Code Stories · why the rule exists

Hot Water: Hot Enough for Legionella, Safe Enough for Skin

Why it exists

Legionella bacteria multiply in lukewarm water. CDC says they grow best between 77°F and 113°F, and its building water-management guidance recommends storing hot water above 140°F and not letting circulating hot water fall below 120°F. Skin is damaged fast at those temperatures. CPSC data say 150°F causes third-degree burns in about 2 seconds, 140°F in 6 seconds, 130°F in 30 seconds, and even 120°F in about 5 minutes. Most victims are children under 5 and adults over 65, who cannot react or get out quickly. Massachusetts settles the conflict by rule. In residential buildings the whole domestic hot water system stays between 120°F and 130°F, and bathing fixtures deliver no more than 120°F through pressure-balancing or thermostatic valves. So CDC's 140°F storage figure is NOT the MA residential answer.

How it came to be

1. 1976 — Legionnaires' disease is recognized after an outbreak among attendees of an American Legion convention in Philadelphia. cdc.gov
2. 1979 — CPSC opens a proceeding on hot tap water scalds. cpsc.gov
3. 1979 — CPSC data on who dies. cpsc.gov
4. 2012 — CPSC Publication 5098 on burn time vs. temperature. cpsc.gov
5. current — CDC Legionella guidance for building water systems. cdc.gov

Make it stick

On the exam

Expect the MA table (248 CMR 10.14(5)): residential 120°F min / 130°F max; shower valve and tub filler 120°F; public and employee lavatory 120°F; residential lavatory 130°F; commercial-kitchen hand sink 120°F; service and scullery sink 140°F; emergency shower and eyewash 60 to 100°F; water to drain 150°F max. Also expect the valve types (product-accepted thermostatic, pressure-balancing or combination valves, adjusted by the installing plumber before final inspection, 10.10(4)(c)) and the exemption for fixtures that need higher temperatures (dishwashers, hot-water dispensers).

What people get wrong

• For Legionella control, a Massachusetts home should store hot water at 140°F. Not under 248 CMR. CDC's 140°F storage advice comes from building water-management guidance, but 248 CMR 10.14(5)(c) caps domestic hot water in residential buildings at 130°F (minimum 120°F). On the MA exam, the residential answers are 120°F minimum and 130°F maximum.
• 120°F can't burn you. CPSC: a five-minute exposure at 120°F could cause third-degree burns. This matters for anyone who cannot get out of the water.
• Anti-scald valves and Legionella control conflict, so you have to pick one. CDC points to engineering controls that mix at or near the point of use to satisfy both. MA builds this into the code: pressure-balancing or thermostatic valves limited to 120°F at showers and tubs, with fixture-by-fixture maximums in 10.14(5).

Sources

• cdc.gov
• cdc.gov
• cpsc.gov
• cpsc.gov

History is presented as history. The current Massachusetts rule is the box at the top — use it on the exam.