elements and test for continuity with a multimeter. Replace any that show no continuity or visible damage.
  • If both are good, scale in the heat exchanger may be blocking heat. See Scale path.
  • Good: You found and replaced the faulty part. Restore power and water—the unit should heat.
  • Bad: All parts test good but the unit still does not heat—call a technician.

Gas path

Goal: Inspect and replace the igniter on a gas tankless unit. Gas valve work requires a pro.

  • Unplug the unit and shut off the gas supply at the valve. Open the front panel per your model. Check your owner’s manual or search “[brand] [model] tankless water heater service manual” for disassembly steps.
  • Locate the igniter. Inspect for cracks or breaks. Test for continuity if possible.
  • If the igniter is cracked or failed, replace it. Restore power and gas. The igniter should glow and the burner should light.
  • Good: The igniter glows and the flame lights. The unit heats.
  • Bad: The igniter glows but no flame appears—gas valve issue. Call a technician. Do not work on gas valves yourself.

Scale path

Goal: Address mineral buildup in the heat exchanger that blocks heat transfer.

  • Scale from hard water can clog the heat exchanger and block heat transfer. If you have hard water and the unit is several years old, descaling may help.
  • Follow the manufacturer’s descaling procedure (often involves a pump and descaling solution). If you are not comfortable, call a plumber.
  • Good: After descaling, hot water flows normally.
  • Bad: Still no heat after descaling—call a technician.

When to get help

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside. Do not call a technician for an active gas leak—that is an emergency.

Call a plumber or appliance technician if:

  • The unit is gas and the igniter glows but no flame appears (gas valve).
  • You have ruled out flow, scale, and components and the unit still does not heat.
  • Water warms at some faucets but not others (plumbing issue).
  • You are not comfortable with electrical or gas work.

Verification

  • Hot water flows within 30–60 seconds of turning on a faucet.
  • No error codes or unusual noises from the unit.
  • The flow sensor, heating elements, or igniter (if replaced) work and the unit heats consistently.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Confirm symptom Verify hot water never gets warm; rule out a different problem.
  2. Power, gas, and flow Check circuit breaker, gas supply, and water flow; low flow prevents firing.
  3. Electric — flow sensor and heating elements Test and replace flow sensor or heating elements if faulty.
  4. Gas — igniter Inspect and replace igniter if cracked or failed.
  5. Scale in heat exchanger Descale per manufacturer procedure or call a pro.
  6. Call a pro Gas smell—evacuate, call 911 or gas utility from outside. Gas valve work or repeated failures—call a plumber or technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Unit type (electric or gas)
  • Whether water flow meets minimum
  • Inlet filter condition
  • Flow sensor / heating elements / igniter test results
  • Steps already tried

Does hot water never get warm when you turn on a faucet?

Turn on a hot-water faucet and let it run 30–60 seconds. If the water stays cold, the heating system has failed.

Turn on a hot-water faucet and let it run 30–60 seconds. Good: water stays cold—heating fault. Bad: water warms slowly or only at one faucet—different problem (flow, plumbing); check other faucets or plumbing.

Yes No

You can change your answer later.

Is power on, gas on (if gas), and water flow adequate?

Circuit breaker, gas supply, and minimum flow rate (often 0.5–1 GPM) are required. Check inlet filter for clogs.

Check power and circuit breaker (electric) or gas supply (gas). Check water flow—tankless needs minimum flow to fire. Clean the inlet filter if clogged. Good: power on, gas on, strong flow. Bad: breaker trips, no gas, or low flow—fix those first or call a pro.

Yes No

You can change your answer later.

Is the unit electric or gas?

Electric units have a power cord or hardwired connection. Gas units have a gas line and vent pipe.

Check the unit label or connections. Electric: power cord or hardwired. Gas: gas line and vent. Good: you know the type. Bad: unsure—call a technician.

Electric Gas

You can change your answer later.

Does the flow sensor work?

The flow sensor detects water flow and signals the unit to heat. A failed sensor prevents firing.

Check the flow sensor. Test per manufacturer procedure or replace if error code indicates failure. Working: unit fires when flow detected. Failed: replace sensor or call a pro.

Yes No

You can change your answer later.

Replace flow sensor and test

Replace the flow sensor with a matching part. Restore power and water. The unit should fire when flow is detected. If it still does not heat, check the heating elements or call a pro.

Done

Do the heating elements have continuity?

Electric tankless units use heating elements. Test with a multimeter for continuity.

Unplug the unit and shut off water. Open the service panel. Locate the heating elements and test for continuity. No continuity or damage: replace the element. Good: elements intact—check for scale in the heat exchanger.

Yes No

You can change your answer later.

Replace heating elements and test

Replace the faulty heating element with a matching part. Restore power and water. The unit should heat. If scale may be present, descale per manufacturer procedure.

Done

Is scale likely in the heat exchanger?

Hard water causes mineral buildup. Scale can block heat transfer. Descale per manufacturer procedure.

If you have hard water and the unit is several years old, scale may have built up in the heat exchanger. Follow the manufacturer's descaling procedure. If not comfortable, call a plumber.

Done

Is the igniter cracked or showing no continuity?

The igniter glows when the unit calls for heat. A cracked or failed igniter prevents the burner from lighting.

Unplug the unit and shut off the gas. Open the front panel. Locate the igniter. Inspect for cracks. Test for continuity if possible. Cracked or no continuity: replace igniter, restore power and gas, test. Good: igniter glows and flame lights. Bad: igniter glows but no flame—gas valve issue; call a pro. Do not work on gas valves yourself.

Yes No (glows but no flame)

You can change your answer later.

Replace igniter and test

Replace the igniter with a matching part. Restore power and gas. The igniter should glow and the burner should light. If the igniter glows but no flame appears, the gas valve has failed—call a pro. Never work on gas valves yourself.

Done

Call a technician

If you smell gas, evacuate immediately. Do not turn on lights or appliances. Call 911 or your gas utility from outside—do not call a technician for an active gas leak. Call a plumber or appliance technician if: hot water warms at some faucets but not others (plumbing); the gas igniter glows but no flame (gas valve); you have ruled out flow, scale, and components and it still does not heat; or you are not comfortable with electrical or gas work. Never work on gas valves or gas lines yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a tankless water heater run but not heat?
Low water flow below the minimum (often 0.5–1 GPM) prevents the unit from firing. A failed flow sensor, scale in the heat exchanger, or faulty heating components (electric: heating elements; gas: igniter or gas valve) can also cause no heat. Check flow and inlet filter first.
Can I fix a tankless water heater that will not heat myself?
Yes, for electric units you can often check the inlet filter, flow sensor, and heating elements. Gas units: you can inspect the igniter, but gas valve work requires a professional. Descaling may need a pro if you are not comfortable. Never work on gas lines yourself.
When should I call a technician for a tankless water heater that will not heat?
If you smell gas, evacuate immediately and call 911 or your gas utility from outside—do not call a technician for an active leak. Call a technician if the unit is gas and the igniter glows but no flame appears (gas valve), if you have ruled out flow and scale and it still does not heat, or if you are not comfortable with electrical or gas work. Gas valve repairs always need a pro.

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