I have a refrigerated "cellar" in the garage where the temperature is quite high during the afternoon. I would like to get a system to send me an alert of some kind when the temperature in the cellar gets above, say, 70F / 20C. The form that alert takes doesn't matter: email, app, text, whatever. The things I don't want, in descending priority: batteries that need replacing, expensive monthly service fees ($20 a year is fine, but I'm not paying for a whole security system), wired Ethernet connection. I am reasonably handy, if someone has a solution that requires soldering stuff together that's fine.
For a while I was thinking the Elgato Eve was going to be the right solution, but it seemed like Apple Homekit didn't support temperature alarms. It could report the temperature from a sensor, but would not alarm. What are people using in their refrigerated storage?