We've touched on it before: showers are important. Like... really important. Not only in keeping us acceptable in the eyes (and noses) of our peers, but in maintaining our own sense of well-being and confidence. Beyond hygiene, showers offer us respite, peace, and luxury. But the utter perfection of a good shower can all too easily be ruined by something as silly as someone starting a load of laundry, or flushing a toilet. These incidents can range from annoying to highly dangerous: extremely cold or hot water can make us jump, panic, and scramble. In the chaos, it's entirely possible to slip and fall. With hot enough water, serious scalding can occur in seconds.
Thermal shock and scalding can happen very easily. Safeguarding against them is one of the most important things you can do for your household, especially if that includes young children or elderly adults. The most effective way to do so is by installing anti-scald devices at fixtures, especially the shower. The two primary types are pressure-balance valves and thermostatic valves.
The chief distinction between the two? A thermostatic valve senses and controls the actual water temperature, while a pressure-balance valve senses and controls only the ratio of hot water to cold. Which works best? As always, it depends on preference and budget.
How do these shower valves work?
Pressure Balance Valves
A pressure-balance valve features just one handle controlling both volume and temperature, as well as a dial or set-screw that sets the stop-point for the handle (the maximum ratio of hot to cold). Pressure is balanced by way of either a sliding disc on a piston, or a spool, that react to changes and maintain the pressure ratio. When some cruel soul flushes the toilet while you're busy rinsing, cold water is sent to the toilet tank, reducing the cold water pressure arriving at the shower valve. The mechanism inside the pressure-balance valve will move to reduce (or cut off) the hot water, maintaining the balance between hot and cold flows: temperature should not fluctuate more than a few degrees. Always keep in mind that a pressure-balance valve doesn't pay attention to temperature, so with the valve set at maximum temperature, and the handle turned all the way to "hot", the water coming out of the shower will be as hot as the water heater has to offer (which could be even higher than the setting on the thermostat).
Thermostatic valves have two handles: one controlling the volume, one for the water temperature. This is because thermostatic valves react to the temperature, not the pressure of the water. With this valve, you can change the flow volume without affecting the temperature, which makes conservation easy: just turn down the flow while shampooing or shaving! This is all possible due to a wax element inside the valve that expands or contracts in reaction to heat. When water exceeds the maximum set temperature, the element expands to reduce the flow of hot water, and allow more cold into the mix. Should either hot or cold supply fail, the valve will close off flow from the other side. The advantage with thermostatic valves is the direct control over output temperature; while your water heater can be set to 140° F to protect against bacterial contamination, the water coming out of your shower head can always be 100° F (a maximum output temperature is set on the valve).
Note: Older anti-scald valves may be designed for higher-flow fixtures (e.g., a 2.5gpm shower head); low-flow fixtures can still present a scald risk when used with these older, uncalibrated valves due to the relatively low flow through them (temperature/pressure changes are felt much more acutely). Make sure that yours is rated for the appropriate flow rate.
How can I choose the right valve?
Either valve presents clear benefits to those caring for children, the elderly, and those with sensory disabilities. Consider your "audience": if it's just you, a partner, and/or some housemates, a pressure-balance valve should present no difficulties. All users can easily adjust temperature and volume to their preference, and can understand how the one handle controls both. Plus, they're usually much cheaper than their thermostatic counterparts. But for those who may not have such awareness and precision, a thermostatic valve may be the better choice: with the max temperature set, even if the handle is inadvertently turned all the way around to 12-o'clock, the water temperature will not exceed the chosen limit. With a pressure-balance valve, although a maximum is set, cranking the handle can result in scalding temperatures if the water heater is set at a high temperature (remember that only the ratio of hot to cold is controlled by these valves). Also consider the environment: if the home is one with wild and frequent water pressure fluctuations, a pressure-balance valve may not perform at its best.
Do I really need a pressure balance or thermostatic shower valve?
Sometimes customers ask if they really need a pressure balance or thermostatic valve, why can't you simply turn down your water heater to avoid burns? While this is a valid question, it's based on a faulty premise that the only thing in your water heater is pure water. If you think you can stay a step ahead of the game and protect your family by turning down your water heater's thermostat, think again: tank heater designs allow for wide temperature variations, and changing the thermostat will do little-to-nothing for consistent outlet temperatures (referred to as the "stacking effect"). In fact, turning that heat down could place your home in a different kind of danger: the microbial kind.
Legionella bacteria (responsible for Legionnaire's Disease, a type of pneumonia) apparently love water heaters, and can proliferate at temperatures up to 120° F – exactly where many energy-conscious folks recommend setting the thermostat. For these and other bacteria, the game's over around 140° F. Since temperatures within the tank are stratified, there's a good chance that if Legionella were introduced, they would thrive in a tank set at 120° F; at 140° F, this is unlikely.
The best solution is to set the thermostat to the higher temperature, and install anti-scald valves at fixtures. A mixing valve to temper the extra-hot water at the heater outlet is also a good idea, especially if you have a number of fixtures (changing all those valves can get pricey). If only a mixing valve at the water heater is used, be aware that it will do nothing for swings in temperature: when that toilet is flushed, you'll likely lose cold water in the shower. However, provided the mixing valve has been set at a safe temperature, the hot water that hits you will not be a scalding risk (it will still, however, be super annoying).
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