Angela, over the years I have relied on two devices.
The low-cost version was a LaCrosse Alerts device. It was very reliable for the 4 years I used it before selling the greenhouse. Beyond that I cannot say.
The more sophisticated device was a home-built sensor/sender coupled to a Sensaphone.
The sensor was a Thermalarm, which is nothing more than a bimetallic thermometer with electrical contacts - one each for low-, and high limits; with the needle being the other. When contact is made on either end, a circuit is closed. To send the signal, I found a place that sells radio frequency (RF) devices. Same technology as a garage door opener. The (wired) "key fob" was in the greenhouse, and when the Thermalarm circuit closed, the fob sent out its signal. In my home office was an antenna stuck to the window (the aluminum siding was good at blocking the signal) connected to a receiver relay that, when activated, would close a circuit connected to the Sensaphone.
The Sensaphone required a connection to a land line, and would first sound an audible alarm in the house. If that was not acknowledged within a selected time period, it would call up to 4 phone numbers sequentially, each dialed 4 times before going to the next.
