NIST water heater patent: 70 C tank against Legionella

The National Institute of Standards and Technology (NIST) has been granted a patent for a water heater system designed to kill Legionella bacteria while reducing the risk of scalding, according to nist.gov nist.gov. The inventor is NIST mechanical engineer David Yashar. His team built a residential-size prototype by modifying an off-the-shelf water heater with around $100 of additional components nist.gov.

Why warm pipes allow bacteria to grow

Legionella pneumophila lives naturally at low concentrations in rivers and soil and thrives at 20 to 45 degrees Celsius (68-113 degrees Fahrenheit) nist.gov. Modern fixtures use less water, but the pipes that supply them have not changed to match. As a result, water stays in a building's pipes for longer and creates conditions in which the bacteria grow nist.gov.

If water with a high concentration of the bacteria is aerosolized and inhaled, it can cause Legionnaires' disease, a severe type of pneumonia with a fatality rate of 10% nist.gov. There are a little over 6,000 confirmed cases per year in the U.S., and NIST notes that the real number could be higher because the disease is not routinely tested for nist.gov.

Residential water heaters are typically set to 49 C (120 F). That slows bacterial growth and may kill some Legionella, but not completely. Once the water enters the piping network, it cools to the temperature at which surviving bacteria multiply nist.gov.

How the design works

The patent describes a water heater that can safely raise the tank temperature to 70 C (160 F), hot enough to kill nearly all Legionella almost instantly nist.gov. Some heat-resistant microbes can survive at that temperature, but they are not a major health concern, according to NIST nist.gov.

Immediately after the water leaves the tank, a heat exchanger cools it to a safe temperature before it goes to faucets and showers. A heat exchanger transfers heat between two fluids without direct contact, so no live Legionella is introduced from another supply nist.gov.

According to NIST, the system does not waste energy because the extra heat is returned to the water heater. The pipes are arranged so that hot water cannot leave without first being cooled, which prevents scalding nist.gov.

Yashar says the design would work in single-family homes but is likely to be most useful in larger facilities such as nursing homes and hotels. Managers of such buildings generally take costly, active steps to prevent bacterial buildup, and this design could offer a passive alternative. He added that a manufacturer could bring the component costs down nist.gov.

NIST patents a water heater with a 70 C tank and heat exchanger against Legionella; prototype built

Why warm pipes allow bacteria to grow

How the design works

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