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Designing and Maintaining a Domestic Hot Water System

When designing a domestic hot water system, careful consideration must be put into the design to maintain safety for all occupants of the building. The two major safety considerations in a design are the scalding of occupants and Legionella forming in the hot water system.


Reducing the Risk of Burns

Most adults will suffer third-degree burns if exposed to 150-degree water for two seconds. Burns will also occur with a six-second exposure to 140-degree water or with a thirty-second exposure to 130-degree water. Even if the temperature is 120 degrees, a five-minute exposure could cause third-degree burns. A child can suffer a third-degree burn in 124°F water in less than three minutes.


Most scald burns occur to children, the elderly, and disabled persons because their physical or mental limitations prevent them from getting out of the scalding water quickly.

Burns/Scalding can be classified into three categories:

  • First-degree burns are primarily minor. This type of burn could result from small amounts of hot liquids or grease hitting the skin. This type of burn involves some redness of the skin and swelling, but only the outermost layer is affected.

  • A second-degree burn is more severe and can be caused by sunburn or chemical exposure. This type of burn involves the second layer of skin being affected and results in some blistering of the skin.

  • Third-degree burns are caused by radiation or corrosive chemical exposure to the skin. These types of burns affect the third layer of skin and result in scarring. Medical attention for 3rd-degree burns is required.


Typically, domestic hot water will be stored at 140 degrees and distributed at 120 degrees. The temperature will be reduced from 140 to 120 at a master mixing valve. One option for a master mixing valve is a one-piece, single body valve that contains a liquid-filled thermostat to regulate the temperature. The second option is a digital mixing valve. A digital mixing valve maintains the recirculation loop temperature by energizing a recirculation pump when the recirculation loop temperature drops below a specified return limit and de-energizes the pump when proper loop temperature is re-established. The heart of the digital mixing valve is the 3-way valve and high-speed actuator which precisely maintains the selected outlet temperature. The digital mixing valve can also be connected to a building automation system (BAS) to provide computer access to a range of temperature and pressure measurements.

As discussed above, the distribution temperature of the hot water is 120 degrees, but water at that temperature scalding can still occur in 5 minutes. To drop the temperatures to a more manageable level, single-handle faucets and tub/showers with limit stops need to be adjusted to deliver a manageable temperature. Faucets that do not have limit stops, such as two-handle faucets, need to have an individual mixing valve to reduce the temperature to 110 degrees. Scalding occurs when temperatures are too hot and Legionnaires disease forms when the water temperatures exist within the legionella growth range when water is stagnant or improperly designed, installed, and maintained hot water systems.


Reducing the Risk of Legionella

Legionella has various growth rates and kill ranges/times depending on the temperature of the water. Below 68 degrees legionella is dormant, 68 degrees to 122 degrees is the growth range, but the prime range is 95 degrees to 115 degrees. Above 122 degrees legionella can survive but will not multiply, at 131 degrees legionella will die within 5-6 hours, at 140 degrees legionella will die within 32 minutes, at 151 degrees legionella will die within 2 minutes and 158 degrees and above legionella is killed rapidly. Maintaining domestic hot water temperatures in the temperature zones where legionella will either not grow or will die is the most effective option in terms of reducing the risk of legionella.


Stagnation of the domestic hot water system can be prevented using the recirculation pump. The pump can either run continuously, be turned on by an aquastat, or through the use of the electronic controller of a digital mixing valve. Also eliminating dead legs in conjunction with recirculating the hot water system will not completely eliminate the chances of Legionella forming but will reduce it significantly.


Maintaining Hot Water Systems

Once the domestic hot water system is installed and commissioned, the next step is to maintain the system as a whole. This is incumbent on the building's maintenance staff to perform periodic flushing, disinfection, and equipment monitoring.


The owner must be vigilant in maintaining the domestic hot water system as improper maintenance and operation of the system can cause the forming of legionella and, as a result, lawsuits. The cost to maintain a hot water system is pennies compared to the cost of a lawsuit.




Written by:

Richard Barbera

Plumbing Engineering Manager

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