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.