Heating. Hot-water boilers connected to water pipes and burning coal, coke or oil are commonly used for all large houses and many small ones. Pipes may be 2 or 4 in. in diameter; the latter are preferable. They must have a steady rise from the boiler to an expansion tank at the highest point and a similar fall back to the boiler. Pipes are usually accommodated at the side of the house, low down beneath the stages, if any. Sometimes in carnation houses, where heat is required more to circulate than to warm the atmosphere, the pipes are slung on brackets about a foot below the glass, midway between ridge and eaves.
Coke, coal and oil Heating are still the most economical, but small boilers are sometimes difficult to control. Improvement is effected by placing the boiler in a pit or shed so that it is not directly exposed to the wind. Automatic stokers are obtainable for large coke and coal boilers.
A single 4-in. flow and return down one side is sufficient for cool greenhouses not exceeding 8 ft. in width. For wider houses, pipes may be continued across one end, while if over 12 ft. in width they should be on three sides. For hothouses, the number of pipes is increased Often the return pipes are in duplicate.
Hot-water boilers can also be obtained to be heated by gas. Care must be taken that there is no leakage of gas or escape fumes into the. In other respects, details of installation are similar.
Houses can also be heated directly with oil lamps without water pipes. These should be specially designed to give off no fumes. Bluetflame lamps are more economical than those of the whitetflame type, but are more liable to give off harmful fumes if out of adjustment. Cleanliness is of vital importance and only hightgrade paraffin should be used. Oil heaters are more serviceable for occasional use, to keep out frost from otherwise unheated or slightly heated houses, than for constant operation.
Electrical heating is very reliable and satisfactory, but comparatively costly unless current can be obtained at a reasonable price. Ordinary domestic heaters are not satisfactory, as the source of heat is too concentrated. Low-temperature radiators, low-voltage strip heaters or fantassisted heaters should be used. Specialinstallations are made by many manufacturers of electrical equipment, and expert advice should always be obtained regarding their installation.
Thermostatic control can be applied very easily both to electrical heaters and to gas-heated boilers. The instrument is set to keep the heat of the greenhouse constant within certain limits, usually three or four degrees. Thermostatic control cannot prevent rises of temperature caused by sun heat.
Capacity of Electrical Heaters. This is expressed in terms of the watts or kilowatts (1000 watts or units) of electricity the heater consumes. Approximately 10 watts is required for every square foot of glass and 5 watts for every square foot of masonry or woodwork to raise the inside temperature of a greenhouse 25° above the outside temperature. Since this is about what is required in cool greenhouses in winter in most parts of Britain, except the coldest and most exposed, it is a useful formula upon which to base calculations.
Make all measurements in feet or in fractions of feet. Measure .each section of sides, ends and roof separately. Multiply the length and width of each section to obtain its area in square feet. Add together all the sections composed of glass and multiply by Add all sections made of masonry or wood and multiply by The sum of these two figures will give the number of watts required in a heater adequate to maintain cool house temperatures. This may need to be doubled for an intermediate house and tripled, or even quadrupled, for a hothouse.- It can be reduced a little if frost protection only is required.