Every greenhouse heating system should be operated with an alarm system to sound a warning if the temperature falls below or rises above a pre-determined setting. A lean-to attached to a friend's home seemed safe "for the time being" and installation of a warning system was delayed. One night the temperature unexpectedly dropped to below freezing, and she lost her entire orchid collection in a matter of hours. I was more fortunate! The one time my heater failed, it apparently went out about an hour before my usual morning visit. When I checked, the thermometer registered 32 F, but at that time my greenhouse was filled mostly with cymbidium orchids that can endure such cold temporarily. I did lose one cattleya orchid, and certainly would have lost more if the failure had occurred during the night and the plants had been chilled for a longer time. Installation of an alarm system is simple. A high-low temperature alarm, battery-operated, can be set to sound a warning bell or buzzer in your home whenever the temperature reaches a danger point.
It is always a good idea to have available some means of auxiliary heat. A small kerosene heater will do, and it is simple to use as a temporary measure. Thermostats available for kerosene heaters are not calibrated properly for greenhouse use, so remember that if you have automatic ventilation you will have to operate it by hand during the time you are using a manually controlled kerosene heater. In an emergency when no means of auxiliary heat is available, cover plants with newspaper or plastic. If the heat failure is of brief duration, this may work. Obviously, such covering will not save plants if the greenhouse is without heat for an extended period.
In any case, schedule a summer check-up of your heating system every year. Minor adjustments made at this time—when the heat is turned off—prevents serious winter trouble. Don't let efficient operation give you a false sense of security. My own difficulty occurred because I neglected the check-up. My heater had operated reliably for five years. The one failure was due to a deposit of carbon that eventually extinguished the pilot light. A summer check-up would have revealed this and avoided a dangerous situation. Don't take your heater for granted.
Check it before it checks out!