Water supply to dishwasher
canadiansue
| Posted in Green Products and Materials on
We are just about to install a new dishwasher. During our extensive renovations we have been using Pex for everything. In the installation instructions for the dishwasher they say “use copper piping only”. We wonder why this would be. Can we safely use Pex if it is permitted in our area? We haven’t checked local regulations but will do so before we progress any further. Thanks.
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They are presumably referring to the connection to the dishwasher itself, which is typically 3/8" copper line, which can bend as you push the dishwasher into place during installation. It's simple to switch from pex to copper at the shut-off valve.
I suspect they are worried about a rubber hose being used to make the connection, and that rubber hose bursting. The electrically controlled valves used in typical appliances ("solenoid valves") snap shut very quickly when they turn off. When the flow of water is interrupted rapidly, its momentum produces a pulse of high pressure that can burst a hose. In addition, it might get hot behind a dishwasher if you use the heated drying cycle. (Nobody who reads GBA would use that energy-wasting cycle, but perhaps a future occupant of the house would...)
I personally have seen a rubber hose supplying a dishwasher burst, and a cheap clothes washer hose from home depot bulge out and get ready to burst from overly hot (155 F?) domestic hot water. So these are real concerns.
The natural question becomes whether PEX would be subject to the same problems. I think it's typically rated at 180 F and 100 PSI. I doubt the inside of the cabinet behind the dishwasher would exceed 180 F, but I'd still play it safe by making the transition from PEX to copper under the sink and using the copper from there to the dishwasher.
I think copper plumbing is also typically rated for a little higher pressure, but the elasticity of PEX helps mitigate the water hammer effect, so I think both are adequately robust for that. I believe that "water hammer arrestors," which are little dead-end tubes with air bladders in them, are required by code in some jurisdictions, and in some cases required for copper but not for PEX. Regardless of whether they are needed for making sure a pipe never bursts, they are nice to have on appliance connections because they make the operation quieter.
If you do a water hammer arrestor, be sure to get a real one with a rubber bladder inside, rather than just adding a capped-off stub pipe. The capped stub will start out filled with air, and it works for a while, but eventually the air in it gets dissolved in the water and it stops working.