Thermostatic and Pressure Balanced Valves
Thermostatic and pressure balanced mixing valves help to further regulate water temperatures in places like the bathroom shower or kitchen sink. These valves mix hot and cold water to ensure a near constant temperature and pressure of the water output from the fixture. Fluctuation from variations in both the temperature and pressure of the incoming water supplies is compensated for by this safety device.
- Consumer Safety Device - Plumbing
- Water Temperature Regulation
- Scald Prevention
Contents
Pressure Balanced Mixing Valve
A pressure balance shower valve addresses temperature fluctuation only due to inlet pressure changes. The internal mechanism is designed to maintain water temperature despite pressure fluctuation in the hot or cold supply. As an example, when someone flushes the toilet it will not cause the water temperature in the shower to suddenly get too hot. The pressure balance valve does not sense the actual temperature of the water, just the ratio of hot and cold water supplied to the valve.
In consideration of the pressure balanced mixing valve in a home shower fixture, there is a cartridge in the single lever design. Once the lever is removed the cartridge is located inside where the hot and cold supply lines converge. Adjusting the shower water temperature is accomplished by changing the Rotational Limit Stop (RSL) position which limits the shower handle range of movement. This is a mechanical piece located between the handle and the inner cartridge. The Rotational Stop Limit (RLS) sets how far the shower handle can be moved toward the hot or cold side. The mixing cartridge is behind the RLS, and it is in the mixing cartridge that the pressures are regulated.
Hard water and mineral deposits as well as rust from aging galvanized pipes commonly causes the mixing valve in the cartridge to fail prematurely. This is because the inner device within the cartridge is mechanical and constantly moving back and forth while you are showering. The hot and cold water enters from both sides via the supply lines and is mixed in the cartridge cap then flows out the base to the outlet in back of the shower valve body. Rubber flapper type backflow prevention valves are necessary for the automatic pressure balance valve inside the lower housing to operate.
Variations in design have resulted from pressure valve evolution. In the early design a balancing disk was in the center (where the cartridge is on the modern design) and the disc would slide back and forth on a piston. It moves very quickly grabbing a little hot from one line and a little cold from the other, constantly mixing it and doing so quickly enough that you feel a constant temperature.
Disc piston as a design was replaced by a spool type pressure balance valve. In the spool design the spool itself slides towards the supply with the lower pressure. This cuts off the volume of water entering the valve on the high pressure side and allows more water from the low pressure side. Output water temp cannot vary more than three degrees. There tends to be much less mechanical motion in this design.
Thermostatic Mixing Valve
A thermostatic control valve prevents high output temperature regardless of input and outlet variations and demand variations. This type of valve can be installed directly on the hot water heater. There are also exists a "local to the fixture" type, such as directly at the shower or sink. High end shower controls now use a thermostatic control valve design.
When installed at the fixture, such as in the shower, the thermostatic valve will have two handles. Of the two handles one controls the mix of hot and cold, and the other controls the volume of water passing through the valve. The water volume control is also how the water is turned completely off. The mixing valve in the thermostatic valve senses the temperature of the water, and constantly adjusts the mix to maintain the temperature selected. It is not necessary to turn the mix handle to turn off or on the water. Once the bather selects a temperature, the valve will remain at that temperature again when the water is turned on. You leave that control set at the temperature you like, and unless another family member moves it, the temperature will be the same next time you shower.
Shutting off the water as a quick response to cold or hot pressure loss is another function of the thermostatic mixing valve. They often use a wax thermostat for regulation. If pressure suddenly drops on the cold water supply line, hot water will be shut off at the valve preventing the bather from getting burned.
- Hot Water Heater Control by Thermostatic Mixing Valve - These provide a uniform distribution temperature for all hot water outlets in a household.
- Fixture design by use of a local Thermostatic Mixing Valve - These are single Outlet Thermostatic Mixing Valves, often called "thermostatic faucets", "thermostat taps" or "thermostat valves". They are designed for individual showers, hand wash basin mixers, bath or tub fillers.
When a mixing valve is installed on the hot water heater, it will open to mix in a little cold when the temperate of the hot water exceeds a certain threshold. The homeowner can inadvertently have a hot water heater temperature set higher than this threshold. The valve then is constantly allowing some cold water to flow into the hot water line to compensate when there is water flow movement. This results in wasted energy. The homeowner is paying for water to be heated higher than what is necessary while the mixing valve is removing the excess heat after water exits from the hot water heater to the fixture.
Criticism
These devices are not without their pitfalls. One major concern is the increase in overall expense to the homeowner. There are many homes where the hot water heater temperature is not set high enough to cause dangerous scalding. It is not necessary to have a hot water heater with a setting above 120 degrees, and it is actually quite sensible to have it set at 110 degrees. This saves the homeowner money. It is a myth that showering or washing dishes at higher temperatures within the limitations of what human skin can tolerate will be any more effective at killing bacteria as opposed to water at a safe lower temperature.
The most common regulatory standard for the maximum temperature of water delivered by residential water heaters to the tap is 120 degrees Fahrenheit/48 degrees Celsius. At this temperature, the skin of adults requires an average of five minutes of exposure for a full thickness burn to occur. Most adults will suffer third-degree burns if exposed to 150 degree water for two seconds. If a homeowner has a commonsense temperature setting on the hot water heater, then a special mixing valve is not necessary at the hot water heater or shower. Keep it turned down, you don't need the water near boiling when it comes out of the tap!
Another problem with the mixing valves is an increased failure rate due to impurities in many urban or rural water systems. High calcium content in water causes the moving mechanism in the pressure balance mixing system to fail, leaving the homeowner without water pressure in the shower or other fixture. Frequent disassembly, cleaning, and possible replacement is necessary in some areas where hard water is unavoidable. Some homeowners have had to install a home water softener or filtration system to help reduce failure of these mixing valves.
- Increased fixture cost
- Higher failure rate
- Energy waste
- Other added expenses such as a water softener
This is another example where the government wants to be our parent in a nanny state nation. The intelligent homeowner is not going to become injured by scalding if commonsense is used. However, because some dumb citizens might get hurt, the government is going to step in and force everyone by building code to update to more expensive bathroom fixtures or water heater safety devices and make us all pay more as well as over-complicate something that is frustrating enough, home plumbing.
