HEATING BY HOT WATER. 169 
a warm greenhouse, ‘‘ intermediate’? house, or early 
vinery ; 60ft. to 80ft. for stoves, and 80ft. to 100ft. for 
forcing houses. 
The most generally employed, and best size of piping 
for all ordinary purposes is the 4in. (internal diameter). 
This gauge contains a greater bulk of water in proportion 
to its diameter than any smaller sizes, and consequently 
retains its heat longer, while pipes of this size of course 
‘carry’ considerably farther than anything smaller. 
The 3in. and Qin. sizes are chiefly employed for connections, 
etc., or as heating pipes in small houses, or for outside 
rows in those of moderate size (see p. 160). The cir- 
cumference of a pipe of any size being always in the 
same proportion to the diameter (roughly speaking, as 
3 to 1), a foot of 2in. piping affords as nearly as possible 
half the heating surface of a foot of the 4in. gauge, or 
half a square foot and one square foot respectively, while 
the 3in. size gives three-quarters of a foot, three rows of 
this gauge affording rather more actual heating surface 
than two rows of 4in., or as 9: 8. 
In all cases it is advisable to allow rather more piping 
than is really necessary for the purpose in view, both in 
order to avoid having to make the pipes very hot at any 
time (this being considered injurious), and also to enable 
a somewhat higher temperature to be afforded, should 
such be necessary at any time. For the same reason the 
boiler also ought always to be well above its work, rather 
than below it, having to force the fire in severe weather 
being at once a wasteful and dangerous proceeding. The 
temperature of the pipes in any low-pressure hot-water 
apparatus cannot rise above 212°— the point at which 
