The outdoor temperature is about -4°F (-20°C) right now (8:30 AM). This time of year, the main focus of concern in the greenhouses is heat. We have three different systems in use, for three different situations.
For the big greenhouse (ca. 2700 sq. ft., which we call Number 4), with all the clivias there is an environmental control computer that regulates heating and cooling and potentially other things as well. There are two identical large overhead gas heaters, capacity around 100,00 to 150,000 BTUs each. Either one could heat my house easily. In the big greenhouse, the controller starts the number 2 furnace whenever the inside temperature drops to the set point. If the one furnace alone is not sufficient, when the temperature drops 2 degrees below the set point, the controller turns on furnace number one as well. This whole greenhouse and some adjacent structures are connected to a large emergency generator that automatically starts when there is a power interruption of more than a few seconds. The clivia greenhouse is probably very well protected in cold weather.
The equipment shed, an oversized garage, and its adjoining hoop house (Greenhouse Number 3) are each heated by a small standard home gas furnace controlled by a simple thermostat for each furnace. Since they also get their power in emergencies from the generator, they are also well protected. A similar small gas furnace and plain thermostat provide heat for the lean-to glass house, Greenhouse Number 1, attached to my home. My home and hence this lean-to glass house are not on any emergency generator, so in case of power outages we have to scramble. I keep a couple of small kerosene heaters around, but we do not like to use them.
The third system is for the small (ca. 250 sq. ft.) freestanding glass greenhouse which is not far from our home. This one, Number 2 in our terms, is heated by two vented natural gas heaters on millivolt controllers. They do not use any electricity from the main system. Rather, the millivolt electric potentials generated by the bimetallic thermostats control these heaters. They have manual pilot lights which have to be restarted by hand when they go out. I went out to greenhouse number 2 to check the pilot lights as soon as I got out of bed this morning. Both were running and the greenhouse temperature was an acceptable 39°F.
So there you have it. Originally -- 30 years ago -- we had no gas out here, so Greenhouse Number 1 was heated by a couple of large electric heaters. Those heaters are still around, and as natural gas prices have approached the cost of electricity for heating, they have been set to take over some of the heating load in Number 1.
If you have a greenhouse that contains valuable plants, you need to provide backups for your heating system. The least satisfactory backup heating system, in my opinion, is one or more small portable kerosene heaters. They are plainly fire hazards and, in poorly ventilated structures, dangerous carbon monoxide sources.
Using millivolt gas heaters is not an optimum solution, but is preferable to using kerosene heaters in emergencies. Be sure to use vented gas heaters and to provide adequate air inlets into any greenhouse heated by gas heaters. No matter what system you choose, if you live in a cold climate, you need to provide for emergency heat in case of power failures. Don't risk your valuable plants,
Good gardening, from here in central Indiana
Look up technical terms in the Glossary of Plant Biology