SMART HOME BUTLER · BATHROOM
Let the bathroom clear its own damp air
No walking back to hit the fan switch after a shower, and nobody forgetting to turn it off. It starts when the air gets humid and stops once it is dry.
That bathroom smell is damp air with nowhere to go
After a shower, humidity in a bathroom often passes 90%. If that moisture has no way out, it sits there for hours. Mould needs three things to take hold: moisture, warmth and something to feed on. A bathroom offers all three.
The usual answer is "remember to run the fan". But the person who just showered is the last person who wants to walk back and press a switch, and almost nobody remembers to return and switch it off. So the fan either never runs, or runs all night.
Hand it to the system and it gets simple. Humidity climbs to 80%, the fan starts. It falls back to 60%, the fan stops. You are not involved at all.
Why the on and off numbers have to be different
Set both to the same number and humidity hovers right around it, so the fan switches on and off over and over. That is noisy and hard on the motor. Pull the two numbers apart and one run clears the moisture properly, then the fan waits quietly for the next time. This is called hysteresis, and it is the heart of the whole setup.
What it actually did that evening
Here is the same evening as the chart above, written out.
- 21:00
Nothing happening
Humidity sits at 65%, comfortably in range. The fan is off and the system is simply watching.
- 21:16
Someone starts a shower
Humidity climbs past 80%, so the fan comes on, along with the dehumidifier if there is one. You would not notice from inside — it is doing what it should.
- 21:26
Peak
93%, mirror completely fogged. The fan keeps going. This is the stretch where it earns its keep.
- 22:00
Cleared
Humidity drops to 60% and the fan switches off. Forty-five minutes start to finish, with nobody touching a switch.
- After 22:00
Quietly on watch
Humidity drifts between 58% and 66%, all of it safe, and the fan never restarts. That is exactly what separating the two numbers buys you.
Four things it takes care of
Health
Held under 60%, neither mould nor dust mites can establish themselves. If anyone at home has allergies or a sensitive chest, this is the one they will notice.
Safety
Floors stop staying wet for hours. In a home with older adults or small children, one fewer chance to slip is worth having.
Comfort
No musty smell and none of that closed-in damp feeling. Towels are properly dry by morning rather than half-damp.
Running cost
It runs when useful and stops when not, so nothing spins all night. The fan lasts longer, and sealant does not blacken within a couple of years.
Before and after
| When | How it goes now | Once installed |
|---|---|---|
| After a shower | Someone has to remember the switch | Starts as humidity rises |
| Switching it off | Usually forgotten, runs till morning | Stops once the air is dry |
| Humid season | Noticed once mould appears | Ventilates twice daily on schedule |
| Sensor fails | You would never find out | Sends you a message |
| Walls and sealant | Blackened in a couple of years | Humidity controlled, lasts longer |
What you need to have
Home Assistant calls every device in your home an "entity". The brand and model do not matter — what matters is the category it falls into. Have these and you are set.
| What it does | Category in the system | Essential? |
|---|---|---|
| Measure humidity | sensor | Yes |
| Extractor fan | fan, or a switch | Yes |
| Dehumidifier | switch | Nice to have |
| Know if the door is open | binary_sensor | Nice to have |
| Receive alerts | notify (phone app) | Recommended |
How the whole thing works
There are only three roles: something that measures, something that decides, and things that act.
Use the ready-made decision, do not build one
The hub ships with 22 tested automation blueprints. Think of a blueprint as a fill-in-the-blanks form: the structure is done, you supply the content. This scenario needs exactly one of them, called greenhouse, which exists to handle the two-number problem.
| Field | What it means | Suggested |
|---|---|---|
| Which number to watch | The bathroom humidity sensor | — |
| Start above | Above this, the fan comes on | 80% |
| Stop below | Below this, the fan goes off | 60% |
| What to control | The fan, plus a dehumidifier if any | — |
| Notifications | Whether to tell you when it acts | On |
Fill in those five and both starting and stopping are handled. You do not write two separate rules, and you do not have to work out what to do about a fan that keeps cycling.
Different homes, different numbers
The values above are a good general starting point. If your situation is unusual, adjust like this.
| Your situation | Start | Stop | Why |
|---|---|---|---|
| Studio, windowless bathroom | 75% | 55% | Poor airflow, so start earlier and clear more |
| Typical home, small window | 80% | 60% | The general case — use this unless you have a reason not to |
| Older adults at home | 75% | 55% | Floors dry sooner, fewer chances to slip |
| Large or wet-room bathroom | 85% | 65% | Moisture disperses on its own, no need to be aggressive |
| Humid season, temporarily | 75% | 55% | Ambient humidity is high all season, so shift both down |
Three optional extras
One blueprint covers the core. These three are additions — take whichever you actually want.
| If you want | Blueprint | What it does |
|---|---|---|
| Scheduled ventilation | adascene_multi | Up to ten slots a day. Twenty minutes at 9am and 5pm works well, and earns its place in the humid season |
| Wait a bit after a shower | binary_sensor_trigger_things | Starts five minutes after the door closes, so it never roars into life while someone is still in there |
| Tell me if it runs too long | entity_state_trigger_notify | Messages you if the fan runs past 45 minutes. That usually means the sensor is stuck and its reading is not updating |
Two one-tap modes
Boost
Fan at full speed, dehumidifier alongside it, target dropped to 50% for the duration. Straight after a shower, or through the humid season.
Daily
Fan at half speed, dehumidifier on standby, target 60%. Ordinary days and rainy weather.
Just say the word
The WOOWTECH hub ships with HA-MCP, which lets AI connect straight into your system: it finds the bathroom humidity sensor and the extractor fan itself, builds the rule, then tests it once and confirms the fan really turns before reporting back. All you have to do is describe the result you want.
Prompts you can copy straight across
Every block below can be copied whole with the button on the right. Pick the one closest to what you want — you do not need all five.
Find the humidity sensor and the extractor fan in my bathroom, then build me automatic dehumidifying. Above 80% humidity, start the fan. Below 60%, stop it. Use two different numbers for on and off, so the fan does not sit there cycling around a single point.
Build the bathroom dehumidifying properly. Start the fan above 80% humidity and stop it at 60%, keeping the two numbers apart so it does not switch on and off around one threshold. If there is a dehumidifier here, run it alongside the fan and stop both together. Also ventilate automatically for twenty minutes at 9am and 5pm every day, which I want for the humid season. If the fan runs more than 45 minutes without stopping, send me a phone alert, since that usually means the humidity sensor is stuck. Keep every notification on phone push, and use the ready-made blueprints on the hub rather than writing your own. When it is built, test it yourself, confirm it really fires, then tell me which devices you used.
I live in a studio and the bathroom has no window at all, so the damp hangs around for ages. Move the thresholds down: start the fan above 75% and only stop it at 55%. Since there is no window to open, add a third scheduled airing as well, twenty minutes each at 9am, 5pm and 11pm.
Change the bathroom dehumidifying rule. Between 10pm and 7am, cap the fan speed at 50%, because it carries into the next bedroom. And for the humid season, drop the start threshold to 75% and the stop threshold to 55% for now. List the four numbers back to me once you have changed them.
The fan does come on when it should, so that part is fine. What bothers me is how slowly the humidity falls: often still in the seventies after half an hour of running. Pull the humidity curve from the last few dehumidifying runs and work out how many points it drops per ten minutes. A quick start that flattens out later means my stop threshold is set too low and the fan is doing pointless work. Slow the whole way through means the airflow itself is too weak. Do not change anything yet, just tell me which of the two it is.
Common questions
Humidity never comes down
The reading jumps around
The fan has got louder
The dehumidifier does nothing
Nothing happens at all
Do I have to rebuild it to change the numbers?
A month from now
The smell is gone, the mirror clears faster, towels are dry by morning and the sealant is still white.
And in that whole month, you never once touched the fan switch.