Tankless Water Heater Banging Noise
On this page (6 sections)
The Short Answer: The timing of the bang tells you the cause. A bang or “whoomp” right as the burner lights is delayed ignition: stop using the heater and have it inspected. A bang when a faucet or appliance valve closes is water hammer in the pipes. Knocking while the heater runs, with weaker hot water, usually points to scale in the heat exchanger.
Three different bangs
| When it happens | What it is | What to do |
|---|---|---|
| The instant the burner lights | Delayed ignition: gas built up before it lit | Turn the heater off; call a gas technician |
| When a faucet, washer, or dishwasher valve snaps shut | Water hammer: a pressure surge in the pipes | Plumbing fix; not urgent unless pipes shake hard |
| Knocking or popping while hot water runs | Scale causing local boiling in the heat exchanger | Descale; check water hardness |
| Thump on a schedule with no tap open | Recirculation pump or a check valve slamming | Test with recirculation off |
| Ticks as pipes heat and cool | Pipes expanding against straps | Usually harmless |
Delayed ignition
When gas reaches the burner and does not light right away, it collects for a moment and then lights all at once. Causes include a weak or misplaced igniter, low gas pressure or an undersized gas line, a dirty burner, a blocked air intake or exhaust, or wind at an outdoor vent. Repeated delayed ignition can crack parts of the combustion chamber and damage the heat exchanger. It can also be accompanied by an ignition or flame-loss code on the display.
What you can do: turn the heater off at its power switch, photograph any error code, and look from outside at the vent terminal for snow, leaves, or a nest. Do not keep running hot water to see if it happens again, and do not adjust gas fittings.
What the technician will check: gas pressure at the heater against the rating plate, gas line sizing for the heater’s maximum input, the igniter and flame sensor, burner cleanliness, and the venting. Fixing an undersized gas line is common on retrofit installations where a tank was replaced with a much higher-input tankless unit.
Water hammer
Tankless heaters do not cause water hammer, but they often coincide with it: the heater sits on the line when a fast-closing valve on a washing machine, dishwasher, or single-lever faucet stops flow suddenly. The pressure wave can make pipes bang and the heater’s cabinet shake. High household water pressure makes it worse.
What you can check: whether the bang happens when a specific appliance shuts off. A plumber can measure your water pressure, adjust or install a pressure-reducing valve, secure loose pipes, and add water hammer arrestors near the appliances that trigger it.
Scale knocking
In hard-water areas, minerals build up inside the heat exchanger. Water passing over the hot, coated surfaces can flash to steam in small pockets, which sounds like knocking or popping and usually comes with less hot water or overheat codes. Regular descaling through the isolation valves, using the heater manufacturer’s procedure, prevents it. If the heater has no isolation valves, a plumber can add them.
Tank-style sediment noise is a different issue; see water heater sounds like rocks.
Related guides
For other sounds, see tankless water heater groaning noise, tankless water heater clicking noise, and the water heater noises hub. For a single loud bang from a tank heater, see water heater made a loud bang.
Frequently Asked Questions
Q: Is a bang at ignition ever normal? A: No. A soft whoosh as the burner lights is normal. A bang that shakes the cabinet or the wall is delayed ignition and should be inspected.
Q: Will flushing fix every banging sound? A: No. Flushing helps only with scale. It does nothing for delayed ignition or water hammer.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- CPSC: carbon monoxide fact sheet (opens in a new tab)Carbon monoxide hazards and emergency response.
Editorial standards and corrections · Prepare a useful service request