For an attached greenhouse, or for a detached unit no farther than 50 feet from your residence boiler, you may find it practical to adapt your present heating system to include the greenhouse. One greenhouse manufacturer offers a Heating Extension Kit for this purpose. It can be used on a hot-water or steam installation to provide an independent, thermostatically controlled second zone of heat for the greenhouse. With this extension, it is not necessary to replace your present boiler or to add a second unit for the greenhouse. Installation costs are moderate. This extension provides one of the least expensive greenhouse heating systems, and it does not reduce the heat available for your home. With it you will be operating a two-zone system, each with its own thermostatic control.
If you are considering this extension of an existing heating system, have your local heating contractor check the BTU output of your present boiler to be sure there is sufficient capacity for the extra demand.
With a low-pressure steam-boiler, the boiler is tapped below the water line to provide the greenhouse heat. Install an aqua stat on the boiler to maintain a minimum of 180 F water temperature inside; also install a high-duty circulator. This system provides hot-water heat for the greenhouse and steam heat for your home. The hot water is circulated throughout the greenhouse by means of fin-tube radiation. With hot-water systems the aqua stat is set high enough to maintain water in the boiler at 180 F or higher. Most forced-circulation systems will do this. Low-temperature gravity-flow hot-water systems are not suitable for use with extension kits, and a vacuum-steam system cannot be adapted to two-zone heating. When you extend a heating system to a detached unit, all heating lines between house and greenhouse are buried in a trench to a depth below the local frost level.
If your home has a warm-air system that you wish to utilize for the greenhouse, it will be wise to attach your greenhouse to your house and as close to the furnace as possible. Supply and return ducts for warm and cold air must cover the shortest possible distance between furnace and greenhouse. Motor-operated damper controls are necessary in the heating ducts of both home and greenhouse. Each should be controlled by its own thermostat so as to operate independently. If you find the warm-air supply is too drying for the greenhouse, install some sort of humidifier, or compensate for the drying out by extra watering and syringing of plants.
Heating extension kits can be used for glass-to-ground designs or in partitioned greenhouses but extra pipe or finned tube for additional radiation is necessary.
Most greenhouse manufacturers and local heating contractors have BTU tables, conversion factors, and other information for estimating sizes and ratings of boilers, heating systems, and heaters for greenhouse use. Don't decide on a heater or the conversion of a system without consulting one of these specialists. An error in judgment can be costly.