Anti-Scald Valve — How It Works and What Code Requires

An anti-scald valve is a plumbing device that automatically limits the maximum temperature of water delivered at a fixture to prevent burns from sudden hot-water surges.

Anti-Scald Valve diagram — labeled parts, dimensions, and installation context

What It Is

Anti-scald valves work by blending hot and cold water in a pressure-balancing or thermostatic cartridge. When the cold-water supply pressure drops suddenly — such as when a toilet flushes — the valve reduces hot water flow proportionally to prevent the shower temperature from spiking. A separate adjustable temperature limit stop prevents the handle from being turned to a position that would deliver dangerously hot water.

Scalding injuries are a significant household hazard, particularly for young children and elderly adults, who have thinner skin and slower reaction times. Building codes in the United States have required anti-scald protection in shower and tub fixtures for new construction since the late 1990s.

Types

Pressure-balancing anti-scald valves (also called pressure-balance valves) maintain a fixed hot-to-cold ratio when supply pressures change but do not actively regulate temperature. Thermostatic valves use a wax or bimetal element to maintain a set temperature regardless of pressure or incoming water temperature changes. Thermostatic valves offer more precise control and are used in higher-end installations.

Where It Is Used

Anti-scald valves are installed in shower valves, tub/shower combination valves, and at the water heater as a mixing valve on the outlet. Code requires them in showers and tub-shower combos in new residential construction throughout the US and Canada.

How to Identify One

Most modern single-handle shower valves have a built-in anti-scald feature. Look for a plastic tab or limit stop behind the handle that prevents the handle from rotating fully into the hot range. A standalone thermostatic mixing valve is a separate device with hot and cold inlets and a mixed outlet, often installed in a valve box in the wall.

Replacement

The temperature limit stop is adjustable and rarely needs replacement. The valve cartridge inside is the component that wears — symptoms include temperature instability, difficulty maintaining a consistent temperature, or the valve no longer holding a set temperature. Cartridges are model-specific and available from the original manufacturer.

Anti-Scald Valve — FAQ

What temperature should an anti-scald valve be set to?

Most manufacturers and plumbing codes recommend a maximum delivered temperature of 120°F (49°C) at the fixture. The water heater can be set higher — typically 120–140°F to reduce Legionella risk — and the anti-scald valve or mixing valve blends cold water to bring the delivered temperature down to a safe level.

What is the difference between a pressure-balance valve and a thermostatic valve?

A pressure-balance valve maintains the ratio of hot to cold water when supply pressures shift, preventing temperature spikes when other fixtures run. A [thermostatic valve](/wiki/thermostatic-valve/) actively monitors and adjusts to maintain a precise set temperature regardless of changes in both pressure and incoming water temperature. Thermostatic valves are more precise and are often used in multi-outlet shower systems.

Can I adjust the anti-scald limit on my existing shower valve?

Yes. On most single-handle shower valves, removing the handle exposes a rotating plastic limit stop. Turning the stop clockwise allows hotter water; counterclockwise restricts it. The exact procedure varies by valve brand, but it typically requires only a screwdriver. The valve does not need to be shut off for this adjustment.

Does the law require anti-scald valves?

Yes, in new residential construction throughout the United States. The International Residential Code (IRC) and most state plumbing codes require pressure-balancing or thermostatic shower valves in all new shower and tub-shower installations. Older homes may have unprotected mixing valves that can be upgraded.

How do I know if my shower valve has anti-scald protection?

Check for a limit stop on the handle — a plastic tab that prevents the handle from turning fully into the hot zone. You can also look up the valve brand and model number (often stamped inside the escutcheon) to confirm. If your shower frequently produces dangerous temperature spikes when other water is used, the valve likely lacks or has a worn anti-scald cartridge.