214 GREENHOUSES 



competitors, it is but fair that buyers 

 should be recommended to exercise consider- 

 able caution. Probably most boilers will, 

 under favorable conditions, develop the num- 

 ber of heat units for which they are rated, 

 but for the sake of safety and to prevent the 

 necessity of forcing, it is best to select boil- 

 ers with ratings at least 20 per cent, in ex- 

 cess of the theoretical needs. 



When boilers are rated according to the 

 number of linear feet of radiating pipe they 

 will supply, it is usually given in terms of 

 either 3%-inch cast-iron pipe or in 2-inch 

 wrought-iron pipe. The following table 

 gives the length of pipes of other sizes equiv- 

 alent to i linear foot of 2 and 3%-inch pipe. 



Most boiler ratings are given for a mini- 

 mum outside temperature of zero degrees, 

 Fahrenheit. For localities subject to a tem- 

 perature of 10 degrees below zero a boiler 

 of 10 per cent, greater capacity should be se- 



