Hi there. I am looking for a simple reliable solution to turning on an extractor fan when the electric shower is used, and turning off the fan a while after the shower has stopped being used. I cant rely on the person using the shower to turn on the light during the day and I don’t want the fan to turn on if somebody is not using the room specifically to have a shower which rules out a PIR. I use Shelly relays and home assistant but the relays don’t work because of leakage voltages causing false switch signal and anyway its not safe to tap off the high current electric shower line without a fused spur etc. Doesn’t have to be a home automation product/solution.
Two parts. The trigger and the action.
Trigger can be heat in pipe, or power to electric power or PIR
You will need something to turn the fan on and off. The action
They are separate things.
I think the most classic way is to use humidity sensor as trigger and relay to control the fan. They can be separate devices and you can set the logic to humidity or time based on HA.
I don’t see any reason why shelly 1 would not work to switch the fan on/off.
The humidity response is very slow. I did try that thank you.
The problem with the PIR is that it does not distinguish between someone using the toilet and someone using the shower. Plus it does not detect person through shower glass screen.
Are there other smart relays out there that don’t suffer form Shelly’s problem of having a very low voltage switch threshold?
Yeah, if you have gadget that reads the sensor every 5min.
Common SHT4x has response time ~4s.
Explain that better.
Shelly relay input will trigger as low as ~3V so any stray induced voltage will trigger it.
SHT4x is a sensor but i dont want to start building my own ccts.
Thank you for your responce. 
My fan is on a smart plug on the light circuit. This is coded in esphome so when the light turns on it waits 1min then turns the fan on for 10min. So this operates even if HA is down.
I also added an automation so that if the light is on (smart plug available) and gas hot water starts it turns the fan on. Can you get a trigger that electric heater has switched on?
Shelly doesn’t trigger any different way from others. If you get false triggers from parasitic coupling your wiring is far from ideal and you probably get falses with other devices as well. Bleed resistor might help.
I didn’t mean that, just pick a device that has decent update interval on data sheet. Some battery powered ones read only once in a while to save battery.
Note a humidity sensor on the ceiling and near the shower will trigger more quickly than a humidity sensor in a smart switch near the door far from the shower.