Cheapest way to replace thermostat bulb locked at 65°F with more intelligence?

Kat Rabun wrote the following:

Sure

I knew programmable thermostats existed, but are they pin-for-pin compatible with the simple bubble thermostats?

In other words, can I just remove the bubble thermostat from the wall somehow and plug in the programmable thermostat in its place?

It doesn't "plug in", it has screw terminals. Make a note of what color wire goes on which terminal (they are lettered) and put them on the same terminals on the new thermostat.

Hi, New thermostat comes with intall. manual. Just don't buy El Cheapo.

I didn't realize it was that easy to substitute a smart thermostat for a dumb one (like how does it get its electricity?).

Do you have a recommended smart thermostat you're happy with?

batteries

My bubble thermostat has batteries? I've never changed them. Seems to me a thermostat should be wired to the wall. Aren't they?

This is all so confusing.

No, your old thermostat does not have batteries. The bubble completes the circuit when the temp goes down and breaks the circuit when the temp goes up. The programmable thermostats need batteries for their lcd displays and memories.

They can recharge via the voltage on the circuit they control as long as they don't have to run 100% of the time.

The first one I had did neither of those. It required an additional wire from the furnace (the 24V return wire, normally not connected).

It varies. Nowadays, most have such low power requirements, they simply use a battery such as a 9v. I have a setback thermostat on it's 3rd year on the same battery. Back in the 70's, my dad put in one that recharged itself, but had a mechanical timer. It frequently ran down and I ended up running an AC adapter to it to keep it charged.