Shower Valves: Pressure-Balance, Thermostatic and Digital Controls

The shower valve is the part behind the wall that mixes hot and cold water and sends it to the showerhead. You only see its handle, but it decides whether the shower scalds when someone flushes a toilet, how many outlets you can run and where the controls sit. It is also one of the first things that has to be chosen, because it is built into the wall before tile. This page explains the types and what to decide.

The valve types compared

What the code requires

Every new shower and tub-shower valve must protect against scalding. The 2021 Uniform Plumbing Code, section 408.3, which Washington adopts, requires individual control valves of the pressure-balance, thermostatic or combination type, installed at the point of use and meeting ASSE 1016/ASME A112.1016/CSA B125.16 or ASME A112.18.1/CSA B125.1. It also limits the water leaving a showerhead to 120°F.

The ASSE 1016 standard defines three valve types: Type P (pressure-balancing), Type T (thermostatic) and Type T/P (combination). Each has a limit stop, and ASSE notes it "must be set to the desired temperature by the installer". The stop can be set lower than the maximum if you prefer. A valve installed with its stop never adjusted is a common miss, so ask that the setting be checked at the end of the job. The full code text is on the bathroom plumbing code page.

How each type feels in use

Pressure-balance

A pressure-balance valve has a spool or diaphragm that responds when hot or cold pressure changes, keeping the mix close to where you set it. On most models one handle turns the water on and sets the temperature, so you cannot set a volume separately: the flow is either on or off. It is reliable, compact and the standard choice for most showers.

Thermostatic

A thermostatic valve senses the outgoing water temperature and adjusts the mix to hold it, so you can leave the temperature handle at your setting and use a separate control to turn the water on and off or switch outlets. That is convenient in a shower with several outlets, and in a shared bathroom where people want different temperatures.

Digital

Digital systems put the mixing unit somewhere accessible outside the shower, such as a vanity or closet, and run the controls from a keypad on the wall. They can store presets for different people and switch outlets at a touch. They need power and a place for the mixing unit, and service means a technician for that system. Ask how the system meets the same scald-protection requirement and what happens if it loses power.

Diverters, volume controls and multiple outlets

A diverter sends water to one outlet or another: showerhead, handheld or tub spout. A two-way diverter serves two outlets; a three-way serves three, sometimes with a position that runs two at once. On a thermostatic setup the diverter is usually its own handle; on pressure-balance trim it can be built into the handle.

Running several outlets at the same time is limited in Washington. The state's plumbing code (WAC 51-56-0400, UPC 408.2.1) caps the combined flow of multiple shower outlets on one valve at 1.8 gallons per minute at 80 psi, unless the valve lets only one outlet run at a time. That shapes what kind of diverter you need, and it is explained on showerheads, rain heads and body sprays.

Where the valve goes

The plumbing code expects controls and showerheads on a sidewall, or otherwise arranged so the head does not spray at the entrance and you can adjust the valve before stepping into the spray (2021 UPC section 408.9). In practice:

• Put the controls near the entry, on the side wall or the wall you face as you step in, so you can reach them from outside the spray.
• Offset the controls from the head in a larger shower, so you are not standing under cold water while it warms.
• Height: the National Kitchen and Bath Association's planning guidelines suggest shower controls 38 to 48 inches above the floor and reachable from inside and outside the spray. That is a designer guideline, not code.
• Avoid exterior walls where you can, so the pipes stay inside the insulated part of the house.

If you are planning for someone who sits to shower, put the controls and handheld where they can be reached from the seat. See aging-in-place bathrooms.

Why the valve is chosen before tile

• The body sits in the wall. It is set at a depth that matches the finished wall thickness, tile included. Each valve has a range of acceptable depth, and a body set too deep or too shallow can leave the trim not fitting.
• The trim must match the body. Handles and plates fit only their own valve system, so the look you want limits the valves you can use.
• Outlet count is set in the rough-in. Adding a handheld or a second head later means opening tile.
• Tile layout works around it. A good tile layout centers the trim plate on a tile or a grout line, which needs the valve location fixed first.

What to check before you buy

• The valve is a pressure-balance, thermostatic or combination type listed to ASSE 1016 or ASME A112.18.1.
• The trim you like is made for that valve body, in the finish that matches the rest of the room.
• How many outlets you want, and whether they run one at a time.
• Whether you want separate temperature and volume controls.
• Where the cartridge is serviced from, and whether replacement cartridges are stocked locally.
• For digital: where the mixing unit goes, its power needs and its service plan.

JDI Construction selects the valve with you during design, before demolition, so it arrives in time for the rough-in and the tile layout can be drawn around it. To plan your shower, start on our Vancouver shower remodeling page or see what a full project includes on the bathroom remodel cost page.