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I asked the inventor about his design and when consumers might see the tech in their homes.
One challenge of current water heater designs is staying hot enough to kill harmful bacteria like Legionella pneumophila, which can cause Legionnaires’ disease. A new water heater design, developed and patented by researchers at the National Institute of Standards and Technology, aims to raise tank temperatures to 160 degrees Fahrenheit, which spells instant doom for Legionella bacteria.
Legionella bacteria love warm water, so while they struggle to survive in a water heater tank set to 120 F, they’re more than comfortable elsewhere in a home’s hot water pipes. It’s technically possible to achieve sufficient temperatures with existing water heaters, but those temperatures are also likely to scald human skin whenever you turn on the faucet.
The biggest innovation of this new design is how it finds a new use for heat exchangers, the same technology behind air conditioners, heat pumps, furnaces and refrigerators. I wanted to know more, so I reached out to David Yashar, a NIST mechanical engineer behind the new design.
As Yashar and his team researched ways to kill Legionella bacteria, they needed to figure out how to make tank temperatures around 160 F safe and practical for humans. They ultimately landed on the concept of using cold water entering the water heater to cool the hot water leaving the tank with an unmixed heat exchanger that contains the water inlet and outlet.
The principle behind unmixed heat exchangers—where the fluids don’t mix—is that incoming cold water gets a heating boost from the hot-water outlet while cooling the water to safe levels for people. As a result, the tank can maintain the high temperatures required to prevent pathogen growth behind illnesses like Legionnaires’ disease, which has a fatality rate of around 10%, according to the Centers for Disease Control and Prevention. Beyond the safety benefits, the process recaptures some waste heat and can improve heating efficiency for the cold water, since it enters the tank warmer.
I’ve covered various heating, ventilation and air conditioning tech over the years, so I immediately recognized similarities between the new design’s unmixed heat exchanger and those used in HVAC and refrigerators. For example, air conditioners use a heat exchanger for the refrigeration cycle (also called the Carnot cycle) to pull heat from air inside your home (evaporator coil) and push it outside (condenser coil) without pulling in outside air. Additionally, heat pumps may use a heat exchanger for defrosting ice buildup when in heating mode.
When I asked about the similarities, Yashar noted that it wasn’t a coincidence that his patent took inspiration from HVAC and refrigeration tech. Yashar said, “My background is in HVAC&R, and it is very common to use unmixed heat exchangers like this to manipulate the bounds of the Carnot cycle to achieve some specific condition. I came up with the idea when I first learned about Legionella and the existing mitigation strategies.” He goes on to explain that using an unmixed heat exchanger “across the inlet and outlet was a very simple, effective, and inexpensive way to push the water temperature up to that condition and back down.”
It’s fitting that Yashar discovered better ways for heat exchangers not only to kill pathogens, but also to improve energy efficiency. Finding new applications for existing technology is certainly an underrated aspect of invention.
As exciting as this tech is, a patent doesn’t necessarily mean it will hit the market immediately. Yashar notes that while his research “proved the concept using a 60-gallon tank,” it depends on when manufacturers license the technology and get their engineers working on designing consumer products suitable for homes. This process could happen quickly or take several years.
It may be years before we see consumer heaters that use the promising new technology.
Because Yashar’s concept essentially modified an existing water heater, I asked if it would be possible to retrofit products in people’s homes. He notes that NIST’s project started with the intention of modifying the tank’s hardware and controls. “The steps we took might not be simple tasks for an existing application,” said Yashar. “It really depends on a lot of things, so it is best if people learn how this works and understand how to achieve it within their existing application.”
He brings up one important consideration: Retrofit tanks will need extra insulation to be worth it. It’s “absolutely necessary for a retrofit because the tank temperature will be about 40 degrees hotter than what the existing system was designed for. If this isn’t done, the standby losses would be very large.” Without appropriate insulation, any efficiency gained by the heat exchanger would be lost to heat loss in standby mode.
So, while a retrofit might be possible, it seems more likely to engineer a product from the ground up based on this design, especially if consumers want to keep utility costs manageable.
For more tips, see the best temperature to set your thermostat this winter and learn how to find air leaks in your home for more efficient heating.