Short definition
An anti-scald valve is the shower or tub-shower control valve that prevents sudden hot-water surges when someone flushes a toilet or starts the washing machine. It uses an internal spool that balances hot and cold pressure (or, in fancier versions, holds an absolute temperature). Code-required at every shower, with output capped at 120 degrees Fahrenheit.
What it is
Inside the wall, the valve body has a moving spool that senses pressure on both supply lines. When the cold-water side drops — a toilet refill steals from the cold trunk — the spool throttles the hot side proportionally so the temperature at the showerhead barely shifts. That’s the pressure-balance type, and it’s what most residential showers use.
A thermostatic valve uses a wax-element cartridge that reacts to absolute temperature instead of pressure differential. A combination valve does both. All three types satisfy the code requirement; pressure-balance is the cheapest and most common. The relevant standard is ASSE 1016 / ASME A112.1016 / CSA B125.16, and a compliant valve must hold output at or below 120 degrees Fahrenheit at the showerhead.
The high-temperature limit stop — a small notched ring inside the trim — is what enforces the 120-degree cap. Setting it correctly is the last step of every cartridge replacement.
Why it matters to a homeowner
If your shower goes scalding the moment a toilet flushes, the pressure-balance spool has failed or the valve was never installed. That’s not just unpleasant — third-degree burns happen at 130 degrees in about thirty seconds, and faster for kids and seniors. Older WA homes with three-handle showers (separate hot, cold, and diverter) usually have no anti-scald valve at all, which is one of the items pre-purchase inspectors flag most often.
The fix is usually a cartridge swap (Moen 1222 or 1225 for Moen valves, Delta RP46074 or RP19804 for Delta) — a $25 to $60 part and an honest DIY job for someone comfortable shutting off water and pulling a handle. Pro install runs $150 to $300. A whole-valve replacement, which means opening the wall behind the shower, is several times that.
When a contractor quotes “scald valve” or “tub/shower mixing valve” or “pressure-balance cartridge,” they are talking about this part.
Common failure modes
- Spool stuck in mid-position. The classic “scalding when toilet flushes” symptom. Cartridge replacement.
- High-temp stop never set. After a cartridge swap, max output drifts above 120 degrees because nobody set the limit. Set it before declaring the job done.
- Mineral scale on spool ports. Hard-water service makes the balancer sluggish; temperature swings get worse over time.
- Thermostatic wax cartridge fails. Output drifts cold-only or hot-only regardless of handle position.
Common variants and what it isn’t
- ASSE 1016 (this entry) is the showers-and-tub-showers standard with thermal-shock protection.
- ASSE 1017 is a master mixing valve at the water-heater outlet, used to keep the heater hot enough to kill Legionella while distributing safe water.
- ASSE 1070 is a point-of-use limiter for sinks, lavatories, and bathtubs without thermal-shock protection.
- Pressure-balance vs. thermostatic. Pressure-balance reacts to pressure differential; thermostatic reacts to absolute temperature. Combination units do both.
Washington note
Washington adopts the 2021 Uniform Plumbing Code through WAC 51-56, effective March 15, 2024. UPC 408.3 requires a pressure-balance, thermostatic, or combination control valve on every shower and tub-shower combination, with maximum showerhead output of 120 degrees Fahrenheit. Any new shower installed in WA — new construction or remodel — must meet this. Older three-handle setups do not have to be upgraded just because you live in the house, but they trigger the requirement when you remodel.