HOT WATER INSTALLATION 171 



valve 



majority of greenhouse oper- 



ators seem to be in accord 



with this view. Practically 



speaking there appears to be 



but little difference in the 



efficiency of the two systems 



and the convenience and the 



arrangement of the house de- 



termines the choice to a con- 



siderable extent. 



Estimating Radiation. 



The calculations for green- 

 house heating are based on 

 certain fundamental facts which for hot 

 water may be stated briefly as follows: A 

 square foot of glass will give off, under or- 

 dinary greenhouse conditions in winter 

 weather, approximately i B. T. U-* of heat 

 per hour, for each degree difference in tem- 

 perature between the air inside the green- 

 house and that outside. A good wood, brick 

 or concrete wall will give off about a sixth 

 as much, or a sixth B. T. U. per square foot 

 per hour. It is customary to divide the total 

 wall surface by six and consider it as equiva- 

 lent to glass. 



*British Thermal Unit; the amount of heat required to 

 raise one pound of distilled water from 62 to 63 

 degrees F. 



