The cold frame can be used with an automatic door opener or without one.
The main door of the coldframe is connected to the smaller door on the opposite end with a small cord. The cord is set loose enough so the main door is open about 1/3 - 1/2 open before the cord is tight enough to begin to open the smaller door on the opposite end of the coldframe. This is so the temperature stays high in the coldframe when the main door is only partially open. If the main door is more open then about 1/2 then the temperature is building fast and the smaller door will begin to open as the main door continues to open. At this point cross circulation of air from one end of the cold frame to the other begins and increases the cooling rate dramatically. If you make sure the end of the coldframe with the main door is taller than the other end then the hot air will naturally rise to the taller end and add to this flow.
The coldframe can be operated automatically or by hand. If operating the door by hand, open the coldframe each morning and set it as open as you expect to need for the day (ie: full open on a hot sunny day, partially open on cloudy day). Check the max-min thermometer after each day to see the range of temperatures for the day to make sure you are staying in a good range. It works even better if you are around all day to keep an eye on the temperature and adjust as needed. If it gets really cold at night, cover the coldframe (really carefully, uncovered corners can get cold fast). The closer to air tight the cold frame is (ie: with good seals around the doors) the better it hold the heat.
The automatic Door Opener has a cylinder filled with a material that expands and contracts with changes in temperature (expands slowly when hot). The spring on the door opener closes the door as the temperature cools. The cylinder is adjusted to set when the door begins to open. Careful testing will determine the best setting for your situation.
If you decide to buy the automatic door opener then you can set the door to open so the temperature never gets above about 85 or 90 degrees F. The opener is expensive (about $50 - $60), we bought one at Territorial Seed because it seems to have the most range in adjustment.Territorial Seed Co. Door Opener Listed in catalog under "hard goods" - "Greenhouse and Cold Frame Supplies" - "automatic opener for cold frame".
The advantage with the cold frame Door Opener is that you set it once and it adjusts itself all day as the temperature changes. If it gets cloudy and cools, the cold frame closes until the temperature rises. If it gets really hot and sunny, the doors will open full to keep the plants inside from over heating. The coldframe Door Opener can keep the temperature within a good growing range day to day. Cover the cold frame with a blanket if there is a danger of frost. Open the plastic cover of the cold frame from one side to water.
Updated: March 20, 2000