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Fix a tankless water heater that has no hot water

We'll confirm flow and demand, check gas or power, then isolate the cause—flow sensor, scale, or ignition—or tell you when to call a pro.

Troubleshooting · Home plumbing
15–60 min
What you'll need
  • Owner manual for your tankless model
  • Descaling kit (pump, vinegar or descaling solution) if scale is suspected
  • Adjustable wrench
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Steps

Goal: Confirm flow and demand, check gas or power, then isolate the cause or call a pro.

  • Open the hot faucet fully. Tankless units need a minimum flow rate to activate.
  • Good: Strong flow. Proceed to Check gas or power.
  • Bad: Weak flow—open valves, remove low-flow fixtures, check inlet filter. See Check flow.

Check flow

Goal: Rule out low flow as the cause—tankless needs minimum flow to turn on.

  • Open the hot faucet fully. Remove low-flow aerators or showerheads temporarily.
  • Check that other hot fixtures are not running. Check the water inlet filter for debris.
  • Good: Adequate flow. Proceed to Check gas or power.
  • Bad: Flow still too low—plumbing may be undersized; call a plumber.

Check gas or power

Goal: Rule out gas supply or electrical issues.

  • Gas units: confirm the gas shutoff is fully open (parallel to the pipe).
  • Electric units: check the circuit breaker. Reset if tripped. If it trips again, call an electrician.
  • Good: Gas or power on. Proceed to Descale or check error codes.
  • Bad: Gas off or breaker tripped—fix and retry.

Descale

Goal: Remove scale from the heat exchanger if it is blocking flow or reducing heat.

  • Follow the manufacturer descaling procedure. Use a pump and vinegar or descaling solution. Circulate through the heat exchanger, then flush with clean water.
  • Good: Improved flow and heat.
  • Bad: Still no hot water—call a pro.

When to get help

If you smell gas, evacuate immediately. Call 911 or your gas utility from outside.

Call a plumber or HVAC technician if:

  • The unit still has no hot water after checking flow, gas or power, and descaling.
  • Error codes indicate an internal fault you cannot resolve.
  • You are not comfortable with the repair.

Verification

  • Hot water flows at the fixtures when the faucet is fully open.
  • No error codes on the display.
  • Water temperature is consistent.

Escalation ladder

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

  1. Confirm flow Verify adequate flow at the fixture—tankless needs minimum flow to activate.
  2. Gas or power Check gas supply (gas units) or circuit breaker (electric units).
  3. Error codes Check display or status lights for error codes.
  4. Descale Flush or descale the unit per manufacturer procedure.
  5. Call a pro Gas smell—evacuate, call 911 or gas utility. Error codes or no fix—call a plumber or HVAC technician.

What to capture if you need help

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

  • Tankless model and type (gas or electric)
  • Flow rate at fixture
  • Error codes or status lights
  • Steps already tried

Is there adequate flow at the hot faucet?

Tankless units need a minimum flow rate (often 0.5–1.0 gpm) to activate. Low flow prevents the unit from firing.

Open the hot faucet fully. Remove low-flow aerators or showerheads temporarily. Check that other hot fixtures are not running. Good: strong flow. Bad: weak flow—fix flow first (open valves, check inlet filter).

Yes No

You can change your answer later.

Fix flow and retry

Open all valves fully. Remove low-flow fixtures temporarily. Check the inlet filter for debris. Clean or replace. Retry. If flow is still too low, the plumbing may be undersized—call a plumber.

Done

Is gas (or power) on?

Gas units need gas supply. Electric units need the circuit breaker on.

Gas: check the gas shutoff is fully open (parallel to pipe). Electric: check the circuit breaker for the water heater. Reset if tripped. If it trips again, do not reset—call an electrician. Good: gas or power on. Bad: gas off or breaker tripped—fix and retry.

Yes No

You can change your answer later.

Turn on gas or power and retry

Open the gas valve fully. Or reset the circuit breaker. Retry the hot faucet. If the breaker trips again, call an electrician. If you smell gas, evacuate and call 911 or your gas utility from outside.

Done

Are there error codes on the display?

Many tankless units show error codes. Check the owner manual for your model.

Check the display or status lights. Look up the code in your owner manual. Ignition failure, flow sensor fault, or over-temperature are common. Good: code indicates a fix (e.g. descale). Bad: code indicates internal fault—call a pro.

Yes No

You can change your answer later.

Look up error code and follow guidance

Check your owner manual for the error code. Follow the recommended action—descaling, reset, or call for service. If the code indicates ignition or flow sensor fault and you cannot fix it, call a plumber or HVAC technician.

Done

Has the unit been descaled recently?

Scale in the heat exchanger can block flow or reduce heat. Descaling may restore performance.

Follow the manufacturer descaling procedure. Use a pump and vinegar or descaling solution. Circulate through the unit, then flush with clean water. Good: improved flow and heat. Bad: still no hot water—call a pro.

Yes No

You can change your answer later.

Descale the unit

Descale per manufacturer procedure. Use a pump and descaling solution. Circulate through the heat exchanger, then flush with clean water. Retry. If still no hot water, call a pro.

Done

Call a pro

If you smell gas, evacuate and call 911 or your gas utility from outside. Call a plumber or HVAC technician if: the unit still has no hot water after checking flow, gas or power, and descaling; error codes indicate internal fault; or you are not comfortable with the repair.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a tankless water heater have no hot water?
Common causes: flow rate too low (tankless needs a minimum flow to turn on), gas supply off or low (gas units), tripped breaker (electric units), scale buildup blocking the heat exchanger, or a faulty flow sensor or ignition. Check flow and gas or power first.
How much flow does a tankless water heater need?
Most tankless units need at least 0.5 to 1.0 gallons per minute to activate. Low-flow fixtures or partially closed valves can prevent the unit from firing. Open the faucet fully and check that other fixtures are not drawing cold water.
When should I call a pro for a tankless water heater?
If you smell gas, evacuate and call 911 or your gas utility from outside. Call a plumber or HVAC technician if the unit still has no hot water after checking flow, gas or power, and descaling; or if you see error codes you cannot resolve.

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