No free lunch with PEX water system

March 1, 2002
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

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.

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.

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.

In this home, the total run in length to the third-floor master bath with the high-flow shower valve is a little more than 75 ft. So, we multiply 75 by .208 (see chart) and add 10.4 (head loss), which is 15.6 + 10.4 for a lost delivery pressure of 26 lb. from our 30/50 pump switch setting. Codes typically require a minimum of 8 PSI residual pressure, which can’t be maintained in this system.

Now add the barbed fitting interior dimension restrictions and someone using a fixture or fixtures on a lower floor. Mr. Jones, who paid a cool 500 G’s for his family dream home, was breathing fire!

Unless careful consideration is given to increased line sizing in the distribution network of potable piping systems using PEX tubing, you must avoid long runs that serve multiple fixtures. The preferred method of piping is a "home run" type installation, with each fixture receiving its own line from a manifold. The manifold should be served using larger diameter tubing and situated close to the domestic hot water storage tank. Little or no pressure fluctuations will be experienced, provided you have relatively short lengths of larger diameter tubing installed on the inlet side of the manifold.

Potable water PEX systems are available from many manufacturers. The tubing can be purchased in a variety of colors to aid in identification between hot or cold runs, which can be a benefit when handling runs through concealed areas.

Although some systems rely on crushing the PEX against the barbs via compression rings, others stretch the PEX to receive brass barbed fittings that have the same I.D. as the tubing itself, thereby avoiding additional flow restriction.

PEX is clean and smooth for good performance and offers less chance of having things stick to the interior. It can be installed with no hidden joints or connections; and when installed with a "home run" manifold, most pressure problems can be easily avoided.

There was no free lunch for the builder who avoided the "unnecessary" expense of using a plumber on this job. He had to repipe the home, which involved tearing up floors and opening walls in a house less than three months old. You see, Mr. Jones is a litigator for a large law firm and earns his living in courtrooms arguing cases before juries.

Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at [email protected].