A slightly different approach. Does it need to be a smart switch? I have the following timer switch in my master bathroom. Motor-rated.
It works as a timer and can’t be left on indefinitely.The timer settings are 1, 5, 10, 20, 30, and 60 min.
P.S. It controls a Panasonic vent/heater combo similar to one linked upthread.
It is okay under code to have 20A outlet protected by a 15A breaker… Just not in a bathroom! As others have stated, an 1800W hairdryer pulls 15A at 115V, and should trip a working 15A breaker.
However, it is absolutely safer to swap to a 15A breaker than it is to run a 15A switch on a 20A breaker. So if you’re going to step out of code, this would be the safer option in my opinion.
Sometimes code is about safety (no 15A plugs on 20A breakers) but sometimes code is about convenience (you must have a 20A plug in your bathroom for your hairdryer).
Yeah, if a compromise needs to be made, this is the one I would do.
As long as this compromise doesn’t lead to using extension cords for hairdryers, I totally agree.
Nah… I’m gonna remove all those pesky breakers… They just impede my flow!
Ha ha, yeah!
Similar to the first couple of posts, I have a CoRE piston that handles this for myself…
For weekdays I have a “Morning” mode that’s triggered via motion in a time window. When in that mode, the fan stays on. Once my wife leaves for the day (I work from home), the house mode switches to “Home.” My piston is as follows:
IF
fan is on
AND
Bathroom temperature is less than or equal to 75 degrees (it’s the hottest room in my house, baked by the sun from noon until sunset! I don’t have central air, and even with the door closed, it gets stuffy in the bathroom)
AND
mode is home
THEN
Using fan
As far as code is concern. If you have a single receptacle on a 20amp breaker. Then you must have 20 amp receptacle. You are allow to have 15amp receptacle on a 20amp breaker if there are more than one receptacle on that 20amp line. Reason is the that you won’t be able to plug a 20amp equipment into a 15amp socket because of the special tab on that cord and plug.
So that hair dryer that’s using more than 1800watts will have a 20amp cord and plug and won’t fit in the 15 amp receptacle.
So what happens if EVERY breaker in your house is 20amp? Does that mean I can’t use any of these GE switches anywhere?
I guess I’m screwed!
To Get back to the OP…
Yes, you can use a GE smart switch.
Yes, there are alternatives for programming.
There are lots of apps you can use.
An on/off switch is usually fine for an on/off fan as long as all the other specs (amps, etc.) are OK.
Never use a dimmer switch for a fan.
If the fan has low/medium/high settings or variable speed, then you need to get a fan switch.
As far as networked versus non-networked with a timer, The nonnetworked will usually be cheaper and more reliable, but you won’t be able to set up fancy automations with all kinds of different logic like light sensors and humidity sensors and that stuff.
I ended up going with a $20 nonnetworked motion sensor switch for a small bathroom because it has foil wallpaper, tile on the shower, and a big mirror, and I was having a terrible time getting signal into the room, regardless of protocol.
So you may find that you use different solutions for different rooms, just depending on the specific conditions. 
A strict electrical inspector would say this is true. You may not use a 15A switch protected by a 20A breaker.
Again, your own tolerance for risk is your own tolerance. A 15A switch doesn’t immediately burst into flames when you pass 15.2A through it. Reputable manufacturers build tolerances into their switched for edge cases. Overloaded switches fail over time because the switch pads are not designed for the stress of opening and closing under high loads.
The real danger is 14gague wire on a 20A breaker. If your house is wired with 20A breakers but with 14gague wire, that’s a big problem. Far bigger than a few 15A switches with hard wired loads under their rated capacity.
It may not be relevant to this thread as I’m in the UK, but in my experience it can be OK to use a dimmer with an extractor fan.
This is because extractor fans often have a separate power supply to ‘run on’ after the light is off, meaning they don’t take their supply from the lighting circuit.
Not sure if that applies to the USA though.
You should never use a light dimmer switch for a fan motor. If the fan is wired in such a way that it is not actually drawing current from the circuit controlled by the fan then that’s an entirely different situation.
If the fan is just on/off, not variable speed, you can probably use a regular on/off switch as long as all the other ratings match.
This applies in the UK as well:
Yes, I’m in the UK and that is how I have wired mine.
The lighting circuit can just be a trigger to turn on the fan and as @John_Crighton has said that fan then has it’s own supply.
So I have wired my Dimmer into the light ‘trigger’ and use a Humidity Sensor to turn on/off the light trigger.
The fan then runs on for the minimum settable time (2 minutes for mine) when the Dimmer turns off and then turns off.
I put this one in, absolutely love it!
If you are building your house. You have control of how you want the wires going into each room. I would never have lights and receptacles in the same circuit even if my municipal allowed it. Last thing you want is your hair blower trips the breaker and you are in the dark trying to get out of the room to reset the circuit.
Highly doubt it that your inspector is going to give you a hard time having a 15amp switch on a 20amp breaker knowing the max load of that switch is not going over 80% of that 15amp switch. I don’t think there is a problem with 15amp zwave switch on 20amp breaker as long as you know the fan/light is not drawing crazy amount of current. I would check with your local law of course. Don’t want house fire on this forum caused by smart switches.
FWIW - I’s nearly impossible to find a timer/fan timer / etc that is 20 amps. Even looking at the Broan ones, which are specifically designed for bath fans all I saw were 15. They do have one of those 3 switches in one gang that is 20 , surprised they can get 3 20 amp switches in their.
Not saying their are none - but I spent some time looking and saw none. I have one of the solid state count down ones, I doubt if it’s 20, nor do I care 
I have an old one where you that is spring powered, bought it for a project and never used it, will need to check it.
They’re all 20A breakers with 12AWG wire, the switch is ONLY connected to the extractor fan which is similar to what others have described so like 1/2hp or very low wattage. I assumed because this is the only load on the switch and is significantly lower than what the switch is rated for this would be fine.
The comments in this thread though have got me worried though that this is not up to code.