Quick Answer: A tankless water heater running normally is not silent, but it should be quiet — gas units hum around 45–55 dB while firing (Rinnai’s own spec sheet lists the RXP199iN Sensei at 49 dB, roughly a running dishwasher), while electric units like Stiebel Eltron’s Tempra Plus have no combustion fan or mechanical switches and run close to silent. A sudden roar, rumble, or buzzing that wasn’t there at install is different from that baseline hum and typically traces to one of five fixable causes: high water pressure, gas pressure, insects or debris in the fan, a partially blocked vent, or a loose fan mount.
Every tankless water heater makes some sound — a modulating gas burner and a combustion fan aren’t silent hardware. The question that actually matters isn’t “does it make noise,” it’s whether the noise you’re hearing matches the normal operating hum or signals something worth fixing before it gets worse.
What normal sounds like, by fuel type
| Type | Typical noise level | Source of sound |
|---|---|---|
| Gas tankless (condensing) | ~45–55 dB | Combustion fan + burner modulation |
| Rinnai RXP199iN Sensei | 49 dB (manufacturer spec) | Combustion fan + burner |
| Electric tankless (Stiebel Eltron Tempra Plus) | Near-silent | Water flow only — no fan, no combustion |
| Reference: running dishwasher | ~50 dB | — |
That 45–55 dB range for gas units — confirmed at 49 dB on Rinnai’s own RXP199iN spec sheet — is the fan spinning up and the burner modulating to match demand, and it’s audible mainly if the unit is mounted on or near a shared interior wall. Our heat pump vs. tankless comparison puts a heat-pump water heater’s compressor noise in the same 45–60 dB range — so a gas tankless unit isn’t unusually loud by comparison, it’s a genuinely quieter appliance category than a heat-pump tank overall. If you’re choosing between fuel types specifically to avoid mechanical noise near a bedroom or home office, electric tankless is the category built around eliminating the fan and burner entirely, not just muffling them.
By the numbers
- 49 dB — the sound level Rinnai’s own spec sheet lists for the RXP199iN Sensei condensing model, roughly matching a running dishwasher. — Rinnai
- 75–80 psi — the incoming water pressure threshold above which plumbing-service reports document vibration and buzzing as a common tankless side effect. — Plumbing service data
- 25–75 psi — the adjustable range of the Watts LFN45B pressure-reducing valve, factory-set to 50 psi, the standard fix for pressure-driven vibration noise. — Watts
Five causes of abnormal noise, and what fixes each one
A unit that suddenly sounds different from how it did at install is describing a real mechanical condition, not settling in. In rough order of how often each shows up in plumbing-service reports:
- High incoming water pressure. Pressure above roughly 75–80 psi is a documented source of buzzing and vibration in tankless units. A pressure-reducing valve set to the standard 50 psi residential target resolves it without any work on the water heater itself.
- Excessive gas pressure. Gas supplied above the unit’s rated pressure can cause water inside the heat exchanger to flash to steam in bursts, producing a rumble or roar. This is a gas-valve adjustment job for a licensed technician — not a DIY fix, and not something to leave running.
- Insects or debris in the combustion fan or vent. Wasps, bees, and in some climates small lizards nesting in an outdoor unit’s fan housing or vent terminal is a recurring cause of buzzing, rattling, or roaring documented across plumbing forums. A vent insect screen is the standard prevention.
- A partially blocked vent terminal. Snow, leaves, or a stored object too close to the vent restricts airflow the combustion fan has to work harder against, raising both noise and — per our error codes guide — the odds of a vent-blockage fault code.
- A loose fan mount or seal. Vibration that develops gradually, rather than appearing suddenly, points to a fan assembly working loose from its mount or a housing seal degrading and letting “tramp air” into the burner chamber — a technician-level fix, typically covered under the heat-exchanger or parts warranty if the unit is still within its term.
Watts LFN45B Lead-Free Pressure-Reducing Valve
- The direct fix for the single most common noise cause on this list — incoming pressure above 75–80 psi vibrating the unit and its connected piping.
- Installs on the main supply line ahead of the water heater; serviceable without removing it from the line.
- Lowering pressure to 50 psi also reduces wear on every fixture and appliance downstream, not just the tankless unit.
Ordering a pressure-reducing valve or a vent insect screen before the noise gets worse — instead of after a service call confirms what you already suspected? Get it there in two days — try Amazon Prime free for 30 days.
Stainless Steel Vent Insect Screen
- Prevents the fan-housing and vent-terminal obstructions that plumbing-service reports repeatedly document as a buzzing or roaring noise source on outdoor and vent-terminated units.
- A five-minute preventive install versus a service call once a nest has already formed inside the vent.
When to call a technician instead of troubleshooting it yourself
Water pressure and insect screens are owner-fixable in minutes. Gas pressure adjustments, fan-mount repairs, and housing-seal replacement are not — they involve the internal gas valve or combustion assembly, the same category our error codes guide flags as licensed-technician-only work. If the noise coincides with a blinking fault code, start there; if it doesn’t, and pressure isn’t the cause, it’s a service call rather than a parts order.
The bottom line
A tankless water heater humming at 45–55 dB while it fires — 49 dB on Rinnai’s own RXP199iN spec sheet — is working exactly as designed, about as loud as a dishwasher and audible mainly right next to the unit. Electric tankless units skip that noise source entirely since there’s no fan or burner to modulate. Anything louder than that baseline, especially a sudden roar, rumble, or buzz, points to one of five documented causes — water pressure, gas pressure, pests, a blocked vent, or a loose fan mount — and checking incoming water pressure first is the fastest, cheapest place to start before assuming the unit itself is failing.
Shop tankless water heater noise-fix accessories on Amazon →