I am converting my 12x16' green house into a large cold frame. I don't want the temp to drop much lower than 30*. I have a 240 volt, potable heater but the temp only lest it go as low as 45*. Is there a way I can install a Thermostat switch externally to only let the heater kick on at a lower temp?

Like this?

https://www.amazon.com/s/?ie=UTF8&ke...l_20zsd2z64j_b

https://images-na.ssl-images-amazon....S327_QL65_.jpg

Or simply spring for a heater with a programmable thermostat built in.

Search "240VAC line thermostat". Lots of options