The red thermostatically controlled outlet is supposed to come on at 78* and got off at 70*. I was actually outside one day and heard the fans shut off. I checked the temperature in the greenhouse and it was 67*. That's 3 degrees off from what it's supposed to be (assuming my other thermometer is correct). I can live with that for fans, not for the heat.
As a result of this I've had to manually turn the heater on at night and off in the morning. That's really inconvenient.
I found the solution in the home brewing community. Home brewers need to keep their fermentation vessels at a specific temperature and there's a nifty device called an STC-1000 that will do just that. There are a lot of forum posts, blogs, and youtube videos on how to use the thing. I went by this one.
The STC-1000 has a temperature sensor and two relays. It turns one one for heat and other to cool. It's exactly what I needed.
I also needed an old computer power cord, a receptacle, a wall plate, wire, a wire nut, and a 6x4x2 project box from Radio Shack.
I set the temperature on it to 4.4*C which is 40*F. I plugged a light into the heat outlet, put the sensor in the freezer and waited. When the panel hit the desired 4.4* the light came on.
My solution was to continue using the red thermostatically controlled outlet that I already had. I plugged it into the cold outlet. I plugged the heater and a light bulb into the hot outlet. The light will make it easy for me to know when the heat is on.
Bonus tomato picture: