A pressure-balancing valve keeps the hot-to-cold ratio steady when pressure changes, so a flush won't scald you. A thermostatic valve reads the water's actual temperature and holds a set point, with separate volume control. Both meet code (ASSE 1016). Pressure-balancing costs less; thermostatic gives tighter, preset temperature.
How each valve keeps your shower from scalding you
Both valves solve one problem. Something changes elsewhere in the house, and what comes out of your showerhead changes with it. They just sense different things.
### Pressure-balancing: it reacts to pressure
Inside a pressure-balancing valve there's a balancing spool or piston that sits between the hot and cold inlets. Push on one side harder than the other and it slides, choking down the side with the extra pressure. Symmons, which makes the Temptrol line, describes the effect this way: "as cold water pressure drops, the valve drops hot water pressure, too."
That's why your spray gets weaker for a second when a toilet refills. The valve is trading flow for safety. It's also why a pressure-balancing valve can't fix everything. It keeps the ratio of hot to cold steady. But say the water arriving at the valve changes temperature, because the tank runs low or the cold line warms up in summer. The ratio stays the same, so the shower still drifts.
Most pressure-balancing valves are single-function: one handle controls temperature and you get full flow once it's on. Delta's Monitor 13 and 14 Series and Moen's PosiTemp work that way. Some are dual-function, with volume and temperature separated, like Delta's Monitor 17 Series.
### Thermostatic: it reacts to temperature
A thermostatic valve has a temperature-sensing element in the mixing chamber. IAPMO's code commentary, describing thermostatic master valves, says a moving piston responds to temperature changes through an expanding or moving bellows, a bimetallic unit or a heat-conducting fluid. The element moves the mixing mechanism on its own. So if the incoming hot water gets hotter or cooler, the valve re-blends to hold the set point. Delta says its TempAssure thermostatic valves read water temperature "to automatically adjust for seasonal ground water temperature changes."
The typical layout is two controls: one sets temperature, the other turns the water on and controls volume. Symmons points out the payoff: you can pre-set the temperature and leave it, so the next shower starts where the last one ended. Moen's ExactTemp line is its thermostatic option. Moen notes that a correctly calibrated 3/4-inch ExactTemp valve hits a stop at 105 degrees. You push an override button to go warmer.
Here's the deal: pressure-balancing reacts to what the pipes are doing, thermostatic reacts to what the water feels like.
What the plumbing code requires
You don't get to skip this part. Every individual shower and tub-shower combo in a home needs one of these valves, and it has to be limited to 120 degrees.
### IPC 412.3 (2024 edition)
Phoenix adopted the 2024 International Plumbing Code (IPC), with local amendments, effective August 1, 2025 under Ordinance G-7397. Section 412.3 of the 2024 IPC says individual shower and tub-shower valves "shall be balanced-pressure, thermostatic or combination balanced-pressure/thermostatic valves." Each one must also conform to ASSE 1016/ASME A112.1016/CSA B125.16 or ASME A112.18.1/CSA B125.1. The same section adds three rules worth knowing:
- The valve must be installed at the point of use and rated for the flow of the showerhead you actually install.
- It must have a means to limit the maximum setting to 120°F (49°C), field adjusted per the manufacturer's instructions.
- "In-line thermostatic valves shall not be utilized for compliance with this section." A mixing valve buried somewhere upstream doesn't count. The protection has to be in the shower valve.
### UPC 408.3
The other major model code, the Uniform Plumbing Code (UPC), covers the same ground in Section 408.3. IAPMO's training commentary on UPC Section 408.3 lays out the same idea: showers and tub-showers need individual control valves "of the pressure balance, thermostatic, or combination pressure balance/ thermostatic mixing valve type that provide scald and thermal shock protection." Handle position stops are set to deliver a maximum of 120°F, and IAPMO's text is blunt that "Water heater thermostats shall not be considered a suitable control for meeting this provision."
IAPMO also explains why showers get special treatment: an abrupt temperature change can trigger an involuntary reflex that causes a fall on a slippery surface. Seated bathers don't face that risk, which is why a tub without a shower riser isn't covered by this section.
### ASSE 1016, ASME A112.1016 and CSA B125.16
These three names are one harmonized standard. ASSE lists it as the performance requirements for automatic compensating valves for individual showers and tub/shower combinations. Per ASSE, it covers valves installed at the point of use "where the user has access to flow or final temperature controls." Both valve types can be certified to it. Look for the standard on the valve's spec sheet or box. That's your proof the valve counts.
Why 120°F? The U.S. Consumer Product Safety Commission says most adults will suffer third-degree burns from a six-second exposure to 140 degree water, or a thirty second exposure to 130 degree water. At 120 degrees it takes about five minutes. That gap is the whole point of the limit stop.
Pros and cons, side by side
Neither valve is the winner in every bathroom. This table compares the two on the things homeowners actually notice.
| Factor | Pressure-balancing | Thermostatic |
|---|---|---|
| What it senses | Pressure difference between hot and cold | Actual mixed water temperature |
| Reaction to a toilet flush | Holds temperature; flow dips briefly | Holds temperature by re-blending |
| Reaction to a hot supply that cools or heats | Temperature drifts with it | Re-blends to hold the set point |
| Seasonal adjustment | Delta says Monitor valves may need manual adjustment | Delta says TempAssure adjusts automatically |
| Controls | Usually one handle (temperature only) | Usually two (temperature and volume) |
| Preset temperature | Only on dual-function models | Standard feature |
| Multiple outlets (body sprays, several heads) | Check the valve's flow rating | Check the valve's flow rating; volume control is separate |
| Relative cost | Symmons calls it "usually the less expensive option" | Symmons says up to twice the cost |
| Retrofit fit | Usually fits the existing opening | May need a wider wall opening or moved piping, per Symmons |
| Hard-water sensitivity | Spool can stick with scale or debris | Symmons notes thermostatic elements "are susceptible to mineral buildup and scale" |
| Code status | Meets IPC 412.3 and UPC 408.3 if listed to ASSE 1016 | Same |
Delta publishes one useful number for comparison: all of its tub and shower faucets are engineered to hold water temperature within ±3.6°F, with a footnote that this "Requires inlet water temperature be set correctly. May require seasonal adjustments."
### The Phoenix angle: seasons and scale
Seasonal adjustment matters more here than in most places. The City of Phoenix 2025 water quality report lists distribution water temperature ranging from 47 to 92 degrees Fahrenheit across the year. That's a big swing in what the cold side brings to your valve. With a pressure-balancing valve, a handle position that felt right in January can feel noticeably warmer in July, and you may end up nudging the handle or the limit stop. A thermostatic valve compensates for that on its own.
Hardness cuts the other way. The same report puts Phoenix hardness at 10 to 17.6 grains per gallon. It says hard water can cause scaling in pipes and water heaters, and on fixtures such as faucets and showerheads. Both valve types have moving parts that scale can bind. Thermostatic cartridges tend to be pricier and more precise, so a water softener or regular cartridge service protects the bigger investment.
Retrofit notes and brand specifics
Swapping valve types is rarely a trim-only job. The valve body lives inside the wall, and each brand's cartridge fits its own body.
### Delta
Delta's MultiChoice Universal rough-in valve is built for upgrades. Delta says any 13, 14, 17 or 17T Series trim kit it currently makes will work with this valve. It also says the valve lets you upgrade "without altering the plumbing behind the wall." In practice:
- 13 and 14 Series: single-function Monitor pressure balance, temperature only.
- 17 Series: dual-function Monitor pressure balance, separate volume and temperature.
- 17T Series: TempAssure thermostatic. Delta notes thermostatic is only available for dual-function faucets.
If your wall already has a MultiChoice body, moving from a 14 to a 17T is a cartridge-and-trim change. Look for the model number on the old trim or the valve body before you shop.
### Moen
Moen splits its lines clearly. PosiTemp is its common pressure-balancing valve, with a rotate-only handle and the 1222 cartridge. Moentrol adds volume control and uses a separate 1423 balancing spool. ExactTemp is the thermostatic option. These bodies are not interchangeable, so going from PosiTemp to ExactTemp means a new valve in the wall. Our guide to identifying your Moen shower valve walks through the handle and screw-position clues.
### Symmons
Symmons says it invented the first anti-scald, pressure-balancing valve. It introduced the Temptrol shower valve in 1968 and calls it the leading pressure-balancing shower valve in North America. Temptrol bodies take Temptrol parts, so replace like with like and match the model before ordering a cartridge.
### When to open the wall
Plan on opening the wall, or working from an access panel behind it, in three cases. You're changing brands. You're going thermostatic on a body that doesn't support it. Or the old valve body is leaking or corroded. That work involves soldered or crimped connections behind finished tile. It's also when you set the new limit stop at 120°F and test it with a thermometer, as Moen's instructions call for.
If your shower is just drifting hot and cold, you might not need a new valve at all. Our page on why the water pressure drops when a toilet flushes covers the normal dip, and how to replace a shower cartridge covers the most common repair.
If you only want to change the handle and plate, our page on how to replace shower valve trim explains why the trim has to match your valve and how to reset the hot limit stop.
Which one should you choose?
Use this as a quick decision guide.
- Choose pressure-balancing if you're replacing a valve in an existing wall, you want one simple handle, and budget matters. It meets code and handles the classic flush problem.
- Choose thermostatic if you want a preset temperature, separate volume control, multiple outlets like body sprays, or you have a household member who's sensitive to temperature swings. Symmons notes the preset feature can help elderly users.
- Choose a dual-function pressure-balance valve (like Delta's 17 Series) if you want separate volume and temperature controls without paying for a thermostatic cartridge.
- Either way, confirm the valve lists ASSE 1016/ASME A112.1016/CSA B125.16, and set the limit stop so the hottest water at the head is 120°F or less.
That's the play. Match the valve to how your household showers, then set the stop.
HQ Plumbing & Air is licensed and insured, gives upfront pricing and free estimates on shower valve replacements, and offers 24/7 emergency service when a valve fails behind the wall.
Commercial buildings also use thermostatic mixing valves at the water heater and at fixtures, explained in what a thermostatic mixing valve is and why commercial buildings use one.