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Bathroom fan automation: switch with light and moisture sensor?

Grzegorz214214 1287 16
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Can I automate a bathroom/toilet fan with the light and humidity using one switch or a built-in sensor module?

The simplest solution is not a custom switch with built-in sensors, but a ready-made bathroom fan with a humidity sensor and timer, connected to the lighting circuit [#21425088] In a suspended ceiling, you can also just run the wires from the lamp to the fan grille, so there is no need to build extra automation [#21425071] For the toilet, a regular fan with delayed shut-off is enough; the bathroom can additionally use humidity-based control if you want [#21426560] The thread’s practical advice is that a DIY Arduino/ESP switch is possible, but it is more time-consuming than using an off-the-shelf fan with the needed functions already built in [#21425205]
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  • #1 21424777
    Grzegorz214214
    Level 4  
    Posts: 6
    Hi,
    I have a request for experienced forumers.
    I am starting the adventure of automating devices in the house. At the moment I have an HA and a sub switch on zigbee for a light on the stairs.

    I would now like to automate the fan in the bathroom and toilet.
    This is where the question arises. Is there a switch with a built-in light and possibly moisture sensor?

    Stopping at the toilet. My expectation for the fan is:
    when the light is switched on "switch on" (I have a toilet without external windows so switching on the light is sufficient signal that the fan should be switched on)
    when the light goes out, "wait a set time and switch off".
    A simple thing assuming we have a total non smart fan.
    One light or presence sensor +switch, e.g. SONOFF Basic.
    And although I don't currently know how to put it all together to make it work, I assume I can figure it out... step by step.
    The problem is that it's a lot of elements, each one looks different, each one has either a battery or a cable. Generally it doesn't look attractive and I'd have to think hard to hide it.
    In the bathroom, additional data such as humidity. so an additional sensor.

    Is there a switch with a built-in sensor? I don't know something that will have an input at 230 and a controlled output.... depending on various parameters... e.g. lighting, humidity etc.
    Assuming of course that... the price is reasonable... or is the option between some expensive smart fan or a sensor sticky?

    Many thanks for your help
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  • #2 21424790
    m2606
    Level 33  
    Posts: 2606
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    Do you have a false ceiling?
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  • #3 21424831
    Grzegorz214214
    Level 4  
    Posts: 6
    >>21424790 .
    In the toilet no, in the bathroom partly yes.
    All in all, right, in the bathroom it can be accommodated on the false ceiling, although this will require a little remodelling at my place, but in the toilet it's a no-go.
  • #4 21425071
    m2606
    Level 33  
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    In a suspended one, you pull the wires from the lamp to the grille and don't have to automate anything.
  • #5 21425088
    balonika3
    Level 43  
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    Why any smarts here at all? After all, there are fans with a humidity sensor, with a timer. Ready-made solutions, simply connect to the lighting.
  • #6 21425185
    Grzegorz214214
    Level 4  
    Posts: 6
    and the spell was gone :P but you know, it would be ..smart ;P
    well, maybe I'm right, maybe I've overcomplicated a simple thing and want to make it more complicated
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  • #7 21425205
    sussyamongussi2
    Level 6  
    Posts: 38
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    >>21425185 I think it is possible to make such a switch myself, using Arduino or ESP, but is it worth it, and you need to have the time to mess around with it.
  • #8 21425343
    balonika3
    Level 43  
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    Grzegorz214214 wrote:
    but you know, it would be ..smart
    Would you like to use it or brag about it? Because you see - when I start the car, the radio comes on. And I don't care whether it's a smart or an ordinary relay, the important thing is that it turns on, which means it works.
  • #9 21426327
    Pan.Kropa
    Level 35  
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    Soon it will be smart cisterns that analyse how much of what is in the bowl and a microprocessor will calculate the amount of water needed to flush it heh.
  • #10 21426337
    m2606
    Level 33  
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    >>21426327 .
    There are already such flush toilets . As well as toilet seats that "talk" to them,and even serve music depending on the mood of the bottom.
  • #11 21426349
    Pan.Kropa
    Level 35  
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    And the toilet probably distinguishes and analyses whether Mr Dot or his wife has sat down. I have nothing against amenities. But many are simply an embarrassment.
  • #12 21426364
    m2606
    Level 33  
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    Exactly so,Sedes analyses. My wife used to work for a company that designs and distributes this.
    She'll rinse the back of it,remember the water temperature,sing it,dry it and even illuminate it.
    You're right,it's embarrassing.
  • #13 21426373
    bratHanki
    Level 39  
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    After a certain limit, the house can already become so intelligent that it considers the owner to be dumber than it is and does not let him in. :-)
  • #14 21426456
    paszczakq
    Level 27  
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    Flat block or detached house ? As a general rule, forced mechanical ventilation is not allowed in blocks/apartments .

    A friend installed such a fan. The effect was that it was blowing half the night. In the morning he went into the bathroom and did a triple flip on wet tiles. Somehow when there was gravity ventilation for 50 years there were no problems.
  • #15 21426460
    Pan.Kropa
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    paszczakq wrote:
    A friend installed such a fan . The effect was that half the night he blew . In the morning he went into the bathroom and did a triple flip on the wet tiles . Somehow when there was gravity 50 years there were no problems .
    I do not understand why the working fan could cause moisture in the bathroom tiles?
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  • #16 21426537
    m2606
    Level 33  
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    Because the ventilation shafts in a block of flats are shared.If you pressurise the chimney, your neighbour loses his ventilation.
    You will make him fungus,or even poison him by injecting various gases.
  • #17 21426560
    Grzegorz214214
    Level 4  
    Posts: 6
    Yes, that's right. In a block of flats, if someone has designed gravity ventilation, then switching on a fan in one flat can throw excess air at a neighbour. I don't know if it's so bad that the dampness comes through, but it's a fact that you generally shouldn't do that.
    I live in a house so I don't have a problem.
    Ok, thanks for the answers. I'm going to install a regular fan in the toilet with a delayed shut-off, in the bathroom I'll probably play with something.... maybe overkill but the word "play" is key here :) .
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Topic summary

✨ The discussion centers on automating bathroom and toilet fans using a switch integrated with a light and possibly a moisture sensor. The primary goal is to have the fan activate when the light is turned on and shut off after a delay once the light is off, suitable for non-smart fans. Considerations include installation constraints such as the presence or absence of a false ceiling, which affects wiring options. Ready-made fans with built-in humidity sensors and timers connected to lighting circuits are suggested as simpler alternatives to custom automation. DIY solutions using microcontrollers like Arduino or ESP are possible but require time and technical skills. The conversation also touches on ventilation issues in apartment blocks, where forced mechanical ventilation can disrupt shared ventilation shafts, potentially causing moisture problems for neighbors. The author plans to install a regular fan with delayed shut-off in the toilet and experiment with more advanced automation in the bathroom for learning purposes.
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FAQ

TL;DR: "for 50 years there were no problems"—but a mistuned fan can run all night. Use a humidity‑sensing fan or a simple timer linked to the light; in flats avoid pressurising shared shafts, in houses a Zigbee/HA relay works cleanly. [Elektroda, paszczakq, post #21426456]

Why it matters: This FAQ helps homeowners choose a clean, code‑friendly way to automate bathroom/toilet ventilation without ugly add‑ons or neighbor issues.

Quick Facts

  • Ventilation target: 50 cfm intermittent or 20 cfm continuous for bathrooms (≈85 or 34 m³/h), per ASHRAE 62.2. [ASHRAE, 2019]
  • Humidity sensing: Typical adjustable setpoint 50–80% RH; built‑in off‑delay about 5–60 minutes on common models. [Broan-NuTone, 2023]
  • Safety zones: Use IPX4‑rated equipment in Zones 1–2; keep control gear outside zones or in suitable enclosures. [IEC 60364-7-701:2017]
  • Noise: Look for ≤1.0 sone quiet fans and HVI Certified ratings for comparable performance. [HVI, 2022]
  • Costs: Basic fans ~$50–100; humidity‑sensing ~$120–250; smart in‑wall switches/relays ~$10–50. [Consumer Reports, 2023]

What’s the simplest way to make a toilet fan come on with the light and turn off after a delay?

Use a fan with a built‑in timer or a wall fan‑timer switch. Feed it from the light circuit so the timer starts when the light goes off. Set 10–30 minutes run‑on to clear odors and moisture. “Run the fan for 20 minutes after a shower.” This keeps wiring tidy and avoids batteries. It works well with non‑smart fans. [HVI, 2022]

Is there a single wall switch with built‑in humidity sensor (and maybe a light) for 230 V?

Practical path: pick a bathroom fan with an integrated humidity sensor and timer, then connect it to the lighting circuit. As one expert put it, “there are fans with a humidity sensor, with a timer. Ready‑made solutions, simply connect to the lighting.” That keeps hardware minimal and neat. If you truly need a wall control, choose a humidistat switch rated for your voltage and load. [Elektroda, balonika3, post #21425088]

Can I just wire the fan to the light in a false (suspended) ceiling?

Yes. In a suspended ceiling, route the fan feed from the lamp and you may not need any automation. This makes the fan run with the light, especially useful in windowless toilets. Add a delayed‑off fan or timer switch if you need post‑run. Ensure connections are in accessible junction boxes. [Elektroda, m2606, post #21425071]

I live in a block of flats—can I install a bathroom fan?

Be careful. Shared ventilation shafts can backfeed neighbors when you pressurize the duct. You could cause mold or even backdraft gases. Many buildings restrict mechanical exhaust into natural‑draft shafts. Check building management and local rules before installing. Detached homes with dedicated ducts don’t have this issue. [Elektroda, m2606, post #21426537]

How do I set ‘fan on with light’ automation in Home Assistant using a Zigbee relay?

  1. Pair a Zigbee fan‑rated relay/switch to ZHA or Zigbee2MQTT in Home Assistant.
  2. Create an automation: trigger = light turns on; action = turn on fan entity.
  3. Add another automation: trigger = light turns off; action = delay (e.g., 15–20 minutes), then turn off fan. [Home Assistant, 2024]

What humidity setpoint should I use so the fan doesn’t run forever?

Start around 60–70% RH for bathrooms. Typical humidity controls let you adjust between 50% and 80% RH. If the fan lingers, raise the setpoint or reduce the built‑in delay. Many controllers also let you cap maximum runtime. “Adjustable 50–80% RH” is common on humidity‑sensing models. [Broan-NuTone, 2023]

How long should the fan run after showering if I just link it to the light?

Aim for 20 minutes of post‑run. That duration flushes moisture to prevent condensation and mildew. Use a wall timer or a fan with a built‑in delay to meet that runtime. Keep the door slightly open or undercut for make‑up air. “Run the fan for 20 minutes after a shower.” [HVI, 2022]

Will a SONOFF Basic or similar module safely switch a 230 V fan?

Check the exact model and ratings. SONOFF BASICR2 is rated 10 A at 100–240 VAC, far above a typical 20–60 W fan. Ensure proper enclosure, neutral availability, and compliance with bathroom zone rules. Verify motor‑load suitability; some relays list resistive ratings only. When unsure, use a relay specified for inductive loads. [SONOFF, 2022]

What noise level should I look for in a bathroom fan?

Select fans rated at or below 1.0 sone for quiet operation. Use HVI Certified ratings to compare airflow and sound on equal terms. Verify the airflow at 0.1 in. w.g. static pressure, which mirrors typical installations. Lower sone ratings improve comfort and encourage users to run the fan long enough. [HVI, 2022]

Where can I place smart relays or sensors in a bathroom with a partial false ceiling?

Mount controls above the false ceiling, outside Zones 1–2 when possible. If equipment must be within Zones 1–2, select at least IPX4‑rated devices. Keep gear accessible for service and away from splash paths. Maintain required clearances and follow manufacturer installation distances. [IEC 60364-7-701:2017]

Are smart fans worth it over timer/humidity models?

For most bathrooms, a timer or humidity‑sensing fan solves the core problem simply. Smart fans add app control, analytics, and integrations but increase cost and complexity. Consider maintenance and reliability before adding radios and batteries. As one pro quipped, “Would you like to use it or brag about it?” Choose function first. [Elektroda, balonika3, post #21425343]

Any gotchas with humidity‑sensing fans keeping you up at night?

If the fan won’t stop, the RH setpoint is likely too low or the delay too long. Raise the threshold, shorten the off‑delay, and verify the room can dry (door undercut for make‑up air). Many models provide 5–60 minute delays and 50–80% RH adjustments. These features prevent nuisance runtime. [Broan-NuTone, 2023]

Why did my bathroom floor get wet despite the fan running?

In buildings with shared gravity shafts, a powered fan can push moist air into connected spaces. That backflow can re‑condense on cool surfaces, leaving floors slick. Avoid pressurizing shared ducts. Use approved solutions and consult building management. This edge case doesn’t apply to dedicated, isolated ducts in houses. [Elektroda, m2606, post #21426537]

How much airflow (CFM or m³/h) should I choose for a bathroom?

Follow ASHRAE 62.2: 50 cfm (≈85 m³/h) intermittent or 20 cfm (≈34 m³/h) continuous local exhaust. Size the fan to deliver that airflow at installed static pressure. Larger rooms or long ducts may need higher rated CFM. Meeting this target cuts moisture fast and limits mold growth. [ASHRAE, 2019]
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