You can't get VC for any consumer hardware product that doesn't generate a continuing revenue stream. That means either charging a monthly fee for cloud connectivity or selling customers' data. Both require Internet connectivity, even though that doesn't really add much objective value (and creates huge security problems) for so-called "smart" home devices.
As far as I can tell, you wouldn't need VC funding for that anyway. A thermostat, as far as I can tell, is really a set of relays that are toggled by a circuit board connected to a thermometer. A quick search on the topic brings up a lot of successful attempts at making thermostats with Raspberry Pis. It doesn't seem like advanced hardware by any stretch of the imagination.
Prototyping should be very easy. I'm sure that a Pi Zero would suffice, along with a board with relays connected to GPIO as well as a thermometer circuit, and an LCD screen. The production product could use a custom built stripped-down ARM board with integrated components, and probably be made for cheap. I'd pay $150 for a dumb thermostat that has a nice LCD screen. Even better if it's hackable(i.e. I can install my own software on it).
RPi is overkill. It could easily be done with an Arduino--including wifi if you wanted. The tough part is building a business around it. But maybe if you focus on sustainability rather than growth and exit, you could do it without VC money.
