coldest night of the winter--so far

So this morning weather.com is telling me that it is -15 degrees outside.  The sun is not yet shining on the greenhouse so I head outside to check on how the greenhouse fared in this cold weather.  It is 28 degrees with the little milkhouse heater running all night.  Last year I used a second heater on really cold nights and when I was checking on things this fall in preparation for cold I discovered that the electronics of that little heater no longer worked and some of the plastic parts were a little misshapen (from melting?) so I am assuming that it died during the really hot weather this summer.  I think the plants will survive an occasional cold night but if it looks like there will be a lot of cold I might want to replace the dead heater with another milkhouse heater.  Our goal has always been to use as few resources as possible in the greenhouse but I want to keep the plants alive and thriving as well.  There are a few more cold nights predicted this weekend and early next week but then it looks to be above zero lows for the rest of the 10 day forecast.  I guess winter has arrived.  It is supposed to be sunny today and that should warm things up nicely.