Last month, we covered the deadlines for code change proposals for the 2027 International Plumbing Code and the 2027 Uniform Plumbing Code, which will have expired when you get this magazine. 

We also began covering how to commission and troubleshoot domestic hot water (DHW) systems. We discussed the importance of design, installation and maintenance of DHW systems. And we began reviewing a checklist for inspection, troubleshooting and maintenance of these systems that started with water heater types and sizing considerations, storage tanks, circulating pumps, aquastats, timers and documenting system temperatures. 

This month, we will continue with Part 2, covering temperature controls. 

A. DHW Temperature Controls

Domestic hot water accounts for more than 25% of all scald burns in children. Children and the elderly typically have significantly thinner skin than adult males and can be burned in less time than the times and temperatures recorded in the Moritz & Henriques burn studies at Harvard Medical College. 

The burn studies were conducted on adult males with up to second-degree burns and baby pigs, with approximately the same skin thickness as adult males. Children, the elderly and the physically handicapped are at increased risk of scald burns because their skin is thinner, and they may not have the ability to respond fast enough to prevent severe burn injuries when water temperatures are more than 120 F. 

The Moritz & Henriques research shows that temperatures at or below 110 F nearly eliminate all scald risk. So, based upon this information, when setting maximum temperature limit stops for a shower or tub-shower, if you set them to a maximum of 110 F, you can practically eliminate the risk of scalding. 

A temperature-actuated mixing valve (TMV) should be installed upstream with a high-temperature alarm designed to alert building occupants and the owner if the distribution temperature is inadvertently adjusted to a dangerously high temperature, requiring readjustment of all the maximum temperature limit stops. 

Many scald injuries are associated with the old-style, noncode-compliant, two-handle tub-shower valves. Eliminating thermal shock and scald injuries was the driving force that brought about the development of various temperature control valves for showers and temperature-actuated mixing valves for distribution systems. 

Each type of temperature control valve or device is covered by an American Society of Sanitary Engineering (ASSE) product performance standard, which lists the specific applications and tests based on the temperature ranges and the conditions where they are intended to be used. 

In some cases, the authority having jurisdiction has the final approval of all installations. However, one must be aware of the applications listed for each valve in the standard and the manufacturer’s installation requirements, which may exceed the prevailing code requirements and the knowledge level of the inspector. 

There have been misapplications of mixing valves where, for example, an untrained individual installed an ASSE 1017 TMV in a combination emergency shower and eyewash application. The ASSE 1017 valve is not intended for this application. When only an eyewash is used, the flow rate can be so low that some styles of mixing valves cannot accurately control the temperature, and the valve does not bypass cold water in a failure mode or an emergency condition. 

It seems simple, but there are valves designed for unique applications. The ASSE product performance standards and applications for each temperature control device are listed below: 

1. ASSE 1016/ASME A112.1016/CSA B125.16, Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations 

(For individual, shower and tub-shower applications.)

2. ASSE 1017 or CSA B125.3, Temperature-Actuated Mixing Valves for Hot Water Distribution Systems 

(For distribution system temperature control at or near the hot water source. Generally, there are three types of temperature-actuated control valves: bi-metal coil, liquid paraffin wax and digital. ASSE is looking into developing a new standard for digital mixing technologies so they can also test all the valve’s electrical motors and electronic control components.)

3. ASSE 1062, Temperature-Actuated Flow Reduction (TAFR) Valves for Individual Fixture Fittings 

(Supplemental thermostatic device to shut down or reduce flow to a trickle when temperatures from a fixture fitting exceed 117 F to 120 F. Scald protection is only at showerheads, faucet outlets, and tub-filler outlet applications.)

4. ASSE 1066, Individual Pressure Balancing In-Line Valves for Individual Fixture Fittings 

(In-line pressure-balancing device in hot and cold branch to fixtures to protect from thermal shock only.)

5. ASSE 1069, Automatic Temperature Control Mixing Valves 

(This valve has tight temperature control tolerances for use near gang shower rooms with a single temperature water supply application.)

6. ASSE 1070/ASME A112.1070/CSA B125.70, Water Temperature-Limiting Devices 

(Temperature-operated mixing valve with a temperature-limiting feature for individual fixtures or groups of fixtures near the point of use.)

7. ASSE 1071, Performance Requirements for Temperature-Actuated Mixing Valves for Plumbed Emergency Equipment 

(These valves are intended for controlling tepid water temperatures for plumbed emergency equipment. They are designed to bypass cold water in the event of a valve failure.)

B. TMV for Supplying the Hot Water Distribution System 

TMVs conforming to the temperature control valve standard ASSE 1017 or the Canadian Standards Association (CSA) B125.3 have also been referred to as master mixing valves, but their purpose is to be located at the beginning of the hot water distribution piping system near the hot water source (water heater) to provide a stable hot water distribution temperature. 

There are several reasons for providing a stable hot water distribution temperature. Hot water distribution systems with a TMV can: 

• Prevent excessive temperatures that contribute to scalding. 

• Prevent excessive temperatures that contribute to erosion in copper pipe above the recommended velocities published in the Copper Development Institute Handbook. 

• Prevent excessive temperatures that lower or reduce the pressure rating of various types of plastic pipe. Many plastic piping systems soften at higher temperatures, so their pressure rating goes down as the temperature goes up. Excessive temperatures have led to piping failures and expensive property damage when the pressure and temperature limits are exceeded. Check with the piping manufacturer for pressure-temperature relationships. 

• Prevent system temperatures dropping into a range that promotes Legionella bacteria and other microorganism growth.

• Maintain a stable hot water distribution system temperature above the Legionella bacteria growth temperature range and without temperature fluctuations that make setting maximum temperature limit stops difficult. Then, with stable system distribution temperatures, it allows setting the maximum temperature limit stops on shower and tub-shower valves at a safe temperature to prevent scalding. 

The code says a 120 F maximum, but I recommend 110 F max. When there is a cast-iron bathtub, the temperature may need to be higher in order to get a hot bath. 

• Provide a system design that will simultaneously protect against Legionella bacteria growth in the storage and distribution piping above 124 F and provide scald protection: about 110 F from the showers/tub/showers, 102 F or 110 F max from bidets, and 115 F or 120 F max from bathtubs and whirlpool bathtubs. This design will protect the piping from extreme thermal damage and occupants from scalding.

C. Point-Of-Use DHW System Temperature Controls 

At the point of use, there should be temperature controls as required by the local code and appropriate for the application: showers and bathtub-shower combination fixtures should have temperature control valves that conform to ASSE 1016/CSA B125-16/ASME A112.18.1-1016. 

Other point-of-use temperature controls include ASSE 1062, TAFR devices in fixture fitting outlet; ASSE 1066, in-line pressure balancing device; ASSE 1069, gang shower mixing valves; ASSE 1070, water temperature-limiting devices; and ASSE 1071, emergency fixture tempering valves.

D. Tankless Water Heater Controls

If tankless water heaters are used, they typically have a lower delivery and temperature setpoint. This is so they can have a higher-rated flow because they must heat water instantly. This lower temperature rise means they generally do not get hot enough to kill bacteria during normal operation. 

Some models have a high-temperature cycle; an overnight cycle raises the temperature enough to disinfect the tankless heater. However, most of those tankless water heaters are not designed to disinfect the biofilm in the piping system unless designed to continuously circulate during the disinfection process for the required contact time at the end of the hot water return system with a disinfecting hot water return temperature. 

Before any thermal disinfection process is done, the engineer or contractor should survey the piping materials in the system to see if they can reach the proposed disinfection temperatures. If a system has a 20-degree temperature drop from the water heater outlet through the circulated distribution pipes and back to the tankless heater, to get 158 F in the return pipe, the supply temperature would need to be 20 degrees higher or 178 F. 

This would generally be higher than the maximum pressure rating for many plastic piping materials at higher pressures. If the hot water system is circulated, the velocity of the circulated hot water may need to be less than 2 feet/second. Based on these facts, thermal eradication is not always the best method of controlling bacteria. 

Most tankless water heater models will have a temperature fall-off as the flow through the tankless heater rises above their rated flow rate and energy input capability, so the system temperatures in a tankless water heater system are generally always maintained in the Legionella bacteria growth temperature range and drop to ideal Legionella bacteria growth temperatures, increasing the risk of a Legionnaires’ Disease outbreak. 

One myth that has been busted for many years is that tank-type water heaters cause Legionella bacteria growth. Stagnant water in the ideal temperature range with ideal water chemistry creates a condition where Legionella bacteria will grow anywhere. The bacteria does not care if it is in a tank, a pipe, a valve or a piece of equipment such as a tankless water heater. If conditions are ideal for growth and bacteria are present, it will grow. 

When tank-type or tankless water heaters are set to 120 F or any temperature below 122 F — the upper limit for Legionella bacteria growth — the bacteria can and typically will grow. Setting the hot water distribution temperature that low causes most of the water to be delivered to a tub-shower through the hot water piping, creating high velocities in the hot water piping and pressure differences between hot and cold water systems. 

In addition, when the hot water distribution temperatures are set so low, the maximum temperature limit stops on all shower and tub-shower valves will need to be wide open. This creates a condition where if the system temperature is ever turned up at the water heater or TMV serving the hot water distribution system, the maximum temperature limit stops are rendered useless and scald conditions will be present. 

This is one of the reasons why ASHRAE has recommended minimum hot water storage and distribution temperature of 140 F and a minimum hot water return temperature above the Legionella bacteria growth temperature of 122 F. Many engineers use a safety factor of a couple of degrees and require 124 F to 125 F as a minimum temperature on the hot water return pipe before connecting to the cold water inlet to the water heater and the cold water inlet or hot water return connection on a TMV station. 

Many tankless water heaters require bypassing temperature controls in their control panel to raise the distribution system above 120 F. 

E. Setting the Maximum Temperature-Limit Stop on a Shower or Tub-Shower Valve

When the domestic hot water system temperature changes for any reason, the maximum temperature limit stops on all shower valves and tub-shower valves on the DHW system need to be readjusted. To readjust all the ASSE 1016 shower and tub-shower valves, they must flow until the maximum hot water temperature is achieved. Then, the limit stop must be set to reduce the hot water temperature to a safe level below 120 F. 

The code maximum is 120 F; this temperature was chosen because, in older cities in the north, where a cold cast-iron bathtub is filled with 120 F hot water, the bathwater temperature is much lower after the heat is drawn out of the water by the cast-iron tub. Using a TMV on the hot water distribution system allows a stable hot water delivery temperature for shower and tub-shower valves with code-compliant maximum temperature limit stops. 

Any bathtub or shower with an old, two-handled filler faucet should be removed and replaced when a new water heater is installed because all codes have a requirement to “not leave a hot water system in a dangerous or hazardous condition.” Working on a hot water system cannot be done if you leave the system unsafe with the potential for scalding or thermal shock. A code-compliant shower or tub-shower valve must be installed when the water heater is replaced or if major renovations are done to the plumbing system. 

There is no grandfather clause that allows an unsafe condition to continue. The grandfather clause only applies to systems where work is done, and the existing nonconforming work can remain as long as there is no hazard present. In all cases, a two-handled tub-shower valve presents an unsafe condition that can injure or kill someone from a thermal shock event that causes a slip and fall or from a scald event that can occur when the cold water pressure drops because of cold water use elsewhere in the building. 

Next month, we’ll continue discussing temperature control valves, with a focus on their use in a DHW system.