A humidity sensor is the one feature that decides when a bathroom fan runs without asking you. It is the best answer to the fan that gets switched off after two minutes, and the Home Ventilating Institute (HVI) recommends running a bathroom fan for about 20 minutes after a shower, which a sensor does by itself. It also means the sone figure that matters is the one at the speed the sensor chooses, at times you did not pick.
So this guide reads the ratings the way a sensor uses the fan: the boost speed it jumps to, the low speed some models idle at in between, and how each maker says its sensor starts and stops. For test pressures, duct sizes and the quietest fans overall, start with our main guide to the quietest bathroom exhaust fans.
Our picks are listed above, with full specifications for each.
What a humidity sensor actually does
The makers describe three kinds of trigger, and the difference shows up in how often the fan runs.
- A set point. The fan starts when relative humidity (RH) rises above a level you set on a dial behind the grille. Delta's BreezSignature H models work this way: "when humidity is above the user-adjustable set-point, fan runs at full speed", with set points "between ~50% to ~90% RH".
- A rapid rise. The fan also starts when humidity climbs quickly, even below the set point. Panasonic's WhisperSense DC submittal calls its sensor "the rapid rise in relative humidity (RH) type". Broan's Lo-Profile DC LP510RS2 and Flex DC AE50110DCS describe the same pair of triggers.
- Dew point. Panasonic's condensation sensors read temperature as well as humidity. Panasonic says they "anticipate dew point", so the fan starts before the mirror and the cold corner of the ceiling fog up.
A rapid-rise sensor catches a shower in a room that was already humid. A set-point-only sensor waits for the room to cross a number, which on a dry winter day can take a while and on a humid summer day can already be true.
Bathroom fans are rated in sones. HVI's definition is that double the sone value is double the loudness, so 1.0 sone sounds twice as loud as 0.5. Decibels are logarithmic, and there is no simple, reliable conversion between the two, so we never convert. Our explainer on sones vs decibels has the details.
The box rating is taken at 0.1 inch of static pressure, an easy duct. Panasonic's sheets say "Industry research indicates static pressure in typical installations ranges from 0.20" to 0.375"", so we rank on the 0.25-inch figure, and only within the same airflow band and the same duct size. Every figure below comes from the maker's page or sheet, and we checked each against HVI's certified products directory (data generated September 23, 2026), as our sibling pages on Panasonic, Broan and Delta Breez fans do.
At 80 CFM, 0.25 inch, on a 4-inch duct:
- Panasonic WhisperGreen Select FV-0511VKS3S, with a sensor module: 0.5 sone.
- Panasonic WhisperSense DC FV-0511VQC1: 0.6 sone at 83 CFM.
- Panasonic WhisperFit DC FV-0511VFC1: 0.8 sone. WhisperValue DC FV-0510VSC1: 0.8 sone at 81 CFM.
- Broan Lo-Profile DC LP510RS2: 1.0 sone at 79 CFM, per HVI.
- Delta BreezSignature SIG80-110H and SIG80-110MH: 1.2 sones.
- Broan EVOLVE PTE511RK with its PES module: 1.4 sones.
At 80 CFM, 0.25 inch, on a 6-inch duct: the WhisperGreen Select is under 0.3 sone, the Delta SIG80-110H 0.5, and the Broan EVOLVE 0.6.
At 110 CFM, 0.25 inch, on a 6-inch duct: the WhisperGreen Select is under 0.3 sone and the Delta 0.8. On a 4-inch duct the WhisperGreen Select is 0.6, the WhisperSense 0.9 at 111 CFM, and the Delta 2.0.
Two lessons sit in those lists. The duct matters as much as the brand: the same Delta goes from 0.5 sone to 1.2 at 80 CFM when the duct shrinks from 6 to 4 inches. And because a sensor always runs the fan at its full-speed setting, choosing that setting is a noise decision. A Delta SIG80-110H set to 80 CFM instead of its factory 110 boosts at 0.5 sone instead of 0.8 on a 6-inch duct. HVI's sizing chart asks for 1 CFM per square foot up to 100 square feet, so many bathrooms do not need the top setting at all.
How each maker says its sensor turns on and off
Delta Breez BreezSignature H. In humidity mode the fan idles at the low speed you preset (0, 30, 50, 60 or 80 CFM) and goes to full speed above the set point. When humidity falls back below it, the manual says the fan "will continue to run at the full speed until a 10-minute time delay has elapsed". It ships at 110 CFM, a 30 CFM low speed and 60% RH. Delta adds: "Humidity tolerance is about ±10%. The humidity switch may need to be adjusted regionally." Its indicator light shows blue in humidity mode and amber when the mode switch forces full speed.
Panasonic condensation sensors. The WhisperFit DC manual says the fan starts when humidity rises above a dial setting between 30% and 80% RH, or "when the humidity rises rapidly but does not reach the humidity trigger setting", and keeps running "for about 20 minutes after the sensor detects that the humidity is ~5 % RH lower than the humidity setting". The WhisperGreen Select's FV-CSVK1 module behaves the same way and, with the multi-speed module, boosts the fan to its high setting rather than switching it on from off. The WhisperValue DC's page says it "will run for 20 minutes before turning off".
Panasonic WhisperSense DC. Both sensors are built in. The manual says it turns on "when motion is detected, when humidity increases rapidly, or when humidity rises above a 30% ~ 80% relative humidity set-point", and its delay runs "0.5 ~ 60 minutes" after the sensors stop detecting.
Broan. The Lo-Profile DC LP510RS2 idles at a low speed you set and boosts "when either of these conditions is detected: a) Humidity above a user-adjustable setpoint (30%-80% relative humidity). b) rapid increases in humidity", returning to low speed after a 20-minute delay. Its humidity dial is factory set to 80%. Broan's EVOLVE PES module page says only that it is a "Modular Humidity Sensor" that can be daisy chained with the motion module; the EVOLVE guide does not describe how it triggers.
Humid summers and false starts
A sensor cannot tell shower steam from a muggy afternoon, and every maker says so in its own words. Panasonic's WhisperGreen Select manual says: "The product may turn ON and OFF due to the unstable environment humidity, but it can be improved by adjusting the Humidity setting selector." Its WhisperFit manual adds that the fan "may cycle on and off if the humidity is constantly near the trigger point. This can be addressed by raising the humidity trigger setting on the dial." Broan's Flex guide says its sensor "may occasionally turn the fan ON when environmental conditions change", and that if it responds too often you should turn the dial toward 80%.
Delta's troubleshooting table lists "The fan keeps running even though the house humidity level is lower than set humidity level", gives its sensor tolerance and outdoor humidity "back drafting to the fan" as causes, and suggests letting it run. Its manual also says not to mount a humidity model near a window, because the "Unit may respond to the outdoor humidity level."
For noise, this matters more than the sone gap between brands. A set-point fan in a house sitting at 65% RH in July, set to 60%, runs at full speed until the air dries, which on a 4-inch Delta set to 110 CFM means 2.0 sones at 0.25 inch. The fixes the makers give are simple: raise the set point in summer, choose a lower full-speed setting, and put a damper cap on the duct. If the whole house is damp, a bathroom fan is the wrong tool; our guide to the quietest dehumidifiers compares the machines built for that job.
Continuous low speed plus boost
Some sensor fans never fully stop. They run at a low speed all day and boost when the sensor trips, and HVI says ventilation "may also be provided on a continuous basis" at other rates. That low speed is the sound you live with, so it gets its own column:
- Delta SIG80-110H: under 0.3 sone at 30 CFM at 0.25 inch on either duct; at 50 CFM, under 0.3 on a 6-inch duct and 0.5 on a 4-inch.
- Panasonic WhisperGreen Select FV-0511VKS3S: 0.3 sone at 30 and 40 CFM at 0.25 inch on a 4-inch duct, under 0.3 on a 6-inch.
- Broan LP510RS2: 0.7 sone at 29 CFM at 0.25 inch, per HVI. Broan ships its low speed set to off.
The WhisperSense, WhisperFit and WhisperValue have no separate low speed: they run at the one speed you select, or stop. If the bathroom shares a wall with a bedroom, a fan that is silent between showers may suit you better than one idling under 0.3 sone. Our quiet bedroom guide covers what a sleeping room asks of the rooms around it.
Motion sensors
A motion sensor starts the fan every time someone walks in. Delta's SIG80-110MH runs at full speed when it detects motion and stays there "until delay time (20 minutes) has passed", within a 10-foot, 90 degree range. Panasonic's FV-MSVK1 module also adds a 20-minute delay. The WhisperSense DC is the flexible one: its delay dial goes down to 30 seconds, and Panasonic's sell sheet lists three ways it can work: motion on and motion off, motion on and humidity off, or humidity on and humidity off. For a guest bath, motion is a fine trigger. For an en-suite visited at 3 a.m., a humidity-only setting is calmer.
Wiring needs
A sensor fan usually wants constant power, and that changes the wall switch.
- Panasonic WhisperFit DC condensation model: "The load/service switch must always be on for the condensation sensor to function." Two red signal wires go to an optional switch for manual on and off, and Panasonic warns not to connect them to live power. Without the red wires, switching the power off for 2 seconds and back on runs the fan for 20 minutes.
- Delta BreezSignature H: the manual wires a power switch and a mode switch. Delta's single-switch sheet shows one switch instead: on runs the fan at full speed, and off returns a humidity model to its low speed once the set humidity level is reached.
- Broan Flex DC humidity fans: "Use an on/off switch or a 2-function wall control", and do not use a speed control.
- Broan LP510RS2: a control switch for power and a boost switch.
If an existing fan is on a switch that people turn off, plan for that switch to stay on, or for a second one.
Other fans we checked
Delta's BreezElite ELT50-110H and ELT50-110MH are listed on Delta's site as "coming soon", so we left them out. HVI lists them at 0.4 sone at 50 CFM and 0.7 at 80 CFM at 0.25 inch on a 4-inch duct. Broan's Flex DC AE50110DCS ($245 on Broan's page) has a good sensor, but its sheet and HVI disagree at 0.25 inch, and HVI lists its 50 setting at 36 CFM there. Broan's QTXEG110SX is 0.4 sone at 110 CFM at 0.1 inch, with no sone figure at 0.25 inch. HVI lists Delta's builder humidity models, the GBR80H and ITG80H, at 0.8 and 1.5 sones at 80 CFM at 0.1 inch, and at 1.5 sones at 0.25 inch, where their airflow falls to 64 and 60 CFM.
The trade-offs
A sensor saves remembering, but it costs control. The quietest setup here needs a module that costs extra and a 6-inch duct. The quietest built-in sensor is far louder on a 4-inch duct. The shallow fans that fit 2x4 and 2x6 ceilings are rated 0.8 to 1.0 sone at 80 CFM at 0.25 inch, against 0.5 for the WhisperGreen Select. And every sensor needs its dial set for your climate, which means taking the grille off at least once. Prices are scarce too: only Broan publishes them. For the full test-pressure story and for fans sized for smaller rooms, see our guides to the quietest bathroom fans for small bathrooms and the quietest bathroom fans with a light.
How we chose
We started from HVI's certified products directory and listed every humidity-sensing bathroom fan from Panasonic, Delta Breez and Broan-NuTone with a rating at about 50, 80 or 110 CFM. We opened each maker's product page, submittal or specification sheet and installation manual, recorded the sone figures with their airflow, static pressure and duct size, and checked each against the directory. We ranked only within the same airflow band, at 0.25 inch, on the same duct size, and read each manual for how the sensor starts, stops and is wired. Broan's own price is the only maker price we could record.
We did not measure any of these fans ourselves. Every sone and CFM figure here is the maker's published rating or HVI's certified listing of it, quoted with the setting, static pressure and duct size it describes, and where only HVI publishes a figure, we say so.
