Code Stories · PressureTest
Verbatim, 10.14(5)(c): "The maximum temperature of domestic hot water in residential buildings shall not exceed 130°F. Plumbing fixtures requiring higher temperatures for their proper use and function, such as dishwashers and hot water dispensers shall be exempted". Table (min/max °F): Residential 120/130*; Water Entering the Sanitary Drainage System -/150; Shower Valve 120/120; Tub Filler 120/120; Public & Employee Lavatory 120/120; Residential Lavatory 120/130; Emergency Showers & Eyewash 60/100; Hand Sink in Commercial Kitchen 120/120; Service Sink & Scullery Sink 120/140.
Residential hot water minimum 120°F (248 CMR 10.14(5))
Residential hot water maximum 130°F (248 CMR 10.14(5))
Shower valve (pressure-balancing/thermostatic) limited to 120°F (248 CMR 10.10(4)(c)4)
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.
Legionnaires' disease is recognized after an outbreak among attendees of an American Legion convention in Philadelphia.
Source: https://www.cdc.gov/legionella/about/history.html
CPSC opens a proceeding on hot tap water scalds.
Source: https://www.cpsc.gov/Newsroom/News-Releases/1979/Commission-Acts-On-Hot-Tap-Water-Scald-Hazard
CPSC data on who dies.
Source: https://www.cpsc.gov/Newsroom/News-Releases/1979/Commission-Acts-On-Hot-Tap-Water-Scald-Hazard
CPSC Publication 5098 on burn time vs. temperature.
Source: https://www.cpsc.gov/s3fs-public/5098.pdf
CDC Legionella guidance for building water systems.
Source: https://www.cdc.gov/control-legionella/php/guidance/monitor-water-guidance.html
Mnemonic: 'Home is 120 to 130; bathing is 120, period.' In a Massachusetts residence, domestic hot water runs 120°F minimum and 130°F maximum, and showers and tub fillers deliver 120°F maximum (248 CMR 10.14(5)). The burn ladder explains the cap (CPSC): 150°F = 2 seconds, 140°F = 6 seconds, 130°F = 30 seconds, 120°F = 5 minutes.
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).
Under 248 CMR 10.14, the minimum hot water temperature for a residential building is:
The 248 CMR 10.14(5) temperature table sets residential hot water at a 120°F minimum and a 130°F maximum. Fixtures such as dishwashers that need hotter water are exempt. The window holds water hot enough for sanitation while limiting scald risk.
Individual shower control valves must be pressure-balancing or thermostatic type with a maximum outlet temperature of:
248 CMR 10.10(4)(c) requires every shower control valve to have a limit device capping mixed-water outlet temperature at 120°F. The installing plumber must set it before final inspection. A central ASSE 1070 mixing valve may be used instead if it also limits supply to the showers to 120°F.
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