daestrom wrote:

... who has space for 100' of 4" pipe (vertical, coiled or otherwise).[?]

The 4" tall spiral hung under a basement ceiling would be about 7'
in diameter. The 6' tall coil would occupy a 2.7' floor circle.
It seems simpler to install and might have better stratification.

10 PI=4*ATN(1)
20 D=4.25'pipe OD (inches)
30 L=100'pipe length (feet)
40 DI=2'coil ID (feet)
50 DO=DI+2*D/12'coil OD (feet)
60 CI=PI*DI'inner circumference (feet)
70 NT=L/CI'number of turns
80 H=NT*D/12'coil height (feet)
90 PRINT D,DI,DO,NT,H

4.25 2 2.708333 15.91549 5.636738

10 PI=4*ATN(1)
20 D=4.25'pipe OD (inches)
30 L=100'pipe length (feet)
40 A=D*L/12'pipe area (ft^2)
50 DI=2'flat spiral ID (feet)
60 DO=2*SQR(A/PI+(DI/2)^2)'spiral OD (feet)
70 NT=12*(DO-DI)/2/D'number of turns
80 PRINT D,DI,DO,NT

4.25 2 7.006704 7.068288

Nick