Quick Reference Guide

8 UNT-PRC015-EN
Riser Selection and Sizing
• Determine gpm through the riser pipes.
Total the flow rates (gpms) of all units on the riser column. Individual units’ flow rates are
determined by their specific selection.
Direct-return riser columns begin with the largest diameter riser pipe carrying the most gpm
at the bottom of the column. As water is diverted out of a riser into a fan coil, water volume is
subtracted and riser pipe diameters will decrease (within specified velocity limits).
The gpm for the second floor is determined by taking the total gpm and subtracting the gpm
from the first floor.
The gpm for the third floor is determined by taking the total gpm and subtracting the gpm from
the first and second floors.
Reiterate this process to determine the gpms for each floor.
• Determine water velocity.
Once the water flow is known for each riser, the proper size of the riser is determined by
calculating the water velocity in the riser. Use this formula to calculate the velocity (ft/sec) for
water:
V = (GPM * 0.102)/R
2
where:
V is water velocity, ft per sec
GPM is the water flow rate, gallons per minute
R is the inside radius of the pipe, inches
The maximum water velocity recommendations are:
6 fps and/or 10 ft head loss/100 ft pipe length.
• Determine riser pipe size
Refer to Figure 5 to determine water velocity and pipe size, or to the Vertical Stack FC Pipe Size
Picker ( http://www.trane.com/commercial/uploads/xls/Vertical%20Stack%20FC%20Pipe%20Size%20Picker.xls )
Note: Figure 5 is Copyright ©2005, ASHRAE (www.ashrae.org
). Used with permission from
ASHRAE (2005 ASHRAE Handbook Fundamentals, Chapter 36, Page 36.7, Fig. 5).
Figure 5. Friction loss for water in copper tubing (Types K, L, M)