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Comment by hedora | Hacker News Reader

The cost of the fan is dwarfed by labor, and you’re not getting a fan you want to live with for $20. Also, at least in California, you have to hook it to a janky humidity sensing switch that costs more than $20.

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I'm just talking about a normal bathroom fan you turn on when you want to have a shower or something. I've honestly never seen one at a hardware store that costs more than about $40. An electrician would take about 15 minutes to install one if you've got a power source nearby, or if you get one with a light built in you could install it yourself by just replacing an existing light fixture with it.

Luckily I don't have the misfortune to live anywhere near California, but that humidity sensor business sounds bizarre. Is that some kind of building-code thing?

> humidity sensing

Anyone have links to working designs for humidity controlled fans or similar - preferably Raspberry Pi?

I want an indoor sensor and an outdoor sensor, and to run the fan only occasionally (maybe 10 minutes a day) and only when outdoor humidity is much lower then indoor humidity. For holiday sleepout, that sometimes gets damp, but outdoors is sometimes high humidity so no point running fan in that case!

I've looked at household ducted fans, but they don't really do what I want.

BME680 sensors for temperature/humidity/pressure should be suitable. Consider the PI's PWM output and a hobby RC BLDC motor/controller combination for "cheap variable-speed fan", or alternatively, a large diameter PWM computer fan from e.g. Noctua. Depends on the power class you need/want to go for (PC fan is low power, RC BLDC is high power).

> Also, at least in California, you have to hook it to a janky humidity sensing switch that costs more than $20.

To make it run automatically?

To have your tight house (and your family) breathe properly you’ll want to couple your smart bathroom exhaust fans with filtered air intakes. HRV/ERVs can do this but are overkill in many mild climates. A set of simple fan+filter pairs does the trick for us, can’t believe we lived so many years in poor ventilation.

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