Guide to Pressure-Balanced and Thermostatic Shower Valves

Water cascading out of handheld shower head.

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Have you ever been taking a shower when the water from the showerhead suddenly become scaldingly hot or icy cold? That sudden temperature change is due to a drop in either the hot or cold side water pressure. This can happen when someone else in the house turns on the water at a fixture, when the washing machine fills, when a toilet flushes, or even when the dishwasher begins a cycle that uses water. Sometimes the situation is just startling and annoying, but a sudden blast of hot water can also create scalding burns.

A pressure-balance shower valve or thermostatic shower valve can remedy this issue, and they are not required by most building codes for new construction or during remodeling.

Two Valve Types

While they are often confused, there are actually two types of shower valve that can serve to balance water temperature and prevent burns.

  • Pressure-balance Shower Valve: This option operates by sensing the volume ratio of hot water to cold water and adjusts them to continually deliver water at your chosen temperature. As you turn on the water with a single handle, you are controlling both the volume and the temperature at the same time. As you twist the control, the volume of the water increases at the same time you are increasing the temperature.
  • Thermostatic Shower Valve. This option operates by sensing the actual temperature of the incoming water and adjusting hot and cold flow to maintain a steady temperature. With these shower valves, you generally set the water temperature and water volume with separate controls. The water temperature remains the same, no matter if the water is trickling or running full blast. This type controls the temperature to within one degree of whatever setting you establish with the temperature control. The chosen temperature will remain at the same setting after you turn the water off, so the next shower will automatically be set exactly where you left it. Thermostatic shower valves are regarded as far superior to pressure-balance valves, but they are also much more expensive.

    How a Pressure-Balance Valve Works

    A pressure-balance shower valve works by maintaining a balanced flow of water between the hot and cold side. They do not actually sense the temperature of the water. The valve has either a balancing spool or a diaphragm inside the valve body that reacts to drops in the pressure of one incoming water supply (either hot or cold) and adjusts the outflow of the opposite water supply to match. This ensures that the water doesn’t get suddenly colder or hotter as it emerges from the spout of the showerhead. Sudden changes in supply water pressure may reduce the outflow of water from the showerhead, but it will not change the temperature.

    How a Thermostatic Shower Valve Works

    Thermostatic shower valves use a more sophisticated mechanism that actually senses the temperature of the water rather than merely reacting to the volume of flow between hot and cold inputs. A wax element inside the shower valve expands or contracts in reaction to heat, changing the amount of water allowed in from each inlet. While this might sound virtually the same as the less expensive pressure-balance valve, there are key advantages:

    • The temperature remains the same no matter what volume you set. This makes water conservation easier.
    • You can set the water heater at a high-temperature setting, yet the shower valve itself can be limited so that the water temperature never reaches a scalding level. This allows washing machines, dishwashers, etc., to use much hotter water than you want in a shower or bathtub.

    Installation Notes

    Pressure-balance shower valves are usually one-handle designs that can fit into the same wall or surround openings where an old standard single-handle faucet fit, but the conversion to a pressure-balance valve sometimes does require some changes to the hot and cold water supply piping and the shower standpipe in the wall to allow the new valve to fit. If you aren't comfortable enough in your plumbing skills, consider hiring a plumber.

    Thermostatic shower valves come in many styles, but typically the separate volume and temperature controls are part of a single-valve body that is covered by a large escutcheon plate. This usually requires some modification to the wall or surround opening to fit the new, larger valve mechanism. Some types, however, use a streamlined profile in which the temperature control is mounted directly on the volume control handle. Alterations to the water supply piping are again sometimes necessary. Many people choose to have a professional plumber install this kind of valve, considering that the cost of the valve itself can easily run $200 or more, making installation mistakes costly.

    Repairing These Shower Valves

    Like older standard shower valves, pressure-balance shower valves have inner cartridges that wear out and eventually go bad. When it does, the cartridge may not mix the water evenly and you might get mostly hot or mostly cold water. Or, you may also get barely any water at all. These are all signs that you need to change out the cartridge. Once a new cartridge is installed in a pressure-balance shower valve, it should work like new. You can extend the life of the shower valve by turning it on regularly, even if that bathroom does not get any use. A valve that sits without use for too long is more likely to go bad that one that is used regularly.

    Thermostatic shower valves also have cartridges that wear out and need replacement, but they are also susceptible to problems originating with dirt or debris in the check valves that control the flow of water from the hot and cold inlets. If your thermostatic shower valve suddenly stops controlling the water temperature or water volume correctly, the first step is to inspect and clear the check valves, following the manufacturer's directions. If this doesn't solve the problem, then replace the cartridge.

    Note: When installing a new pressure-balanced or thermostatic shower valve, save the paperwork because it will make it easier to identify which replacement cartridge you need when the time comes to change it. Sometimes a quick call to the manufacturer can result in a free new cartridge, so keep the phone number handy.