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 THE CALL was routine enough — little to no pressure on the upper floors when water was drawn on the lower ones. This was a well-water system, so my first thoughts turned to pressure setting, pump sizing and depth to water in the well. Then there were thoughts of water conditioning equipment and filters that might be in need of service.One quick glance at the basement ceiling, however, revealed this home’s problems regarding pressure drops and scalding showers. From the outlet of the water softener, the piping was reduced from 1-in. PVC to 1Ú2-in. PEX. Following this was a whole-house spun-fiber filter. After that began the meandering, sloppy array of PEX to joint after joint of brass barbed, crimp-ring fittings that further reduced the interior dimension of that single line of 1Ú2-in. PEX tubing. Adding insult to injury was the 1Ú2-in. branch line to the water heater! PEX tubing has opened a world of opportunity regarding labor savings for potable water installations in new home construction. It also has enabled those who lack basic plumbing skills to install hot and cold potable water lines. Pressure drops through PEX tubing must be carefully considered in applications such as this one, and they should also be examined in "home run" style applications. As plumbers, we know that pressure drops constitute a formula for callbacks, disgruntled customers and increased liability.
Look at the pressure chart provided and check the 1Ú2-in. column. The numbers with asterisks indicate 8 FPS (feet per second flow rates), and each column stops when flow equals 12 FPS. Drop down to the 4 GPM line and you’ll see our pressure drop per foot of tubing is .208. We’ll need to add a head loss of 10.4 PSI for the change in elevation of 24 ft. to our total.
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The math really isn’t very difficult. Common sense tells you a system installed in this manner will be troublesome. For instance, in a 1Ú2-in. PEX tubing application, the pressure drop at 4 GPM (gallons per minute flow rate) equals .208 in PSI/FT. A 4 GPM flow rate equals 8 ft. per second, which some codes list as the allowable limit. But these same charts show a maximum flow rate of 7 GPM for 1Ú2-in. PEX tubing. It’s not hard to understand why our homeowner was experiencing pressure and flow problems in this three-story, 5,000-sq.-ft. home.
