In 2009, I could send and receive text messages though SMS gateways that would control my arduino-controlled hot tub that my friends and I got for $100 on Craigslist. I could adjust the temperature and it would acknowledge and tell me when the programmed temperature would be reached based on the time derivative. This was useful as a grad student in Ann Arbor winters.
I had a python script running on a handmedown computer running Ubuntu 8.10 that just periodically checked an email account, parsed the command, flipped the relay as necessary, read the temperature rise, and replied.
For a second I thought you were one of my roommates, because we had a very similar setup for our (free) hot tub. Ours was a few years earlier and using older hardware.
We had an actual server in the garage, with a X10 firecracker on one serial port and a temperature sensor on the other. We used a 220V relay with a 110V pull, hooked up to a normal X10 switch controller. So from an X10 remote in the house, or via an sms gateway, we could turn on our hot tub remotely.
This had a few advantages including: 1) this setup was more accurate than the ancient thermal sensor/control on the hot tub 2) we never again had a large electric bill because someone forgot to turn it off and 3) really nice for when a date is going well and you can turn on the hot tub ahead of time :)
I had a python script running on a handmedown computer running Ubuntu 8.10 that just periodically checked an email account, parsed the command, flipped the relay as necessary, read the temperature rise, and replied.
Writeup: https://partofthething.com/thoughts/hot-tub-controller/