How a pressure-balance shower valve works

Someone flushes a toilet while you are in the shower, the cold supply pressure drops for a few seconds, and the water at the showerhead should stay at the same temperature. That is the whole job of a pressure-balance valve. Most single-handle tub and shower valves in US homes are one, and the plumbing codes require the protection.

What the standard requires

The performance standard is ASSE 1016. It applies to automatic compensating valves installed at the point of use, where the user controls the final temperature and nothing mixes downstream. Three types of valve meet it:

• Type P, pressure balancing. Senses the hot and cold inlet pressures and compensates for changes in them to hold the outlet temperature.
• Type T, thermostatic. Senses the water temperature at the outlet and compensates for thermal variations.
• Type T/P, combination. Senses inlet pressures before mixing and the temperature at the outlet, and compensates for both.

The code side follows the standard. Under IPC-based codes, individual shower and tub-shower valves must be balanced-pressure, thermostatic or combination valves meeting ASSE 1016 (or ASME A112.18.1/CSA B125.1), with the maximum setting field-adjusted to no more than 120 F. Every valve that meets the standard must have an adjustable means to limit the setting toward hot, capable of limiting the outlet to 120 F or the manufacturer’s lower maximum.

How the pressure-balance cartridge does it

Inside the valve body sits a cartridge. Hot and cold water enter it on opposite sides. Between them is a free-moving part, a spool or a sliding disc on a piston, that reacts to the pressure on each side. When both supplies push equally, it sits centered and both inlets are open the amount the handle calls for. When the cold side loses pressure, the hot side’s pressure pushes the spool toward the cold inlet, which throttles the hot opening by the same proportion. The ratio of hot to cold stays where you set it, so the temperature stays put even though the total flow drops. Delta says its tub and shower valves are engineered to hold the temperature within about plus or minus 3.6 F.

Notice what the valve is not doing: it is balancing pressure, not reading temperature. With the limit stop wide open and the handle turned fully to hot, the water at the showerhead can be as hot as the water heater delivers. That is why the limit stop matters.

The rotational limit stop

Behind the handle of a pressure-balance valve is a notched ring or stop, the rotational limit stop, that physically restricts how far the handle can rotate toward hot. The installer sets it at installation, and it may need periodic adjusting because supply water temperatures vary with the season. On a Delta Monitor valve, moving the stop one tooth counterclockwise lowers the maximum by roughly 6 F. Moen recommends limiting the maximum temperature flowing from the shower to 120 F or less to prevent scalding, and Delta’s instructions warn installers to keep the hottest flow under the temperature allowed by the local plumbing code, which is 120 F in most places and 110 F in some.

If your shower will not get as hot as it used to, check the limit stop before anything else. If the handle stops short of the hot position, the stop needs to be repositioned.

How a thermostatic valve differs

A thermostatic valve reads the temperature of the mixed water itself. Because of that it also corrects for things a pressure-balance valve cannot see: the water heater being turned down, or incoming cold water being colder in February than in August. Thermostatic valves are common on higher-end and European-style showers. Both types satisfy the code; the pressure-balance valve is simply the one most houses have.

Symptoms of a worn cartridge

The cartridge is a wear part. Moen’s own troubleshooting pages give the two classic signs:

• The shower keeps running or dripping with the handle in the off position. The fix is to replace the cartridge.
• The water pressure decreases as you turn from cold toward hot. Again, the cartridge.

Before you buy one, rule out the cheaper explanations the same pages list: a mis-set limit stop; supply lines reversed at installation (hot going to the cold side), which makes the valve behave backwards; clogged inlet screens; or simply no hot water from the heater.

Cartridges are brand- and series-specific. The replacement must match the valve body, not just the brand; a Moen 1222 and a Moen 1225 go in different valves. If you are not sure what you have, the Stems and Cartridges index identifies cartridges by brand and part number, and the old one in your hand is the best reference. Some cartridges, the Moen 1222 among them, are sold with or alongside a removal tool because they can be stubborn to pull from an old valve body.

What you can do, and when to call a plumber

Replacing a cartridge is homeowner work on most valves: shut off the water (many valves have integral stops behind the trim plate; if not, use the house main), then turn the handle on to confirm nothing flows and the pressure is gone. Pull the handle and trim, lift out the retaining clip (do not drop it into the wall), pull the cartridge, fit the new one in the same orientation (backwards and the hot and cold swap), and put the retaining clip back before the water goes on; Moen’s instructions say the water cannot be turned back on until the clip is in, because the clip is what holds the cartridge in the valve body against line pressure. Adjusting the limit stop is also yours to do, with a thermometer in a cup at the showerhead to confirm the maximum.

Call a licensed plumber if the valve body itself leaks inside the wall, if the integral stops are seized, if you find the supply lines reversed, or if the cartridge will not come out even with the removal tool. A valve body replacement means opening the wall and sweating or crimping new connections, and it is the right job to hand off.

Related

• Almanac: March (shower drips) and February (water heater temperature, which sets the ceiling the limit stop works under)
• How a compression faucet works covers the two-handle valves that use stems and washers instead.
• Glossary: pressure-balance valve, thermostatic valve, rotational limit stop, diverter