Repiping a 1928 School: Water Chemistry Picks the Material
A 1928, three-story elementary school (420 students) is being renovated. The existing domestic system is galvanized steel with heavy tuberculation; pressure at top-floor fixtures has fallen to 12 psi while the street main reads 78 psi. The building has a lead gooseneck at the service and 40 feet of service line of unknown material, plus lead-based solder on pre-1986 copper branches in the 1970s addition.
Finished water analysis: pH 6.9, alkalinity 38 mg/L as CaCO3, calcium hardness 42 mg/L as CaCO3, chloride 110 mg/L, sulfate 85 mg/L, Langelier Saturation Index -1.6. The engineer proposes type L copper for the new mains and PEX-A for branch runs above corridor ceilings. A local contractor counters with CPVC because it is cheaper. The school must remain occupied during construction, and the state requires a lead service line inventory.
1. Evaluate the copper main proposal against this water chemistry, and state what must be verified before any material is approved.
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Copper is the weak point, and the chemistry is why.
- An LSI of -1.6 at pH 6.9 with 38 mg/L alkalinity means the water is aggressive and poorly buffered; it will attack surfaces rather than lay down a protective scale.
- The chloride-to-sulfate mass ratio is 110 / 85 = 1.29. Above roughly 0.5, low-alkalinity waters are associated with pitting corrosion of copper, particularly in new fluxed horizontal runs where debris sits. Type L wall thickness does not protect against pitting; pinholes form regardless of schedule.
- That does not automatically disqualify copper, but the engineer owes a corrosion-control basis: utility pH or alkalinity adjustment, an approved inhibitor program, or a coupon/pipe-loop study before the mains are specified.
PEX-A is immune to CSMR but must be listed against the utility's actual disinfectant - chlorine, chloramine and chlorine dioxide carry different ASTM F2023 ratings and temperature derating. Confirm all fittings and valves are NSF/ANSI 61 and 372 certified. CPVC is a legitimate cost alternative, subject to the same listings and to its freeze and brittleness record in an unheated 1928 building. Do not join metal to plastic without dielectric isolation.
2. How do you handle the lead gooseneck and the 40 feet of unknown service line while the school stays occupied?
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Presume lead, and never do a partial replacement. The gooseneck confirms lead at the main, and the unknown 40 feet must be classified as lead in the inventory until an excavation or a verified utility record proves otherwise; unknown status is only defensible with a documented verification plan.
During an occupied renovation the risk is disturbance, not the standing water:
- Full-length replacement from main to meter by a licensed contractor. Partial replacement of only the publicly owned portion is well documented to raise lead at the tap and should be refused.
- Sequence the cut-in and flushing when students are not in the building, and notify parents in advance.
- After any disruption, flush the service and building, then sample. A single first-draw sample does not show the building is clear.
- Provide NSF/ANSI 53 lead-certified filters at every drinking fountain, classroom sink and kitchen tap during construction and for at least six months after, with sampling before filters are removed.
- Test the 1970s addition separately for lead solder. A school is not exempt from the 15 ppb action level because it is not a public water system.
3. The top floor sees 12 psi with 78 psi at the street. What is actually happening, and how do you correct it?
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The 12 psi is a symptom; the loss is in the pipe. Static head for three stories is about 30 ft, or 13 psi. With 78 psi at the street, a sound building should see roughly 65 psi at the top floor. Instead about 66 psi is disappearing, nearly all of it as friction in tuberculated galvanized mains - a collapsed effective diameter, not a fixture problem.
Correction path:
- Repiping may be sufficient on its own. Replacing tuberculated galvanized (C-factor around 80-100 and falling) with PEX or copper (C ≈ 130-150) can restore 40-50 psi of residual. Re-run the hydraulic calculation with the new material and true developed length before buying a booster.
- Check the fixture type. Tank-type water closets need only about 8 psi flowing; flushometer valves need 15-20 psi. If the top floor has flushometers, 12 psi static guarantees complaints no matter what the main reads.
- If calculated residual is still below about 20 psi after repipe, add a booster with a pressure-maintaining tank, not a PRV.
- Confirm the 12 psi was measured flowing, not static. A static reading at a closed hose bib on a tuberculated main overstates what fixtures actually see.