A tankless unit makes some normal operating sound, mainly from the fan and water flow, but distinct new noises usually point to a specific cause.
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Quick answer
A gas tankless water heater produces some normal operating sound, mainly a soft fan hum during combustion and a faint sound of water flowing through the unit, which is generally quiet enough to go unnoticed if the unit is installed away from living spaces like bedrooms. Noises beyond that baseline, such as popping, rattling, buzzing, or a noticeably louder fan than before, usually point to a specific cause like mineral scale, a loose component, or a developing combustion issue, and are worth having checked rather than assumed to be normal.
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Unlike a storage tank, which is mostly silent between heating cycles, a tankless unit runs its fan and burner every time hot water is drawn, which means it does produce some consistent sound during use. Most homeowners describe this as a soft, steady hum from the internal fan drawing combustion air and venting exhaust, along with a faint sound of water moving through the unit. In a mechanical closet, garage, or basement, this level of sound is typically unnoticeable from adjacent living spaces, especially with a properly installed, sealed venting system.
| Sound | Possible cause |
|---|---|
| Popping or cracking during heating | Mineral scale buildup inside the heat exchanger |
| Rattling or buzzing | A loose panel, mounting bracket, or internal component |
| Noticeably louder fan than usual | Airflow restriction, developing fan issue, or venting obstruction |
| A distinct clicking that repeats without the unit firing | Possible ignition system issue worth having checked |
| Water hammer, a loud bang when a fixture shuts off | Often a plumbing issue rather than the unit itself, but worth mentioning to your technician |
Popping or cracking sounds specifically are commonly associated with scale buildup on the heat exchanger, similar to the sound sometimes heard from an old kettle with mineral deposits, and are a reasonable trigger to schedule a descaling visit.
Where the unit is installed has a real effect on how much of its operating sound reaches living spaces. A unit installed in a garage or basement mechanical room, well away from bedrooms, is far less likely to be noticeable during normal use than one mounted on an interior wall shared with a bedroom or living room. If noise is a priority for your household, mentioning this during the site assessment allows your technician to factor sound transmission into the location recommendation alongside the more technical venting and gas line considerations.
If your system includes a recirculation pump, that pump adds its own operating sound, typically a low hum while running, which is separate from the tankless unit's own fan noise. On a continuous recirculation setup, this additional sound runs more often than an on-demand configuration, which is a factor worth weighing if the pump is installed somewhere sound-sensitive.
A new or changing sound, especially popping, rattling, or a noticeably different fan tone than the unit has always had, is worth mentioning to your technician even if the unit is otherwise heating water normally. Catching a developing issue, like early scale buildup or a loosening component, while it's still a minor fix is generally easier and less costly than waiting until the underlying cause has progressed further.
Safety note
Any inspection of a tankless water heater's combustion, venting, or internal components in response to unusual noise should be performed by a Technical Safety BC licensed gas fitter. If you ever smell gas or suspect a carbon monoxide issue, leave the home immediately and call FortisBC at 1-800-663-9911.
Yes, a soft fan hum and faint water flow sound during operation are normal, but distinct new noises like popping or rattling usually point to a specific cause worth checking.
This is commonly associated with mineral scale buildup inside the heat exchanger and is a reasonable reason to schedule a descaling visit.
Yes, installing the unit away from bedrooms and living spaces, such as in a garage or basement mechanical room, generally reduces how noticeable its operating sound is inside the home.
Yes, it adds its own low hum while running, separate from the tankless unit's fan, and runs more often on a continuous recirculation setup than an on-demand one.
That's often a plumbing water hammer issue rather than the tankless unit itself, but it's worth mentioning to your technician so the cause can be properly identified.
Any new or changing sound beyond the normal fan hum and water flow, especially popping, rattling, or a distinctly louder fan, is worth having checked before it develops into a bigger issue.
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