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level. The pressure system is where there is some device which forces the water 

 on its own course over the pipe system. This is usually a mercury generator or 

 some form of pump. Both of these machines have their advocates, and many 

 have success with either. The gravity system has its limitations, however, and 

 the first one is that the water cannot be carried over 150 ft. of coil away from 

 the boiler with good success. Some plants are working to-day at 200 ft. away 

 from the boiler, but the best results are not 'being obtained ^because of overloading 

 the boiler. Some growers claim that the highest point of the heating system 

 should be at the boiler, thus giving the water a drop all the way into the boiler 

 again. The pipes should have a drop oE ten inches per 100 ft., for suc- 

 cessful running of the gravity system. With the mercury generator or some 

 form of circulator the water can be forced oveir 400 ft., this being over twice 

 the distance of the gravity system. The pipes do not need so much grading in this 

 form of heating. In this system the water is forced through at a greater rate 

 than by gravity, and it comes back to the boiler containing more degrees of 

 heat than that which has passed over the system working according to the laws 

 of gravity. This point means more than is often thought, and should be investi- 

 gated thoroughly before any system is selected. The longer the water takes to 

 travel around a heating system the less numher of degrees of heat it contains 

 when it again comes to the boiler. All this water has to be heated over again, 

 and thus it is quite apparent the more degrees the water has to be raised in 

 temperature the more units of heat required and this falls back again and 

 increases the consumption of coal. 



The gravity system is to be recommended for small houses such as 30 or 40 

 by 100 or 150 but may be used up to 30,000 square feet of glass. For 

 larger houses, and where there are several in a range and hot water is 

 wanted, some form of circulator is recommended. There are some good 

 pumps on the market, and as a rule they are run by steam. A small steam 

 boiler is necessary for the pump alone. One of these is giving good service at 

 the Goldwood greenhouse near Cleveland, Ohio. Another method used in the 

 vicinity of Eoehester is that of a centrifugal pump operated by a >^ h.p. electric 

 motor. This system has given entire satisfaction, and the cost for power is very 

 small. This method is not a common one and should be inquired into thoroughly 

 before being attempted. Hot water heating is never recommended for plants 

 having much over 100,000 sq. ft. of glass but this will no doubt be overcome in the 

 future. 



Steam Heating. — For long houses and for large houses or large ranges the 

 use of steam is advised. In these cases where the heat has a long way to travel it 

 can be carried to a better advantage and with a less cost of fuel by the steam 

 system. There are several systems of steam heating which may be installed, but 

 there are only two of thes3 that have b3en satisfactorily applied to greenhouse 

 heating. These are the high pressure and the low pressure systems. 



The low pressure system is used to a great extent and this is one that operates 

 at a pressure of 1-10 lbs., usually 5-6. This works by gravity and the con- 

 densation is carried back to the boiler without any circulator of any kind. This 

 system necessitates that the boiler must be several feet below the lowest return in 

 the house. This is the limiting factor for this system as it is not always convenient 

 to have the boilers in a pit as this entails. This pit may be overcome and the 

 .boilers placed on a level with the houses by the addition of a steam trap which 

 lifts the water into the boiler from the returns. This arrangement is found 



