Code Stories · why the rule exists
Water Heater T&P Relief Valves
The rule in Massachusetts today
Not in facts.json (only pex-wh relates). Verbatim, 10.14(6)(f)2: "All storage water heating equipment capable of heating water in excess of 212°F shall be equipped with a properly sized T&P relief valve." b. "A T&P valve shall be installed in a tapping directly in or on the tank, within 12 inches of the top of a vertical tank, or within six inches of the top of a horizontal tank, with no fittings between the valve and the tank, except that a bushing may be used ... or the valve shall be installed in the hot water outlet pipe as close to the top of the tank as possible but in no case more than five inches from the top of the tank." c. "The discharge outlet if [sic] the T&P relief valve shall be full-size and pipe using non-ferrous material or tubing rated to withstand the maximum relief valve temperature with no shut-off, to a fixture or a point to within twelve inches of the floor."
- No PEX or CPVC within 24 in. of the final connection to a water heater (248 CMR 10.06(f))
Why it exists
A water heater is a closed pressure vessel with a burner or element under it. If the thermostat or gas valve fails, the water keeps heating. Under pressure it can go past 212°F without boiling. If the tank then ruptures, that superheated water flashes to steam all at once, and steam takes up about 1,700 times the volume of the water (National Board). That is an explosion, not a leak. The T&P valve opens on either excessive pressure or excessive temperature. The valve's listed temperature setting, typically 210°F, holds the water below the atmospheric boiling point, so released water cannot flash into an explosion. Its pressure setting matches the tank's working pressure, generally 150 psi per A.O. Smith. These are listing and manufacturer values, not numbers written in 248 CMR. The discharge rules are about the person standing nearby. A full-size pipe with no shut-off, ending at a fixture or within 12 in. of the floor (248 CMR 10.14(6)(f)2.c), sends a sudden release of scalding water and steam at the floor instead of at someone's face, and lets it drain.
How it came to be
- 1982 — Spencer, Oklahoma: a water heater explodes into the Star Elementary School cafeteria, killing a teacher and six students. edweek.org
- 1982 — Investigators find the relief valve could not sense the tank temperature. edweek.org
- 1982 — UPI reports that the valves were mounted on pipe away from the heaters and failed to open. upi.com
- 1982 — CPSC warns of steam explosions in fireplace-powered water heaters without proper relief devices. cpsc.gov
- 1984 — National Board of Boiler and Pressure Vessel Inspectors on the physics. nationalboard.org
Make it stick
Spencer, Oklahoma, January 19, 1982, lunchtime. An 80-gallon water heater exploded into the Star Elementary School cafeteria, killing a teacher and six students. Initial findings reported that the relief valve's temperature probe was missing and that the valve had been improperly installed. A T&P valve only protects you if its probe can sense the hottest water.
On the exam
Expect MA location (within 12 in. of the top of a vertical tank, 6 in. for a horizontal tank, or in the hot outlet no more than 5 in. from the top; no fittings except a bushing), discharge rules (full size, non-ferrous or rated tubing, no shut-off, to a fixture or within 12 in. of the floor), relief rating not less than heater input (up to 200,000 BTU/h; ASME above that), and vacuum relief valves in the cold supply above the tank top. Also know that a check valve is prohibited in the cold-water supply to a water heater (10.14(6)(c)1), and that a closed system from a meter check or backflow device calls for a thermal expansion tank (10.14(3)(j)).
What people get wrong
- A pressure-only relief valve is enough. A runaway thermostat can overheat water without an alarming pressure rise. The temperature element is the part that prevents a flash-steam explosion.
- A dripping T&P should be capped or plugged. Never. Slow weeping usually means thermal expansion in a closed system (A.O. Smith), and a big dump means the thermostat has failed. Fix the cause. In MA a check valve may not be installed in the cold supply to a water heater (10.14(6)(c)1), and a thermal expansion tank is the standard remedy for a closed system.
- Put the T&P anywhere on the hot outlet piping. No. MA requires the T&P in a tank tapping within 12 in. of the top of a vertical tank (6 in. for horizontal), or in the hot outlet no more than 5 in. from the top (10.14(6)(f)2.b). Initial findings at Spencer reported a missing probe and an improperly installed valve.
- 210°F is an arbitrary number. It sits just under water's atmospheric boiling point (212°F), so released water does not flash to steam. This is reasoning from physics, and the value comes from the valve's listing standard. 248 CMR itself triggers the T&P requirement for storage equipment 'capable of heating water in excess of 212°F'.
Try it: 3 real practice questions
Q1. On a vertical storage water heater, a T&P relief valve installed in a tank tapping must be located within what distance of the top of the tank?
The T&P goes in a tapping within 12 inches of the top of a vertical tank (6 inches for a horizontal tank), with no fittings other than a reducing bushing. The hottest water is at the top, which is what the temperature element must sense.
248 CMR 10.14(6)(f)2.bQ2. Which T&P relief valve discharge installation complies with 248 CMR?
The discharge must be full size, non-ferrous (or tubing rated for relief temperature), with no shut-off, piped to a fixture or to within 12 inches of the floor. A valve, a reduced size, or a termination 30 inches up each violates one of those requirements.
248 CMR 10.14(6)(f)2.cQ3. Which of the following is prohibited in the cold-water supply to a hot water heater or storage tank?
A check valve in the heater's cold supply traps expanding hot water and drives pressure up to the relief setting. The code instead requires a shutoff valve on the cold branch and allows a properly sized thermal expansion tank.
248 CMR 10.14(6)(c)1These come from the PressureTest bank. Start 7 days free for the full bank, timed exam simulations and drawn isometric problems.