Speed in Stabilization Stops Scalding


Everyone has an opinion about scalding from sink, shower and bathtub fixtures. While opinions vary as to how to best mitigate the risk of scalding, in the end the object of the game is to effect change—to stabilize temperatures—as quickly as possible following a pressure disturbance.

The potential for scalding accidents will only intensify as the baby-boom generation ages, because scalding incidents affect the elderly as much or more than any other age group. So, finding the requisite safeguards is critical. The key is to make certain that the total system is working as an integrated unit. This can be done by considering your overall system design and not simply each valve separately. Thankfully, the American Society of Sanitary Engineers (ASSE) is in the process of finalizing a comprehensive, integrated set of standards that will segment the various products by type and position within the overall system design, as well as providing guidance about specific applications. Figure 1 identifies and contrasts the various types of ASSE-compliant valves that may be required in differing applications throughout a single system.

Figure 1

The hot water heater in this example is set at the maximum temperature required, which will be 140°F. Since water this hot is generally only required for kitchen and laundry needs, temperatures must be stepped down and limited for other applications throughout the rest of the system as a safety measure. Each of the downstream applications has its own unique requirements and limitations, with the overall first step-down coming after the kitchen/laundry portion of the system through the use of an ASSE 1017 device located near the hot water source. From that point, specific devices control the outlet temperature of each application, with the degree of the potential hazard at each outlet determining which in-line or point-of-use device is needed.

Now that we have established an understanding of the various ASSE devices, it is helpful to review the differences between thermostatic and pressure balancing valves used to minimize scald potential in applications calling for an ASSE 1016 device. ASSE 1016 devices for tubs and showers, particularly those for shower installations, require the ability to have bathers protected from a sudden temperature change resulting from a disturbance in the cold water supply pressure. This is because sudden increases in outlet water temperatures up to the maximum available hot water temperature can easily result in scalding and/or slip and fall accidents as bathers attempt to flee shower water. For this type of application, both pressure balancing and thermostatic control devices have unique strong points that should be considered.

To illustrate, consider the following test: We measured the water temperature changes in a test shower, resulting from a sudden pressure disturbance similar to an adjacent toilet flushing. These tests gauged the performance of a variety of different safety and non-safety shower valves throughout a simulated flush cycle. The showerhead used was a commonly used model, as was the standard water closet flush valve. Structured in this manner, the test sequences were actually even more realistic than the standard ASSE 1016 test protocol.

Controlled criteria for all tests were as follows:

  • The hot water supply temperature was maintained at 140°F.

  • The cold water supply temperature was at 50°F.

  • Static water pressure, with no load, was 58 psi.

  • The average shower temperature was set manually at about 100°F.

Our first test (Test 1, below) illustrated the water temperature change measured at the showerhead when a non-safety shower valve is used. This test used a conventional, single-handle non-safety shower valve with no flow restrictor. As indicated in Test 1 the shower outlet water temperature spiked to over 130°F when the toilet was flushed. A bather in the shower would have been scalded in a matter of seconds.

In the same test sequence, the use of a 2.5-gpm flow restrictor resulted in a similar rapid increase in temperature, with the sustained effects of the temperature increase lasting much longer. The addition of the flow restrictor still resulted in a temperature spike and a potentially dangerous scald and/or slip/fall situation. It also resulted in the mixed water temperature remaining higher for a longer period of time after the pressure disturbance ended.

Next, in Test 2 (right) we compared how well a thermostatically controlled shower valve handled the same pressure disturbance. When we compared the flow-restricted, non-safety valve to a thermostatically controlled shower valve at full flow (+6 gpm), the temperature and time curves were as indicated in Test 2 (below). Even though it was better than a non-safety valve, the thermostatic control valve still showed a significant temperature increase to over 110°F. This result was well above the nominal pain threshold of about 106°F, which means the bather would have recoiled from the heat, possibly slipping or falling in the process. Note that this test was done without a flow restrictor, which provides the thermostatically controlled valve with its best chance to perform. That's because a thermostatic control valve works by sensing the temperature of the water passing over its internal sensor, commonly called a thermal motor. It's a "read and react" process, so the more water passing over the sensor, the quicker it will react to changes. Conversely, the slower the water flow, the slower the reaction.

Next, we tested how the addition of a flow restrictor would impact the thermostatically controlled valve's performance. Test 3 (below) shows the time and outlet temperature reaction of the thermostatic valve, this time with a federally mandated 2.5-gpm flow restrictor added to that same scenario—a sudden loss of cold water pressure simulating an adjacent toilet flushing. The reduced flow caused by the flow restrictor resulted in a maximum outlet temperature of over 120°F before compensation for the scalding hot, slow-moving water occurred.

The pioneers of safety valve development realized the first thing that changes in these scenarios is the valve's inlet water pressure. In all of these tests, a toilet is flushed and inlet water pressure to the cold side of the shower valve drops. The outlet water temperature changes occur because of the inlet water pressure changes. So, responding to outlet water temperature changes with a thermostatic control valve requires water movement over the thermal motor and time. The more movement, the less time and vice versa. Most importantly, during that time, the bather could wind up being doused with very hot water! The most logical choice for controlling this type of scald scenario, especially as the distance between the control valve and the hot water source increases, is to install a pressure balancing mixing valve. A pressure-balancing valve balances either side to the lowest pressure being supplied to the valve and it does it much quicker than a thermostatic control valve.

We ran the same pressure disturbance test again, but this time using a pressure balancing safety valve with a flow restrictor. The outcome was dramatically different. As indicated for Test 4 (below), there is almost no measurable temperature change at the showerhead. That's because the operating dynamics of the pressure-balancing valve provides an instant response. The instant a pressure disturbance occurs, the pressure-balancing valve adjusts, providing the predictable, constant outlet temperature throughout the test sequence.

Proponents of thermostatic control valves argue that pressure-balancing valves require proper adjustment to their limit-stop mechanisms, which is true. Maximum temperatures need to be checked and set upon installation, which is a simple task not unlike setting a thermostatically controlled valve. The manufacturer's pre-set temperatures on thermostatically con-trolled valves must be precisely checked and calibrated during installation as well. Competent installers double-check the temperature settings and limit-stop mechanisms before they finish a job, regardless of the type of control valve used.

Likewise, seasonal differences in cold water supply temperatures can occur, resulting in either type of valve needing to be recalibrated if an exact maximum high-limit temperature is to be maintained.

Some people would argue that for every degree of increase in the ambient cold-water temperature being supplied to a pressure-balancing valve there is an equal amount of increase in the mixed water outlet temperature. Because the hot water is a relative constant and the average mix is about two-thirds hot water to one-third cold, the increase in the mixed water outlet temperature is thereby mitigated. However, comparing ambient cold-water temperatures in Arizona in the summer to the cold-water temperature in the upper peninsula of Michigan in the winter is a distortion of reality.

As stated earlier, each type of anti-scald valve has its strengths and weaknesses. For applications at or very near to the hot water source—where flows are typically higher—thermostatically controlled valves are appropriate and should be specified. However, in point-of-use shower or tub/shower applications, pressure-balancing valves are the most effective, safest choice.

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