Comment by cyberax
9 hours ago
They are typically not powered by batteries. These thermostats either use the common wire or utilize the fan circuit to periodically charge a capacitor powering the MCU.
> I can look at it and evaluate whether I trust it. I cannot do that with some proprietary blob on an MCU.
Your air conditioner/heater likely has a controller. Probably several, at least for thermal protection and overcurrent.
You make a good point about the furnace. It remains to be seen how long it'll last. My only hope is that it was designed to be serviceable.
I was very surprised to find the battery. This thermostat is designed to be compatible with older 3-wire systems, so I suspect they slapped a "10 year" battery in it and hope for the best. It's also marketed fairly deceptively. Or at least enough have fooled me--I thought I was buying an analog device.
EDIT: now I'm looking at the data sheets for an LM57 and getting some ideas