Q&A with Dr. John Straube about radiant floor heating, domestic hot water, and Legionnaire’s Disease.

Q: I am interested in combining the domestic hot water system in my houses with a hydronic floor system. I have heard stories about connecting the two systems causing Legionnaires’ disease. Is this for real?

A: Dr. John Straube, Ph.D., P. E., of Building Science Corp., fills us in: Yes, it is for real, but you can prevent it.

Legionnaires’ disease is actually a more common problem than you might think. The primary place to find legionella bacteria is in residential hot water systems – usually in your shower. 

Q: Holy cow! How common is it?

A: Newspapers report on outbreaks of 20 to 30 people, so it seems sporadic but small. Actually, the number of people who get it and go to the doctor with pneumonia-like symptoms is pretty high. The Centers for Disease Control reports around 5,000 people per year get Legionnaires’ disease. The vast majority got it from their home hot water system or from a hotel’s hot water system.

Q: Creepy. How do you kill legionella?

A: Here are three things to keep in mind when designing a building or community:

1. Keep the water hot. At 130 degrees, the bacteria will die within 5 to 6 hours; at 150 degrees, the bacteria will be killed in a few minutes.

2. Use a tankless gas water heater. Legionella is not a problem with tankless water heaters – especially gas-fired tankless models – because they blast the bacteria, if present, right away.

3. If you don’t install a tankless unit, use an oil or gas-fired water heater.

Researchers who sampled water tanks have learned that fossil-fuel water heaters have much lower rates of legionella bacteria than do electric water heaters.

They believe that’s because the temperature difference between the heat exchanger and the water is well more than 150 degrees in oil and natural gas-fired water heaters, death is instantaneous for legionella bacteria as it slides by these heat exchangers under fire.

Because of this direct correlation between tank temperature and legionella, the DOE recommends to keep water at 130 degrees. If the water never goes below 130 degrees, legionella will not survive.

Temperatures of 130 to 140 degrees are ideal for killing legionella, but can scald bathers in relatively short periods of time. Babies and the elderly are particularly susceptible.

Q: I don’t want to get sued for scalding babies. Can I just knock the tank temperature down to 115 or 120 degrees and call it a day?

A: Bacteria still can live in 115 to 120 degrees temperatures. They just need a slightly cooler temperature to reproduce, such as at the tank inlet and the outlet. Because the inlet is flushed frequently with city water, and the city water doesn’t have the seed-stock for the legionella to take hold, cold water inputs are not a problem.

Exits are the problem. In a 115-degree tank, the temperature at the nozzle is lower than 115 and bacteria can reproduce there. So the legionella can live in the tank and then multiply at the cooler exit, which will be fed with a steady stream of seed-stock bacteria, ready to reproduce.

Q: I feel like puking. Is this a big problem?

A: Bigger than it needs to be. The Centers for Disease Control says that thousands of people die every year because of Legionnaires’ disease because much of it is misdiagnosed as pneumonia.  Most health officials want to keep the water hot, but it is also important to prevent scalding.

The answer is very hot water in the tank and a mixing valve to deliver the water at a temperature that won’t cause scalding.

In Ontario, Canada and many countries in Europe, the standard is to store domestic hot water at 140 degrees or higher, but deliver it at no more than 120 degrees.  This approach avoids scalding, but at the same time makes sure that legionella can’t grow.

Q: So keep it hot in the tank, cooler at the tap, and don’t sweat it?

A: Right.

Q: How does all of this relate to my hydronic floor?

A: Knowing how legionella grows in a domestic hot water system makes it clear why connecting potable water to a hydronic floor is of concern. It also helps us understand solutions.

Consider a hydronic floor with 1,000 feet of PEX tubing in a slab. In the summer, you stop pumping water through the slab because you don’t need heating. So you’ve got 15 gallons of water sitting in the tubing for five months. There are bacteria in there and they spend a lot of time sitting in the tubing. It is not an ideal reproduction temperature, but they can grow and wait.

When the system is turned on, it is often set at 90 degrees because people want a warm floor, not a hot one. These are almost the same conditions you would use in a lab to induce bacteria to grow.

This is why some jurisdictions require builders to separate space heating water from domestic hot water. You don’t want bacteria-friendly water in your shower. To avoid this kind of mixing, codes might specify heat exchangers or even double-wall heat exchangers (the latter is a little extreme, but some require it).

Q: Copper fan coils can trump this requirement.

A: In many places if a fan coil is made of copper and the plumbing lines are short, you don’t have to separate out potable water. Copper pipe has antibiotic properties – slime can’t grow on it. So as long as water only touches copper, you can have potable water in the fan coil. The amount of water stored is also less. This exemption is a scientifically backed up.

If a system waits 5 to 6 months to get back to work, it can rust; so many fan coils have timers to run the pump periodically to flush the system. The fan doesn’t run and not much energy is used, but it does make sure that the water is not stagnant. This is called a pump exerciser, and it helps reduce the chance of stagnant water as the fan coil runs with much hotter water than floor loops.

One last word: Studies clearly show that as the temperature drops to 120 degrees, the amount of bacteria explodes, and that fossil fuel-fired appliances are dramatically safer in this regard than electric water tanks.

Hydronic flooring connected to DHW can work, but too often it starts getting fishy, and that is no condition for potable water.