There has been a lot of confusion about how to properly design, install and maintain a domestic hot water system to simultaneously control both scalding hazards and Legionella bacteria growth. Many people mistakenly believe that controlling hot water system temperatures is like a balancing act performed by simply adjusting the thermostat dial on the water heater to somewhere between scalding temperatures and Legionella bacteria growth temperatures. Unfortunately, there is no middle ground temperature. Any temperature that will minimize scalding in accordance with temperature limits in the model plumbing codes will be in the Legionella bacteria growth temperature range. The only solution to this dilemma is to use hot water temperature control valves to keep storage and distribution temperatures above the Legionella growth temperature range and reduce the temperatures at the fixtures to a safe temperature for bathing, showering and washing.

Water heater thermostat accuracy

Many people have made the mistake of assuming the thermostat dial on a water heater accurately controls the outlet temperature that flows from a water heater, but the thermostat does not accurately control the outlet temperature. Using the thermostat on a water heater to prevent scalding is prohibited by the model plumbing codes because the industry knows the thermostat dial on a storage-type water heater is not designed to accurately control hot water coming from a water heater. There are several types of water heaters on the market and the most common is the storage tank type. Other tankless or instantaneous water heater types have their own unique temperature control versus flow challenges that typically result in the use of temperature actuated control valves as part of the equipment, or a tempering valve is required as part of the system design, or the tankless heaters may not be capable of reaching a disinfecting temperature. The thermostat on a storage-tank-type water heater, however, is not designed to accurately control the outlet temperature because it is located near the bottom of the water heater to sense incoming cold-water temperatures and “turn-on” the energy to the heating element or the fuel to the burner.

The thermostats do not sense or control the hot water that accumulates at the top of a water heater. The water heater thermostats are designed in a way that causes an inherent delay in sensing hot water temperature because the heat must flow from the heated water through a boundary layer of water adhering to the thermostat, through the wall of the thermostat, through an air space in the thermostat, and then to a metal rod that expands and contracts to open and close contacts in the thermostat device.

I was told this heat transfer delay causes a lag time and overshoot temperature of about 11 to 15 degrees depending on the manufacturer. One water heater manufacturer’s representative told me the technicians in their factory had witnessed temperature lags as much as 18 degrees on some water heater models tested. This thermal delay results in a temperature chart that looks like a roller coaster. The water temperature at the thermostat element drops below the set point before the burner comes on and rises above the set point before it shuts off. This is why two different water temperature tests of a water heater at different times can yield vastly different results even though no adjustments have been made to the thermostat setting.

Stacking

In addition to the above mentioned thermal cycling, there can be stacking of hot water at times when there are intermittent short draws of hot water. Stacking occurs when cold water is drawn into the bottom of the water heater intermittently and causes the burner to cycle on even when the hot water at the top of the heater is well above the desired temperature setting. With each consecutive short draw of hot water and resulting burner cycle, the hot water temperature in the top of the water heater continues to rise and the outlet temperature can be as much as 30 degrees or more, above the thermostat set point. Therefore, the model plumbing codes prohibit the thermostat on a water heater to control the outlet temperature for purposes of scald prevention at fixtures.   

What are scalding temperatures?

The scald burn injury studies done many years ago at Harvard Medical School by Dr. Moritz and Dr. Henriques showed that it took approximately 5 to 8 minutes of exposure to temperatures in the range of 120 F for adults to develop a serious, irreversible scald burn. It should be noted that children and the elderly have skin that is thinner than the adult male and baby pig skin from the burn injury studies, and therefore they can develop burns sooner than 5 minutes. Because of these studies, recommendations by various code and standard committees use the 120 F temperature as the maximum allowable shower and bathing temperature for bathers to get out of harm’s way before a serious scald injury occurs. Code change proposals were submitted during the previous cycle to increase the temperature from 120 F to allow the thermostat on the water heater to be set at 130 F to address both Legionella bacteria growth and scalding concerns without requiring temperature actuated mixing valves in the hot water system. The problem is the code change would have significantly increased the chances of scalding and Legionellosis by operating in a temperature range that would not adequately address either hazard. The proposed code change was unanimously voted down at the model plumbing code hearings. The maximum temperature of 120 F has been established by numerous code and standard industry committees for decades based on the Moritz & Henriques burn studies as the maximum safe temperature to minimize scald injuries in showers, bathtubs and combination bathtub/showers.

Chemical and other forms of treatment for control of Legionella bacteria

Commons methods for controlling Legionella bacteria in both cold-water and hot-water distribution systems is by adding supplemental disinfectants to the water. This will maintain water treatment disinfectant chemical residuals at a level that is effective at controlling Legionella bacteria. The most common water treatment chemicals are Chlorine, Sodium Hypochlorite, Chlorine Dioxide, Monochloramines. Other treatment methods include: copper/silver ionization, Ozone and Ultra-Violet Light. These are not chemical treatments, but they are sometimes used in the right applications. If a facility is going to add chemicals or provide any additional physical treatment to treat the water onsite, they may need to register as a water treatment provider with the local jurisdiction. The levels of water treatment chemicals added to the water should be controlled to assure the water treatment chemical residuals do not exceed the safe drinking water act levels and to assure they do not harm the piping system. Many Chlorine-based chemicals can be very corrosive to piping systems if the chemical residuals are too high.

What are some emergency disinfection methods for hot water systems that are contaminated with Legionella bacteria?

Emergency disinfection methods are typically used when a plumbing system tests positive for Legionella bacteria or if a case of Legionellosis has been attributed to someone who was in or around the building and resulting tests of the plumbing system are positive.

Flushing

Flushing is simply flowing lots of water through all parts of the plumbing system to assure any stagnant areas of the plumbing system have been adequately flushed with fresh, treated water from the municipal water supply. It is important to verify that the municipal water supply has an adequate water treatment residual to control Legionella bacteria growth. Testing of the source water supply may be necessary as part of a building water management plan designed to control Legionella and other organisms that may grow and colonize in a building water distribution piping system.

High-temperature heat & flush

High temperature heat and flush is often done to kill Legionella and other bacterial organism growth in the hot-water distribution and circulation system. This method is often done improperly, and some plumbing systems are incapable of performing a heat and flush disinfection. If high temperature heat and flush is to be used, the minimum temperature of 158 F is required to thermally disinfect the entire plumbing system. The temperature loss in the system needs to be accounted for and this is not always done by novice maintenance personnel or inexperienced remediation professionals. Many failed thermal disinfections occur where the facility operators simply set the water heater thermostat to the disinfection temperature and do not take into account the temperature drop in the system.

There are new system components on the market that make it practically impossible to thermally disinfect the hot-water distribution systems because of temperature actuated balancing valves on the hot water return piping, as well as other thermal limiting or mixing devices in the hot-water distribution system.

In a hotel facility that I investigated, that had a prior Legionella problem, the building maintenance personnel decided to disinfect their system with hot water at 150 F.  Figure 1 (see below) shows with the water set at 150 F, the Legionella bacteria should be killed in just over 2 minutes. However, looking at the hot-water return temperature in this large facility, there was a 37-degree temperature drop during peak usage hours and a 42-degree temperature drop during off-peak hours. (The hot water distribution and recirculation system was poorly designed.) This created a condition where the hot water was leaving the water heater at 150 F and returning at 108 F during off-peak hours when the heat and flush operation was performed. The hot-water system was only capable of heating to 160 F because of the heating water heat exchanger. The system was incapable of heating the water in the entire system to a disinfecting temperature because of poor system design. When the temperature was dropped back down to the normal system operating temperature of 122 F, the return temperature would fluctuate between 80 F and 85 F based on the usage in the building. If 158 F is required in all parts of the plumbing system, and the hot-water circulation system is designed for a 20-degree temperature drop from the water heater outlet to the hot water return after the circulating pump, then the water heater outlet temperature should be 178 F. There are other ways to make sure a proper heat and flush can be performed for a building with somewhat lower water heating capabilities. Looking at the chart, Legionella begins to die at 131 F, but it takes 5 to 6 hours at 131 F. So, if the hot water return temperature is maintained for more than 6 hours and slightly above 131 F, the hot-water main circuit should be disinfected, however, the branches and dead legs will not be disinfected. A complete survey of the piping in the building should be performed to identify all branches and dead legs, and water would need to be flowed from each branch for a period of time sufficient to kill the bacteria in the biofilm. Remember, bacteria buried in a biofilm may be insulated from the water temperature flowing in the piping, so additional time flowing at a given temperature would be required to assure killing bacteria buried deep in biofilm and the underlying scale and calcium build-up on the pipe walls.

The flow rate through each branch should be for at least 25 percent more time than shown in the chart in Figure 1, in order to effectively kill bacteria within the scale and biofilm.

For example:

Water flowing from a fixture at 140 F requires about 32 minutes to provide a kill of all Legionella bacteria in laboratory test samples. Adding 50 percent to the 32 minutes means the water would need to flow for about 48 minutes at 140 F (140 degrees = 32 minutes from Figure 1 x 1.5 = 48 minutes).

Large scale flushing test example 140 F: If a large institutional facility has 1,200 fixtures and the water heater has a 20 gallon per minute instantaneous flow capacity, you could flow about 20 fixtures at a time, and they would need to each flow 1 gallon per minute for at least 20 minutes. The hot water system in the building would need to be raised up to assure the return temperature is 158 F, and at least 158 F or higher hot water temperature would need to be flowing from the flushing fixtures for the specified time period. This process could take well in excess of 60 hours while requiring numerous staff to supervise the flushing operations that will most likely take 2 days or so to complete. This would expose building occupants to scalding temperatures during this time period. Many water heaters will not have that kind of heating capacity and/or temperature capability. When planned out, the high-temperature heat and flush option does not work in many buildings because the shear scope and complexity of the high-temperature heat and flush operation can lend itself to cutting corners and cheating on manual disinfection test reports. Digital recording of temperatures with electronic mixing valves tend to eliminate this human factor from the equation.

Large scale flushing test example 151 F: The minimum recommended flushing temperature in the guideline is 158 F, however if flushing at a slightly lower temperature is done because of the temperature or flow limitations of a water heater, water flowing from a fixture at 151 F requires about 2 minutes to provide a kill of all Legionella bacteria in laboratory test samples. However, the minimum flow time from each fixture for thermal disinfection is discussed in ASHRAE Guideline 12, and the minimum required emergency, high temperature thermal flushing of fixture branches can be as much as 20 minutes at 158 F. As the temperatures go lower, the length of time for flushing needs to be longer. Using the method above, with 151 F water flowing from the fixture outlet and adding a percentage to the recommended flushing time of 20 minutes, it could take well over 30 minutes to kill Legionella bacteria embedded deep in a biofilm and scale build-up on the wall of a hot water piping system. The chart in Figure 1 (see above) shows a 2-minute kill, however, industry standards require a 20-minute minimum flushing at the disinfection temperature at 158 F.   

Hyper-chlorination

Hyper-chlorination, also known as chemical shock, is the use of chemical disinfectants such as chlorine or chlorine dioxide for a relatively brief period. (For example, 1 hour to 24 hours at concentrations above maximum levels permitted for potable water.) The objective of hyper-chlorination or chemical shock is remediation of hot and cold potable water systems colonized by Legionella. Typically, the chemical disinfectant is introduced upstream of the area to be treated and is distributed throughout the building potable water system, with sequential flushing of the disinfectant through every fixture for several minutes. Because chemical shock uses chemical concentrations above the maximum levels permitted for potable water, adequate precautions should be taken to prevent use of the water during the chemical shock period and to protect occupants that may be exposed to water with high chemical concentrations. You should notify the occupants of the building prior to implementation of the chemical shock process. Some building use patterns may allow chemical shock treatment to be scheduled when occupancy is low.

Corrosion and other damage can occur to plumbing system components and piping with increases in disinfectant concentration, time, temperature and the frequency with which chemical shock or hyper-chlorination is performed.

Component manufacturers often provide the maximum chemical concentrations to which their products can be exposed. Treatment with higher concentrations may cause damage and invalidate manufacturers’ warranties. High chemical concentrations and durations can damage the plumbing system, and with increased oxidation or corrosion of the plumbing system, it can make future efforts to treat Legionella colonization more difficult.

Hot-water system design to control Legionella bacteria growth and scalding

The proper way to design a domestic hot-water system to control Legionella and scalding, in that order, would be to store the hot water at about 140 F to 150 F. That temperature will kill Legionella bacteria or pasteurize the water in the storage tank and distribution piping system. Then, the proper design would use a digital mixing valve that will deliver hot water to the distribution system at 134 F plus or minus 1 degree and record the temperatures continuously. The circulating pump size/flow rate and hot-water system insulation thickness should be designed with the recirculation piping limiting the flow velocity to a maximum of 3 feet per second at a 10-degree temperature drop in the circulated loop so that the hot-water return temperature is at or above 124 F. The reason for a minimum of 124 F hot water return temperature is because in the new ASHRAE 188 standard, Legionellosis: Control of Legionella in Building Water Systems, suggests a hot water distribution temperature no less than 122 F.  If you look at Figure 1, Legionella bacteria stops growing about 122 F. A safety factor should be applied to assure the temperatures do not drop into the growth temperature range below 122 F.  With 124 F selected as the minimum hot water return temperature, the 124 F hot-water return water returns after the circulating pump where there should be a gauge to monitor the lowest temperature in the hot-water distribution and circulation system. The hot-water return pipe should then split and go to the cold-water inlet of the mixing valve and the cold-water inlet of the water heater. The 124 F hot-water return water mixes with 140 F hot water from the water heater in the digital mixing valve where it constantly monitors and records the system temperatures to deliver 134 F hot water to the distribution system. This is a relatively close temperature margin and some types of thermostatic mixing valves would have trouble mixing when hot water return temperatures get close to the delivery temperature. This is where digital mixing valves perform very well.

Recent discussions related to hot-water system temperature controls   

During several recent industry meetings dealing with control of organisms like Legionella bacteria in building water systems, many people seemed to have misunderstandings about how plumbing systems work. I heard comments from people that said, “You need to set the water heater thermostat to 158 F degrees and flow water from all the fixtures for a 20-minute period to disinfect the hot water system.”

It is important to note, if you need 158 F for disinfection, the water temperature setting at the water heater must be approximately the same number of degrees higher than 158 F as the hot-water return temperature is below the hot-water distribution temperature.

I have also heard comments that said, “the code official or health department requires that we set our water heater at 110 F or 120 F to prevent scalding.” I pointed out that in most cases the plumbing code does not specifically address hot water storage or distribution temperature. And in a few isolated cases, health departments make mistakes and mandate language that creates a health and safety hazard because most health officials do not understand how plumbing systems work. There are some that have mandated low temperatures to try and protect people from scalding, by mandating storage temperatures in the Legionella bacteria growth temperature range. If they would have simply stated the maximum temperature flowing from a shower is 110 F, the limit stop on the shower valve could be set to limit the hot water temperature to 110 F. Instead, they apparently tried to address buildings where there are old non-code tub/shower valves and they mandated low storage temperatures instead of mandating updating the shower valves to code-compliant ones with maximum temperature limit stop adjustments that are set to 110 F.

Another guy said, “my code requires me to store and deliver hot water at 120 F. So, the return temperature is always going to be between 100 F to 110 F.”  I then explained to him that there is no place in the plumbing code that addresses a maximum temperature in the storage or distribution system, except for a requirement for combined heating and domestic hot water systems to have a mixing valve set to a maximum of 140 F. The guy then started to mention sections in the plumbing code and health code requirements that address maximum temperature of the water required in the hot water storage tank and piping. He was pointing to the section on showers and bathtubs. I pointed out to him that section 4 of the model codes is fixture and fitting requirements and addresses maximum hot water delivery temperatures from the fixture and it does not address storage or distribution temperatures.   

During my 40-year career, I have witnessed many domestic hot-water systems that were designed, installed, or maintained poorly. As part of my forensic investigation work, I have investigated many scald incidents, Legionnaires disease outbreaks, and poorly performing or failing plumbing systems.   

Energy conservation

There is no justification for energy or water conservation programs that allow inadequate system performance and contribute to health and safety issues.

An electrician at Queensland Hospital told the news agencies that a state government energy efficiency program may have increased the risk of Legionella bacteria living in the water supplies of homes and businesses across Queensland, Australia.

The Welsley hospital has had its hot water temperatures limited to 45 C, (113 F) in line with Queensland regulations designed to prevent patients from scalding themselves. The Brisbane electrician who worked for the state government’s now defunct Climate Smart Program said anyone who used that service would have had their hot-water system temperatures reduced to 50 C (122 F) to save energy.

He said electricians warned the government at the time that lowering the temperature would encourage harmful bacteria growth, but the warning was ignored.

"Part of the direction given was to turn down all the hot water systems all over Queensland down to 50 C (122 F)," the man, who did not want to be named, said. "Anybody that had the Climate Smart service had the temperature of their hot-water system reduced from 65 to 70 C (149 F – 158 F) down to 50 C (122 F)."

This was an example where an energy conservation program was potentially deadly. I believe energy and water conservation should take a back seat to health and safety.

Water conservation

We seem to be in a water conservation limbo contest with competing green design standards and water and energy conservation programs trying to squeeze every drop of water down to an undetermined minimum flow in order to gain points for a plaque to hang on the wall in a building and save a few drops of water. How low can they go? Water conservation is causing lower flows in water utility mains and building water distribution piping. These lower flows cause water to take longer to arrive at the end of the system. Current ultra-low-flow fixture flow rates are about 20 percent of the overall water flow rates prior to the Energy Policy Act of 1992. This causes significantly lower flows where in the past, it would take days for water from the treatment plant to reach the ends of the distribution system. Now, it takes weeks for water to reach the ends of the distribution system. This allows water treatment chemicals to dissipate to undetectable levels well before reaching the end of the water distribution system and then there are no water treatment chemical residuals to fight-off bacteria and organisms growing in the biofilm of our water distribution pipes.

Recently, Drexel University and Purdue University were awarded EPA grant money to study and report on the effects of water conservation programs on water quality.

In addition to water quality issues, water conservation is also causing drain line transport issues with lower and lower flow rates. Our drain pipes look like rivers during a drought; there is not enough water in the river to float the boats. At a recent industry meeting where water conservation activists made a proposal to further reduce the flow rates for various fixtures, I made a plea to stop the madness of percentage flow reduction every code cycle to save water. Saving water will mean nothing if the water we save goes to nut farmers in the desert who are extracting millions of gallons of water every day to water trees. What we need to also do is pay for research to determine what are the minimum sustainable flow rates for various pipe sizes. We have issues with drain line transport of solids, high-tensile strength toilet paper, baby wipes, adult wipes, feminine products, etc. We need to fund more research through the Plumbing Efficiency Research Coalition (PERC) to know what are the sustainable minimums.