How long does it take to get hot water to a 1.5 gpm faucet if there is 13 ft of 3/4" copper pipe
and 8 ft of 1/2" copper pipe to the faucet?
The time for it to reach full temperature due to pipe temperature is not an issue at this point.
For these calculations, we assume the measurement of the pipe to be inside diameter.
13' of 3/4" copper pipe V = (π) • (3/8 in)2 • (13 ft • 12 in/ft)
V = (3.1416) • (9/64) • (156) in3
V = 68.92 in3
8' of 1/2" copper pipe V = (π) • (1/4 in)2 • (8 ft • 12 in/ft)
V = (3.1416) • (1/16) • (96) in3
V = 18.85 in3
total volume V = 68.92 in3 + 18.85 in3
V = 87.77 in3
Per the conversions in my Chemical Rubber Book, 1 cubic inch holds 0.004329 gallon.
convert to gallons V = 87.77 in3 • 0.004329 gal/in3
V = 0.38 gal
time to flush line T = 0.38 gal / 1.5 gal/min
T = 0.25 min
.