172 GREENHOUSES 



To arrive at an estimate of the possible 

 heat loss from a greenhouse add to the total 

 square feet of exposed glass surface a sixth 

 of the total square feet of exposed wall sur- 

 face, and multiply the sum by the difference 

 between the temperature at which the house 

 is to be kept and the lowest outside tem- 

 perature which will probably be experienced. 

 Suppose, for example, that a house has 

 10,000 square feet of glass and equivalent 

 glass, that it is desired to keep it at a night 

 temperature of 50 degrees, and that the low- 

 est outside night temperature to be expected 

 is 10 degrees. The number of B. T. U. 

 given off by such a house under these con- 

 ditions would be [50 ( 10)] x i x 10 x 

 10,000 or 600,000 B. T. U., and enough heat- 

 ing coils must be provided to supply this 

 amount. 



In hot water heating the coils will give 

 off approximately two B. T. U. per square 

 foot of surface per hour for every degree 

 difference in temperature between that of 

 the coil and that of the surrounding air. The 

 average temperature of the coils may be 

 taken to be 160 degrees, and if the house is 

 to be maintained at 50 degrees the difference 

 will be no degrees. Multiplying no by 2 



