Hot Greenhouse on a cool day

2 weeks past the 1st frost and plants are still alive. Overnight it got down to 25F outside and I heated the greenhouse to 40F on a very low setting on the heaters. I had to change the inflation blower timer to 45 minutes off and 15 mins on, down from 1 hour off 15 min on. That is due to the cold weather making the polyethylene contract. When I finally let the plants die off, I will stop inflating the greenhouse. It will be taught due to the cold and installing it on a warm day.