If you’re preparing to renovate a bathroom, you might have plans to install some new fixtures. One of the things you should know before buying a new shower is the type of valve that’s attached to the water pipes inside the wall. It controls the flow and temperature of the water and is part of the rough-in plumbing behind the shower handle and cover.

You may need a certain type of valve to match up with the configuration of the new shower you have. Since putting in a new valve is usually a plumbing project best left to the pros, you can save this expense if you don’t need a new model. For example, if you currently have a three-port design but you want to install a new fixture that needs four ports, you’ll need to swap out the old device. Even if the valve you have installed is in good shape and fits your new shower installation, it’s helpful to understand what type it is. 

Two designs typically meet modern plumbing codes — balanced pressure and thermostatic. If you recognize that you have an older type of valve inside your wall, you might want to swap it out for a newer model. Here’s how to recognize both the newer and older devices you might find installed inside your bathroom walls.

The pressure balancing valve is the most common type used in modern designs. This model gives the shower a single handle to control the flow and temperature of the water and prevents water pressure from dropping when someone flushes the toilet. The valve senses that pressure on the cold water line drops when a toilet flushes, automatically adjusting the flow on the hot water line to match. A common mistake when installing a single-handle shower faucet is to skip on adding a pressure balancing valve. You can spot this design by looking for a cylinder with a small stem extending from it and the water supply lines on either side. The shower handle attaches to the stem.

Thermostatic valves also regulate both flow and temperature. But unlike pressure balancing valves, these models use a thermostat to control the hot and cold water supply and provide precise control of the temperature. The design has a dual-connector system that ensures the unit can manage the water flow and temperature separately, allowing you to reduce or expand the flow of water without affecting the temperature. To recognize this valve, look for a small cylinder with a stem that extends from the front of the unit. It will have water supplies on either side that are vertically above the stem to provide space for the thermostat. The larger size and positioning of the water supply pipe connectors make this model very distinct from the pressure balancing valve. 

If you haven’t updated your shower or bathroom in a while, you might have some older valves inside the wall that you may want to replace. The shower mixing valve is one such model. Also called a manual valve, it mixes the water from both the hot and cold water pipes. Unfortunately, you could end up with scalding when someone flushes a toilet and causes the cold water pressure to drop suddenly. The valve cannot adjust for these situations like the thermostatic valve. To recognize this simple design, look for inputs on either side and a single cylinder extending out the front piece that connects to the handle.

Another older design found in showers is a compression valve. You’ll commonly see this in a shower that has two separate handles for controlling cold and hot water flow. This is an extremely simple design where you turn the handle counterclockwise to increase the water flow and clockwise to reduce the flow. As it ages, the rubber washer that blocks the water flow can wear out and create leaks. To recognize the compression valve, look for a cylinder with a direct connection to the water supply pipe. It includes a connector that diverts the water upward to the showerhead. The stem that connects to the handle is located on the front of the cylinder. Showers with this design have two of these valves, and the cold and hot water mixes at the showerhead.