The Supply Side: Pressure and PEX

Transcript

SPEAKER_1: Alright, so last lecture we nailed down venting—atmospheric pressure keeping traps sealed. Now I want to flip to the supply side. Where does the material choice actually start? SPEAKER_2: It starts with PEX. Most new NWA residential construction runs cross-linked polyethylene instead of copper. But PEX isn't one thing—there are two types that behave differently in the field. SPEAKER_1: PEX-A versus PEX-B. From the outside they look identical. What's the real mechanical difference? SPEAKER_2: The connection method. PEX-A uses expansion—you expand the pipe end with a tool, slide on a fitting, and the pipe's memory contracts back around it. PEX-B uses a crimp or clamp ring compressed over the outside. Both are code-compliant. But PEX-A has one big field advantage: kink recovery. Heat a kinked PEX-A line with a heat gun and it returns to round. PEX-B stays kinked. SPEAKER_1: So for tight framing runs—lots of corners in an NWA subdivision—PEX-A gives a crew more forgiveness. SPEAKER_2: Exactly. Once the pipe is chosen, focus on distribution layout. Opt for a manifold system over the traditional trunk-and-branch for better efficiency and control. SPEAKER_1: Think of it for someone who hasn't seen a manifold installed. What does it actually look like? SPEAKER_2: Think of a circuit breaker panel, but for water. A central manifold—usually in a utility room—has individual ports, one dedicated line running to each fixture. Kitchen sink gets its own run. Each bathroom gets its own run. Any fixture can be shut off at the manifold without touching anything else. The Arkansas Plumbing Code requires individual shutoff valves on the supply to each fixture anyway—a manifold satisfies that requirement cleanly. SPEAKER_1: Mm-hmm. And supporting those PEX runs—is there a specific interval that matters? SPEAKER_2: Guides recommend supporting horizontal PEX lines as frequently as every 32 inches, and vertical runs about every 10 feet. That's often stricter than older practices. The reason isn't just structural—it's acoustic. Unsupported PEX vibrates under pressure, and in a finished home that becomes a knocking sound inside the wall. SPEAKER_1: Wait—so the support interval is partly about noise, not just keeping the pipe from sagging. SPEAKER_2: Correct. [short pause] After lines are run and stub-outs are set, ensure the system is ready for inspection by documenting the layout and materials used. SPEAKER_1: What does the test actually require in NWA jurisdictions? SPEAKER_2: At minimum, water piping must hold working pressure—or at least 50 psi—for 15 minutes without dropping. But at least one Northwest Arkansas jurisdiction requires underslab supply mains to be air-tested to 75 psi. That's a more stringent bar than the general checklist minimum. If the gauge creeps down during the test, there's a leak. The method is to isolate sections—close manifold ports one by one—and retest each segment until the drop stops. SPEAKER_1: What about testing with air versus water? I've heard air can be more dangerous even at the same pressure reading. SPEAKER_2: That's a real hazard. Water is incompressible—if a joint fails under a water test, you get a leak. Air stores energy like a compressed spring. A fitting that blows under air pressure can become a projectile. That's why most checklists restrict air testing of plastic water piping, with PEX being a limited exception when the manufacturer allows it and freezing conditions make water testing impractical. SPEAKER_1: So not just a preference—there's genuine safety logic behind defaulting to water. SPEAKER_2: Exactly. Now, there's one more piece that gets overlooked during rough-in: thermal expansion. When a water heater heats a closed system—one with a check valve or PRV at the meter—the expanding water has nowhere to go. Pressure spikes. Over time, that stresses every joint in the system. SPEAKER_1: And the fix is an expansion tank on the cold supply line to the heater. SPEAKER_2: Right. In NWA, local rules require a check valve and PRV near the meter box—so a closed system is essentially guaranteed on new construction. That makes an expansion tank necessary, not optional. For everyone following along, the key idea is this: PEX type, manifold layout, support intervals, pressure test results, and expansion tank installation all need to be documented with photos before insulation goes in. That paper trail is what gets a rough-in inspection approved. SPEAKER_1: Supply is in, waste is vented—now the question is whether the structure itself can hold it all. Drilling, notching, fire-stopping—that's next. SPEAKER_2: And that's where a lot of otherwise solid rough-ins get flagged. The framing work is the last line of defense before the inspector walks through.