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Quick answer from a licensed plumber: Match four things from the old valve: pressure setting (the tag; usually 150 psi), thread size (3/4-inch male in, female out on nearly every tank), shank length (extended for foam-insulated tanks) and probe length.
Watts’ residential valve is the 100XL (100XL-8 for the 8-inch probe; L, LL and LLL prefixes for 2, 2 1/2 and 3 inches of insulation; RL for a 4.75-inch shank), and Cash Acme’s is the NCLX (NCLX-1, -5, -5L, -5LX, -8 by element and shank). Watts sets the 100XL at 210°F with standard 75–150 psi settings; Cash Acme’s table lists most NCLX valves at 150 psi, and the NCLX-5LX listing gives 210°F. Never fit a higher pressure rating than the heater’s tag.
Two that were Amazon-sold or shipped on September 25, 2026: the Watts LL100XL-DP-150/210 extended-shank 3/4-inch T&P valve and the Cash Acme NCLX-5LX 3/4-inch T&P valve, 150 psi.
Step 1: Read the Old Valve
A residential T&P valve is defined by five things, and Watts’ model selector lists four of them as its filters: “Thermostat Probe Length Shank Length Max PSI Working Pressure Max. Temp. F.” The fifth is the pipe size, and for tank water heaters that is almost always 3/4-inch.
Watts describes its 100XL as a “bronze body with NPT male inlet and NPT female outlet connections, unique thermostat with special thermo-bonded coating, and a test lever,” with “Temperature Relief: 210 degreeF (99 degreeC). Pressure Relief Range: 75 to 150psi (5.2 to 10.3 bar). Standard Pressure Settings: 75, 100, 125, or 150psi.” Cash Acme’s NCLX is the same idea: “available in male inlets and female outlets with several options in shank and element lengths,” and it “comes standard with a test lever.”
So the matching order is: the pressure stamped on the old tag (a 150 psi water heater takes a 150 psi valve), the thread size (3/4-inch on almost every tank), the shank (short, or extended for a heater with thick foam insulation), and the probe length. Everything else is a brand choice.
| Spec | Where it shows on the old valve | Why it matters |
|---|---|---|
| Pressure setting | Stamped on the tag: 75, 100, 125 or 150 psi | Must match the heater’s rated working pressure; Watts’ standard settings are 75, 100, 125 or 150 psi |
| Temperature | Tag: 210°F on residential T&P valves | Watts and Cash Acme residential valves relieve at 210°F (Watts lists a 180°F option on some models) |
| Inlet / outlet size | Thread on the body: 3/4″ male in, 3/4″ female out | Watts 100XL and Cash Acme NCLX residential models are 3/4-inch; 1/2-inch and 1-inch exist |
| Shank length | The threaded stub between the body and the tank | A long shank reaches through foam insulation; Watts sells “Extended Shank Up to 2 IN”, “2 1/2 IN” and “3 IN Insulation” versions |
| Thermostat probe length | The probe inside the tank | Watts lists a standard extension thermostat, a 4-inch and an 8-inch (100XL-8); Cash Acme numbers its NCLX and FVMX by element (NCLX-1, -5, -8) |
Step 2: Watts 100XL by Model Number
The 100XL is the residential valve, rated for “hot water supply tanks and hot water heaters up to 105,000 BTU/hr,” and it is “ASME rated and CSA listed.” Watts encodes the setting in the number: the digits after the dash are pressure and temperature, so “100XL-150 3/4” is a “3/4 IN Brass Self Closing Temperature and Pressure Relief Valve, 150 psi, 210 degree F, Test Lever, Extension Thermostat.” The L, LL, LLL and RL prefix letters are the shank; DP is a display pack.
| Watts model | Watts description (verbatim) | Use it when |
|---|---|---|
| 100XL-150 3/4 | “150 psi, 210 degree F, Test Lever, Extension Thermostat” | Standard 150 psi residential heater, standard shank |
| 100XL-125210 | “125 psi, 210 degree F, Test Lever, 4 IN Ext Thermostat” | Heater rated 125 psi |
| 100XL-100210 3/4 / 100XL-075210 | “100 psi” / “75 psi, 210 degree F, Test Lever, Extension Thermostat” | Heaters and tanks rated 100 or 75 psi |
| 100XL-8-150210 3/4 (also -100210, -125210) | “Test Lever, 8 IN Extension Thermostat” | Heaters that came with the 8-inch probe; Watts: “Model 100XL-8 is also available with 8 in. (200mm) extension thermostat” |
| L100XL-100210 3/4 / L100XLM7-3-125210 | “Extended Shank Up to 2 IN Insulation” | Tanks with up to 2 inches of foam |
| LL100XLM7-2.5 3/4 | “150 psi, 210 F, Extended Shank Up to 2 1/2 IN Insulation” | Up to 2 1/2 inches of foam |
| LLL100XL-2M7-150210 3/4 / LLL100XL 100/180 | “Extended Shank Up to 3 IN Insulation” (the 100/180 relieves at 180°F) | Up to 3 inches of foam |
| RL100XL-150210 3/4 | “3/4 IN Temperature and Pressure Relief Valve, 4.75 IN Shank” | A 4.75-inch shank, the only shank Watts states as a length |
| DP100XL-TT-150210 | “150 psi, 180 degree F, Test Lever, Extension Thermostat, Display Pack” | Retail pack; note the 180°F rating |
The 40L and 40XL are a different family: Watts says they “provide higher capacity temperature and pressure protection to domestic hot water supply tanks and hot water heaters for various commercial applications,” with the same 210°F and 75–150 psi settings and a “Coated thermostat” in 3-, 4-, 5-, 7- and 8-inch lengths (the 40XL-5 is a 3/4-inch valve with a 5-inch thermostat). If the old tag says 40XL, match it; if it says 100XL, do not upsize to a 40XL without checking the heater’s BTU rating against the valve’s.
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Step 3: Cash Acme NCLX and FVMX by SKU
Cash Acme’s residential line is the NCLX: “designed for the protection of water heaters and storage tanks and are ideal for all domestic water heater applications,” with “a cast bronze body, brass and stainless steel internal parts, silicone seat disc and stainless steel spring.”
Its SKU table pairs a part number with a size and factory setting: “23279-0150 3/4 in. NCLX-1 150 psi,” “23261-0150 3/4 in. NCLX-5 150 psi,” “23354-0150 3/4 in. NCLX-5L 150 psi,” “23266-0150 3/4 in. NCLX-5LX 150 psi,” “23272-0150 3/4 in. NCLX-8 150 psi,” plus 1/2-inch NCLX-1 and NCLX-5S valves and a “NCLX-5LX w/ SharkBite Outlet Connection.” The suffix is the element and shank: the NCLX-5LX listing describes a 2-5/16-inch extended shank, and the discontinued “NCLX-5 for Water Heaters using PEX Tubing, 180 F Element” shows the 180°F option existed for PEX-piped heaters.
The FVX/FVMX is the heavier series: “designed to offer high capacity protection for domestic and commercial hot water heaters,” “available with male (FVMX) or female (FVX) inlet connections and female only outlet connections,” and “All FVX Series valves through 1-1/4″ have coated thermal elements that protect against mineral build-up.” Its 3/4-inch rows include “16934-0150 3/4 in. FVMX-5C Male Inlet 150 psi,” “16933-0125 New 3/4 in. FVMX-3C Male Inlet 125 psi” and “20682-0150 3/4 in. FVMX-5LS Male Inlet, Long Shank 150 psi.” Match the letters (3C, 5C, 8C = element; LS = long shank) and the four-digit pressure at the end of the SKU.
Replacing It
Turn off the gas or the breaker, shut the cold inlet, and drain a few gallons so the water level drops below the valve. Unscrew the discharge pipe, then back the old valve out of the tank with a wrench on the body. Wrap the new valve’s male threads with tape, thread it in so the outlet points where the discharge pipe can reach, and reconnect the pipe.
Refill with a hot tap open until air stops, then check for weeping at the threads and at the seat. Lift the test lever once the tank is up to temperature to confirm it opens and reseats; a valve that will not reseat gets replaced, not tapped.
FAQ
What size T&P valve does a water heater need?
Almost every residential tank uses a 3/4-inch valve with a male NPT inlet and female NPT outlet, set to the pressure on the heater’s tag (usually 150 psi) at 210°F. Match the shank length to the tank’s insulation and the probe length to the old valve.
What is the difference between Watts 100XL and 100XL-8?
The thermostat probe. Watts describes the 100XL-8 as available with an 8-inch extension thermostat; the standard 100XL has a shorter extension thermostat. Both are 3/4-inch residential valves.
What do the L, LL and LLL prefixes mean on a Watts T&P valve?
Extended shank length for insulated tanks: Watts lists L100XL for up to 2 inches of insulation, LL100XL for up to 2 1/2 inches and LLL100XL for up to 3 inches, plus the RL100XL with a 4.75-inch shank.
Is a Cash Acme NCLX the same as a Watts 100XL?
Both are 3/4-inch residential T&P valves with test levers. Watts lists the 100XL at 210°F with standard settings of 75, 100, 125 or 150 psi; Cash Acme’s NCLX table is mostly 150 psi (with 100 and 175 psi rows), and the NCLX-5LX listing gives 210°F. Cash Acme numbers its valves by element and shank (NCLX-1, NCLX-5, NCLX-5L, NCLX-5LX, NCLX-8), and Watts by probe and shank (100XL, 100XL-8, L/LL/LLL100XL). Match the specs, not the brand.
Can I replace a T&P valve myself?
The swap is a wrench job once the heater is off and partly drained, but the discharge piping and the pressure rating must match the heater’s manual and local code. If the new valve keeps opening, have a licensed plumber check supply pressure and thermal expansion rather than replacing the valve again.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, model list, size, setting and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read; product details come from those makers and from the Amazon listings, not from hands-on testing. Each linked product was shipped or sold by Amazon, new and in stock, when we checked on September 25, 2026. Where a link goes to Amazon we may earn a commission at no extra cost to you. Nobody pays to be included here. More on who writes this and how we pick.
Sources
- Watts: 100XL Temperature and Pressure Relief Valves (models and shank lengths)
- Watts: 40L / 40XL Automatic Reseating Temperature and Pressure Relief Valves
- Cash Acme: NCLX Temperature & Pressure Relief Valve (SKU table)
- Cash Acme: FVX/FVMX Temperature & Pressure Relief Valve (SKU table)
Bottom Line
Read the tag, then buy the same pressure, the same thread size, and a shank that clears the insulation. Watts 100XL or Cash Acme NCLX for a residential tank; 100XL-8 or an NCLX-8 if the old probe was the long one; an L/LL/LLL or NCLX-5L/5LX if the tank is foam-insulated. Pipe the discharge properly and fix the reason it was dripping. Replacing the whole heater instead? Our water heater buying guide has a pick for each fuel and space.