The average hot water is a relatively simple appliance to understand. It uses gas or electricity to dump energy into water in the form of heat, keeping it at a pleasant temperature for uses like bathing and cleaning. Basic mechanisms are in place to ensure the water stays at a relatively constant temperature, neither too hot where it could cause burns, nor too cold such that it wouldn’t be fit for purpose.

One of the problems, though, is that sometimes storing water at the desired temperature can create the perfect breeding ground for bacteria. However, a neat little trick developed by NIST could solve that problem rather elegantly.

You’re Hot And You’re Cold

One of the interesting problems of our modern era is that changing our water usage has changed the risk profile for pathogens growing in hot water. NIST has noted for some time that while the pipes in our walls haven’t changed size, things like our shower heads have changed their flow rate to save water. Less flow rate in the same sized pipes means that water stays in the pipes for longer, increasing the time in which harmful pathogens have to grow in that environment.

Chief amongst these pathogens? The one most commonly feared is legionella pneumophila, the bacterium responsible for causing Legionnaires’ disease. The severe form of pneumonia comes with a fatality rate of 10%, and infection typically comes from inhaling aerosolized droplets containing the bacteria.

Of course, a great way to create aerosolized droplets filled with bacteria is to spray not-quite-hot water through a shower head. This is why hot water systems, particularly in large multi-occupancy buildings, are a risk for such infections. Legionnaires’ disease is rare, but it’s always out there, with 6000 confirmed cases showing up in the US each year. It’s possible that the true number is up to 10 times higher because the disease isn’t routinely tested for. The disease’s rarity is in part because engineers and tradespeople put in plenty of work to minimize the ability for the bacterium to breed in hot water systems. As NIST’s work demonstrates, though, there is possibly even more that could be done to tackle this problem.

Ideally, your hot water pipes in your house would not host any nasty bacteria. But often, hot water systems sit around 49 °C (120 °F), a temperature that’s hot enough to be useful and pleasant without a major scalding risk. Legionella pneumophila won’t easily grow at that temperature; the bacteria thrives between 20 °C to 45 °C (68 °F – 113 °F). Unfortunately, though, it’s not hot enough to kill the bacteria completely. Furthermore, water cools as it travels through pipes to an outlet, and thus pipes can be a perfect environment to spur further growth.

The simple solution would be to simply jack up the hot water system to store water at a much higher temperature, say around 70 °C (160 °F).  This could be undesirable in many contexts, though, as water that hot can more easily cause burns. It would be ideal to store the water at a higher temperature, while making sure it was delivered at a slightly lower temperature so it didn’t hurt anybody when it left the tap or shower head.

It turns out that there is a remarkably simple way to achieve this. NIST developed a simple heat exchanger which can be mounted atop any old hot water system. It’s designed such that the hot water leaving the tank passes through a heat exchanger where it’s surrounded by cold water which is running to the water heater’s intake pipe. This has dual benefits. The hot water inside the tank can be stored in the tank at 70 °C, which kills almost all legionella peumophila almost instantly. That water, though, is then cooled to a slightly safer temperature as it passes through the heat exchanger and out to the tap or shower. The hot water remains at a safe temperature for the user, and contains far less harmful bacteria, and cooling it doesn’t introduce any pathogens from another source. In turn, the cold water coming into the tank is pre-warmed slightly by the hot water, so the energy isn’t simply wasted.

This trick isn’t just simple, it’s also cheap. NIST engineers were able to fabricate a prototype unit to fit to a residential water heater using just $100 of components. It could be a useful addition for homeowners looking to make their hot water service a little cleaner. However, the real benefits are likely to be for larger operators of multiple-occupancy facilities, like nursing homes and hotels. These facilities often have to take great care to avoid the build up of harmful bacteria in building-wide hot water systems. Having the ability to increase the tank temperature to the point that legionella pneumophila bacteria simply die off would be a huge boon to this effort.

Sometimes, it’s possible to make great gains with a simple hack. Using the cold water service to cool hotter outgoing water is a smart trick that uses only minimal additional equipment to offer a safer hot water supply. It could yet become a common install for hot water systems in order to fight the good fight against a harmful disease that likes to lurk in our pipes. If you happen to see a heat exchanger atop a hot water heater in future, you can smile that someone has done the work to try and make that hot water supply just a little safer going forward.