thermostatic-mixing-valve-shower
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A thermostatic mixing valve shower uses a valve that senses the water temperature at the outlet and adjusts the mix to hold that temperature. The ASSE International white paper defines the thermostatic type this way: “A compensating valve that senses the water temperature at the outlet and compensates for thermal variations to maintain the water temperature at the outlet.” The valve also has a limit stop, which the same paper says “must be set to the desired temperature by the installer.” The sources do not describe how to set it.
Quick answer
- A thermostatic valve responds to outlet temperature. A pressure balancing valve responds to differences between the hot and cold inlet pressures. The paper names a third, combined type.
- The limit stop controls the maximum temperature at the outlet. The paper says that is its job at “the point of use.”
- The NIH study says thermostatic mixer equipped valves “limit water temperature to a maximum of 120°F.”
- Changes in the incoming water temperature can make a limit stop need resetting. The sources do not give a method for resetting.
What the thermostatic valve does
The ASSE paper names three kinds of shower valve that meet its standard. The table below lists them with the paper’s own definitions. The thermostatic type is the one the phrase “thermostatic mixing valve shower” refers to. The other two types are included because the paper lists them together and because a shower can have either.
| Type (as named in the paper) | What the valve senses, per the paper |
|---|---|
| Type P, pressure balancing | “senses inlet supply hot and cold water pressures and compensates for variations in the inlet supply pressures to maintain the water temperature at the outlet” |
| Type T, thermostatic | “senses the water temperature at the outlet and compensates for thermal variations to maintain the water temperature at the outlet” |
| Type T/P, combination | “senses inlet supply hot and cold water pressures before mixing, senses the water temperature at the outlet, and compensates for pressure and thermal variations” |
The paper’s point is that a thermostatic valve holds the outlet temperature against thermal changes, and that a pressure balancing valve holds it against pressure changes. A combined valve does both. The paper does not say which of these a particular shower has, so the listing or the manufacturer’s literature is the place to check.
Why the limit stop is part of the valve
The paper explains that a compensating valve is not only a mixer. It says: “This type of valve has a temperature-limiting feature that controls the maximum temperature of the bath or shower water at the point of use.” That feature is the limit stop. The paper calls the limit stop “an adjustable and lockable means to limit the maximum setting of a device towards the hot position, limiting the maximum discharge temperature.”
The paper says the installer must set the limit stop, and that the setting is not automatic. It also says that “Every manufacturer includes detailed instructions with their products for the setting of the limit stop.” It adds that installers do not always follow those instructions. The paper lists “Installation errors” and water that is not yet hot at the time of installation among the reasons limit stops are not set properly.
The paper also says that not every shower has this feature. It states: “Not all shower valves have a maximum temperature limit adjustment.” Older valves may be non-compensating types, such as a two-handle shower valve. A reader cannot tell from the handle alone whether a thermostatic valve with a limit stop is installed.
What changes the setting
The paper’s main point about upkeep is that the incoming water can change. It says: “Any significant change in the incoming water temperature, either cold or hot, may require the limit stop to be reset.” It names two examples: a change to the water heater or thermostatic mixing valve setting, and the replacement of the water heater.
Seasonal change is the other factor. The paper says “In some climates, the incoming cold water temperature can change significantly from winter to summer and summer to winter.” It suggests testing the highest temperature available during the coldest and hottest months. If that test exceeds the maximum allowed by local plumbing code, the paper says the limit stop should be considered for resetting. The paper does not describe the test equipment or the procedure beyond checking the outlet temperature with a thermometer.
What the NIH study says about thermostatic mixer valves
The NIH study is about water heater temperatures. Its authors measured the hot water in 976 urban homes and found that “Hot water temperature was unsafe in 41% of homes.” It recommends a specific step: “installing thermostatic mixing valves to ensure a safer temperature.” The study describes what this kind of valve does in the water system: “Thermostatic mixer valves allow hot tap water to be set at a fixed temperature outside of the water heater and without affecting the temperature of stored hot water in the tank.”
The study also discusses a trial. It says “Edwards et al. quantified the effects of a thermostatic control system on the prevalence of dangerous water temperatures in a cluster randomized controlled trial in London.” The study reports that the prevalence of dangerous hot water temperatures “was significantly reduced and gas consumption was not significantly affected.” That trial tested a thermostatic control system. The source does not say it tested a shower valve with a limit stop, so this article does not treat the two as the same device.
Why the paper focuses on setting, not on choosing a model
The paper says it “does not attempt to determine which device should be used in which application, nor does it attempt to determine if one type, style, or manufacturer is better than another.” It is a reference on the limit stop, and it is written for installers and the homeowners who read the instructions.
That is useful for a shopper because it means the paper is not a buying guide. The paper gives the rule that matters: the limit stop has to be set for the water the valve will actually receive, and that setting can drift as the water supply changes.
How we chose
This article uses two sources. The ASSE International paper is a standards-body white paper on how the limit stop on compensating shower valves works and why it must be set. The NIH study is a peer-reviewed paper in the Journal of Burn Care Research that reports water temperatures in urban homes and recommends thermostatic mixing valves. Both were opened on 2026-10-28, and their saved text was checked against every quote used here.
No products were ranked and no brands or models are named. The two sources do not compare products, so the article does not compare them either.
What the sources do not say
- How to set or adjust a limit stop on any valve. The paper says it “does not describe how to adjust the limit stops,” and points to manufacturer literature instead.
- Which thermostatic shower valve to buy. The paper says it does not judge one manufacturer against another.
- A current price for a thermostatic shower valve. The sources give no shower valve price.
- How often the outlet temperature should be checked. The paper describes checks tied to seasons and to water heater changes, without a fixed interval.
- Whether a specific valve on the market is a thermostatic type with a limit stop.
What we could not verify
The paper says the “maximum safe water temperature, usually 120 degrees Fahrenheit, is mandated by the local code”. The two sources do not give that code for any city or state, so the local figure must be confirmed with the local authority. The NIH study does not report whether the homes it sampled had shower valves with limit stops. The paper is a white paper and does not state the edition of the code it refers to, so that edition is not confirmed here.
FAQ
Is a thermostatic mixing valve shower the same as a pressure balancing shower?
No. The paper names them as different types. A thermostatic valve senses outlet temperature, and a pressure balancing valve senses inlet pressures. The paper also names a combined type that senses both.
Does a thermostatic valve have a limit stop?
The paper describes a temperature-limiting feature on compensating valves, and it says the limit stop must be set by the installer. It also says that not all shower valves have this feature, so the valve’s own literature should be checked.
Why would the setting need to change?
The paper says a significant change in the incoming hot or cold water temperature “may require the limit stop to be reset.” It gives the water heater setting, a water heater replacement, and seasonal change in cold water as examples.
What temperature does the NIH study use as a limit?
The study says thermostatic mixer equipped valves “limit water temperature to a maximum of 120°F.” The study’s own definition of safe hot water is 120°F or below.
Can a homeowner reset the limit stop?
The sources do not describe the adjustment. They say the manufacturer’s instructions contain the setting method, and that a licensed master plumber or a certified plumbing engineer can help when literature is not available.
Does a thermostatic valve at the water heater replace a shower valve?
The NIH study describes a thermostatic mixer valve set outside the water heater, which allows hot tap water to be set at a fixed temperature. The sources do not say that a valve at the water heater replaces a limit stop on a shower, so this article does not treat them as interchangeable.
Bottom line
A thermostatic mixing valve shower holds its outlet temperature by sensing the water and adjusting the mix. The limit stop sets the ceiling, and the paper says it must be set at installation and reset when the incoming water temperature changes. Read the manufacturer’s instructions, and use a licensed plumber for installation and setting.
Sources
- ASSE International Scald Awareness Task Group. “Adjustment of Automatic Compensating Valves to Prevent Potential Scald Hazards.” White paper. https://asse-plumbing.org/media/cfnnsz0k/asse_compensating_valves.pdf (opened 2026-10-28)
- Shields WC, McDonald E, Frattaroli S, et al. “Still too hot: Examination of water temperature and water heater characteristics 24 years after manufacturers adopt voluntary temperature setting.” J Burn Care Res. 2013;34(2):281-287. https://pmc.ncbi.nlm.nih.gov/articles/PMC3605550/ (opened 2026-10-28)
