shower-mixer-valve

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A shower mixer valve sets the balance of hot and cold water that reaches the shower head. The type most often discussed in these sources is the pressure-balance mixing valve. This Old House says it keeps the shower temperature steady when pressure changes elsewhere in the house, and Watts says that design does not control outlet temperature on its own. This guide sets out both views, what a Symmons valve specification states, and what this guide does not cover. Replacing a mixer valve means opening the wall and soldering copper, which is beyond swapping a showerhead, so installation steps are not included.

Quick answer

• This Old House says pressure-balance shower valves “keep water at a consistent temperature even when fluctuations occur elsewhere in your home’s plumbing system.”
• Watts says that pressure-balanced valves “require setting of the limit stop by the installer,” and that the limit stop “would need to be re-set” when supply temperatures change.
• Watts says pressure-balanced valves do not automatically control outlet temperature, and calls those limitations “potentially dangerous.”
• The Symmons specification lists a 4-port, pressure-balancing mixing valve with 1/2-inch sweat inlets and 1/2-inch NPS threaded outlets.

What a pressure-balance valve does

This Old House describes the core part as “a stainless steel balancing piston.” The article says the piston’s “shoulders move across the hot- and cold-water inlet and outlet ports, increasing or decreasing the water supply to maintain a stable temperature.” In the article’s example, when the cold supply drops during a flush, “the valve automatically reduces the hot water flow by the same amount.” That is the mechanism behind the claim that the temperature stays steady.

The same article describes a stop spindle in the valve body. It “shuts off the water supply for easy maintenance,” and the article says this lets a reader “service the cartridge without turning off water to the entire house.” That feature is a maintenance point, not a temperature control.

What This Old House claims for it

This Old House says that “Installing a pressure balance shower valve can help homeowners prevent sudden temperature changes in their showers and avoid potential scalding.” The article’s list of benefits includes “Compliance with modern plumbing codes in many areas,” “Consistent shower temperature,” “Enhanced comfort during showering,” “Improved safety,” and “Potential water savings by reducing the need to readjust the temperature.”

The article’s adjustment advice is to “Make sure the water temperature is consistent and does not fluctuate.” It also says that installing one “is a worthwhile upgrade for any bathroom,” which is the article’s opinion, not a test result. The article does not give a code section, a city, or a standard that requires the valve.

Where Watts disagrees

Watts is a maker of thermostatic tempering valves, and its PDF on safety thermostatic tempering valves describes the limits of pressure-balanced designs. The PDF says that “Many shower and combination tub/shower mixing valves in today’s homes use” a pressure-balanced design, and then argues against relying on that design for temperature control.

Watts says pressure-balanced valves do not automatically control outlet temperature. It says they also do not automatically limit outlet temperature “should the supply temperatures dramatically increase or decrease,” and calls such limitations “potentially dangerous.” To prevent a user from adjusting the valve beyond a safe operating temperature, the PDF says that “pressure balanced valves require setting of the limit stop by the installer.” It adds that “to accommodate future changes in the hot or cold water temperature, the limit stop would need to be re-set.”

This is a direct disagreement with the This Old House article. The article says the valve keeps temperature steady and reduces scalding risk. Watts says the valve’s temperature stability depends on a limit stop set by an installer, and that changes in supply temperature can move the limit. The two sources do not say which view applies to a given house, and the sources do not give a test that resolves the question.

Standards named in the sources

Watts describes the standard that covers shower valves at the fixture. It says ASSE 1016 “covers three types of valves: Pressure Balancing, Thermostatic and Combined Pressure Balancing/Thermostatic.” The Watts PDF does not say which standard a particular pressure-balance valve carries unless the product is named.

The Symmons specification names two standards for its shower valves: “ASME A112.18.1/CSA B125.1” and “ASSE 1016.” These are listed on the sheet as the valve’s standards and certifications.

What the Symmons specification states

The Symmons sheet covers the Temptrol shower and tub-shower mixing valve, in models S-46-1-BODY and S-46-2-BODY, and in related versions. The sheet describes the valve as a 4-port, pressure-balancing mixing valve. It says the valve has an adjustable stop screw that limits handle turn, and the dimension drawing labels that screw as the maximum-temperature setting.

• Supply inputs: “Supply input ports 1/2 inch sweat.”
• Outputs: “Output ports NPS threaded 1/2 inch.”
• Model S-46-1-BODY: a shower mixing valve with an integral volume control, and a plug fitted in the tub supply output port.
• Model S-46-2-BODY: a tub-shower mixing valve with an integral diverter.
• Reverse version (-REV): reverse coring, with hot on the right and cold on the left, for back-to-back installations.
• Service stop versions (-X): the sheet lists integral service stops to allow water shut-off for servicing the valve.
• Check stop version (-CHKS): integral check stops, for installations where a positive shut-off is needed.

The sheet gives a warranty of limited lifetime to the original end purchaser in residential and consumer installations, and five years for commercial installations. The sheet does not state a flow rate or a temperature range for the valve, so this guide does not give either.

Is this job for you

This Old House rates its installation as moderate in difficulty and says it requires “Basic plumbing skills required, including soldering.” Its steps involve taking down the tiled wall, cutting the existing valve out of the pipe, and soldering copper pipe to the new valve. Because that work goes beyond swapping a showerhead, this guide does not give steps for it.

Two safety statements from the sources are worth keeping in mind for any plumbing work. First, This Old House quotes Richard Trethewey: “The cardinal rule before starting any plumbing project: Turn off the water.” Second, the article says a bucket of water or a small fire extinguisher should be close by when soldering, as a precaution. A reader who is not comfortable soldering or opening a tiled wall should call a licensed plumber.

Watts recommends regular inspection of mixing valves. It says “Regular inspection of mixing valves is recommended to assure maximum valve life and a properly functioning system,” and that “The frequency of cleaning depends upon local water conditions.”

FAQ

Does a pressure-balance valve keep the shower temperature steady?

This Old House says that pressure-balance shower valves “keep water at a consistent temperature even when fluctuations occur elsewhere in your home’s plumbing system.” Watts gives the opposite caution: pressure-balanced valves “require setting of the limit stop by the installer,” and the limit stop “would need to be re-set” when supply temperatures change. Both views are in the sections above, and the sources do not settle the question for a given house.

Why would someone install one?

This Old House says installing one “can help homeowners prevent sudden temperature changes in their showers.” It describes the aim as avoiding “unexpected bursts of hot water that can cause scalding.”

Can I change the valve without plumbing work?

The sources do not say this. This Old House’s installation involves opening the wall, cutting and soldering pipe, and it is outside the scope of this guide. Its own advice starts with “The cardinal rule before starting any plumbing project: Turn off the water.”

What should I check on the valve over time?

Watts recommends regular inspection of mixing valves, and says “The frequency of cleaning depends upon local water conditions.” The sources do not give an inspection interval in years or months.

How we chose

This guide uses three sources, all opened on 2026-10-28: the This Old House installation article, the Watts PDF on safety thermostatic tempering valves, and the Symmons specification submittal for the Temptrol shower mixing valve. The choice was made to cover one article written for homeowners, one manufacturer that disagrees with the homeowner article on temperature control, and one product specification with stated figures. It is not a ranking of products or brands.

What we could not verify

What the sources do not say

• None of the three sources gives a flow rate or outlet temperature range for the Symmons valve, and the sources do not give a test result for either valve in a home.
• The This Old House article does not discuss the Watts point about limit stops, and the Watts PDF does not discuss the This Old House installation.
• The sources do not say how often a limit stop should be reset in a home where the supply temperature has changed.
• The sources do not give a price for the valve or the installation. This guide does not give one.
• The Watts PDF is written for its own thermostatic products, and its description of pressure-balanced valves is written from that point of view.

Sources

• This Old House, “How to Install a Pressure-Balance Shower Valve,” https://www.thisoldhouse.com/plumbing/how-to-install-a-pressure-balance-shower-valve (opened 2026-10-28)
• Watts, “Safety Thermostatic Tempering Valves,” PDF, https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/24295-source/637360942890000000/pg-mxv-pdf.pdf (opened 2026-10-28)
• Symmons Industries, “Temptrol Shower and Tub-Shower Mixing Valve, Specification Submittal,” PDF, https://discover.symmons.com/wp-content/uploads/2019/11/Symmons-Temptrol-Shower-and-Tub-Shower-Valve-Specification-Submittals.pdf (opened 2026-10-28)