This month, we continue with the last part of the checklist for commissioning and troubleshooting domestic hot water systems. We discuss hot water temperature maintenance cables, designing a DHW distribution system to control Legionella, and flushing and disinfecting domestic hot and cold water systems. 

Hot Water Temperature Maintenance Cables 

The International Plumbing Code (IPC) requires hot water (more than 110 F) to be maintained within 50 feet of the plumbing fixtures; the plumbing (Part VII) and mechanical (Part V) of the International Residential Code stipulates within 100 feet of plumbing fixtures. This is to ensure that hot water will be delivered to and flowing from a faucet or fixture fitting within a reasonable amount of time. 

This distance limitation also limits the amount of previously heated hot water dumped down the drain before hot water arrives at a given fixture. The Uniform Plumbing Code does not address this. 

According to the IPC, hot water temperatures can be maintained near a fixture in two ways: by circulating hot water through a hot water main and branch piping to within 50 feet of the fixture, or by installing hot water temperature maintenance cables on the hot water pipe to within 50 feet of the fixtures. 

When hot water temperature maintenance cables are installed, the following items should be checked when commissioning them: 

 • Heating cable temperature. Verify the temperature range or watts per linear foot of the heating cable. Manufacturers offer different types of cables and control methods: self-regulating, power-limiting, parallel constant watt or series-resistance. These all affect the system design and watts per linear foot for different temperature ranges to maintain.

• Ground-fault protection. Verify the cables are connected to a ground-fault-protected electrical circuit. Many years ago, I witnessed a fire caused by a heating cable that was cut with an insulation knife when it was installed. This caused an arc and eventually led to an electrical fire in a storage room full of legal papers. The heating cable, in that case, was not ground-fault-protected.

• Electrical circuits for heating cables. Make sure there are dedicated electrical circuits for each section of pipe to be covered with electric heating cables. There is a maximum distance or watts per foot with a limitation to the length of each pipe to maintain various temperatures. This may require multiple electrical circuits for one run of piping.

 • Insulation thickness. Verify that the insulation type and thickness required by the heating cable manufacturers is installed. The manufacturer’s insulation thickness or insulation value may be higher than what is specified by the engineer. The specifications for standard insulation values may not require thick enough insulation to allow the heating cable to maintain a given temperature. 

 • Signage on the outer jacket. “Electric Heat Tracing” labels should be spaced no greater than 10 feet apart or as directed by the manufacturer, the code for the jurisdiction or the construction specifications, whichever is more stringent. 

• Electrical resistance integrity. After a heat tracing circuit has been installed and fabricated but before the thermal insulation is installed, the heating cable should be tested to ensure electrical resistance integrity. The cable should be tested with a 500 Vdc megohm meter (megger) between the bus wires and the metallic braid. Test the polymer-coated heating cables per the manufacturer’s instructions and local code requirements. 

• Larger-diameter pipe. Check the heating cable manufacturer’s installation instructions for larger-diameter pipe to see if more than one cable is required when wrapping the cables. 

Hot Water Circulated Closer to Fixtures

To save water, reduce the time it takes to get hot water from a fixture and reduce the amount of previously heated hot water dumped down the drain, innovative new plumbing designs are on the market by several companies: 

1. Kemper Industries (Kemper hygiene systems/flow splitter fittings): sampling valves, flushing valves, flow-splitters tees, cool-flow cold water circulation systems, flushing boxes with automated flushing of ends of mains and branch mains, temperature sensors, flush points, control systems and flushing components (temperature/pressure/flow-metering).

2. Viega Water Management Solutions: double-drop elbows and smart loop system components.

3. GF Piping Systems (Hycleen Automation Systems): Continuous automatic balancing of the hot water system; maintaining water temperatures outside of Legionella growth range; automating dead-leg flushing based on time, flow, pressure and temperature to maximize energy and water efficiency; continuously maintaining water quality and temperature; and controlling comfort and safety at fixtures by accurately controlling distribution temperatures.

Each manufacturer has provided safe and hygienic hot and cold water distribution systems for many years. These systems recognize the need to minimize dead legs by circulating hot and cold water systems as near to the fixtures as possible — the design of the future. 

These systems minimize wasted water and reduce the time wasted waiting for hot water to arrive at the fixture. There is an installation cost increase for a completely automated water distribution system. However, the water quality will be better, and the chances of scalding will be reduced with fully automated controls.

Design and Setup of a DHW Distribution System to Control Legionella 

Use these tips to design and set up a domestic hot water system to simultaneously protect against Legionella bacteria growth and scalding:

• Storage-type water heaters. Hot water should be stored at a minimum temperature of 140 F and distributed at no less than 122 F at the coolest point in the system, which is the end of the hot water recirculation pipe. The industry has no full agreement on the minimum temperature to control Legionella bacteria growth. This is because laboratory tests show that Legionella will grow and reproduce up to 122 F. 

• Tankless electric or gas-fired water heaters. These water heaters generally operate within the Legionella bacteria growth temperature range of 68 F to 122 F. Consideration for chemical disinfection should be given for these types of water heaters.

• Instantaneous water heaters. These water heaters are generally supplied by steam, heating hot water or sometimes high-input gas burners or high KW input electrical water heaters. They can have scalding temperatures under certain conditions; it is a good design practice to add a temperature-actuated mixing valve conforming to ASSE/ANSI 1017 in a hot water distribution pipe after this type of water heater. 

This avoids temperature spikes in the hot water distribution system when there are varying flows.

Legionella Growth at Higher Temperatures 

There have been reports that live Legionella bacteria was found in a piping system after raising hot water temperatures to 150 F briefly. This is possible because Legionella bacteria can go into a cyst form, encapsulating the bacteria. It can be inside an amoeba and deep within layers of calcium and mineral buildup on a pipe wall covered with a biofilm. 

Each layer provides a level of heat insulation that allows bacteria time to survive short-term emergency thermal disinfection. The biofilm, calcium and minerals on the piping system’s wall, the amoeba and the cyst encapsulation can insulate Legionella bacteria from hot water temperatures when it is deep within a pipe wall’s scale and biofilm. 

Short-term thermal disinfection in piping systems with significant biofilm and scale buildup may not effectively eradicate Legionella bacteria if the heat does not penetrate through all layers. Some reports note that after an emergency disinfection of a piping system with significant biofilm and scale, the Legionella bacteria can return in even greater numbers within weeks. 

A thermal disinfection or short-term chemical disinfection may kill bacteria on the surface of the biofilm, but it does not kill bacteria under the biofilm or remove the biofilm. After a short disinfection, the dead biofilm becomes a buffet for bacteria that survive the disinfection or new bacteria entering the distribution system from the public water supply. 

With the right growth-temperature conditions, Legionella bacteria can grow back in even greater numbers within a few weeks if the water management plan does not have proper control measures. This is why we should store hot water at a minimum of 140 F and distribute the hot water so that the return temperature is above the bacterial growth temperature range. 

After the water heater(s), a temperature-actuated mixing valve conforming to ASSE/ANSI 1017, Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems, should be installed to ensure a constant temperature of hot water is delivered to the hot water distribution system. At the fixtures, point-of-use hot water temperature-limiting controls should be used to provide safe temperatures at fixtures. 

Showers and tub-shower valves should use code-compliant valves conforming to the industry standard ASSE 1016/ASME A112.1016/CSA B125.16, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations (reaffirmed in 2021). These shower and tub-shower valves open to cold water first and slowly add hot water. 

They have a maximum temperature limit-stop adjustment restraining the amount of hot water mixed with cold water to limit the maximum water temperature delivered from the fixtures. In addition, these valves compensate for thermal shock, a sudden temperature change caused by pressure changes or changes in hot or cold water delivery temperatures. 

Lavatories, bathtubs, shampoo bowls and pedicure sinks are required by the code to use an ASSE 1070 temperature-actuated, temperature-limiting valve that reduces the maximum temperature delivered from the fixture to a safe temperature. 

Flushing and Disinfecting Domestic Hot and Cold Water Systems

Flushing and disinfecting cold and hot water systems should be done for new buildings within two weeks of significant occupancy and after a building has been shut down for more than two weeks, where water sits stagnant in the pipe. 

This is because water treatment chemicals can dissipate to levels that will not control bacteria growth in about five to 10 days, depending on temperature, pipe materials, water quality, chemical type and chemical residual level. If a pipe has been shut down for more than two weeks, there is a possibility of increased bacteria levels based on the conditions present. 

Caution should be taken when performing emergency flushing and disinfection operations because many new piping systems are damaged. The commonly referenced flushing and disinfection standard, AWWA C651, is for flushing and disinfecting water mains, not building water systems. Chlorine, monochloramines and chlorine dioxide are oxidizing chemicals that can damage pipe materials commonly used in building water systems.

The model plumbing codes and the AWWA C651 standard list disinfection procedures call for 50 parts/million for 24 hours or 200 parts/million for three hours. These are the minimum chlorine levels and minimum contact times, but the standard and the code give no maximum chlorine levels. 

Because of this oversight, many plastic and metal piping systems have been damaged or destroyed by excessive water treatment chemical levels during water system disinfection. 

The water management team, contractor and engineer should evaluate the disinfection water treatment chemical along with the material of the piping, valves, fittings, equipment and appliances. They need to verify the maximum parts per million levels and the maximum contact time for each chemical at various chemical levels to ensure the piping system will not be damaged.