Concerning Legionella, that is why I am designing for minimum 130F supply temp, preferably 135F. I plan on having a small electric instant water heater at the exit of my 30 meter 1" dia corrugated stainless steel heat exchanger pipe located in the top of my 2000 gal storage tank. The heater will make up the last few degrees just in case the geo/desuperheater/solar can't keep up, and I will put some thin silver rods in the storage tank since silver is bacteriostatic (bacteriocidic if the contact time is long enough). The silver is easy to wipe down should it ever become corroded to maintain efficacy.
130F is hot enough to kill Legionella bacteria. Any hotter, they die faster. That is the reason tank style water heaters can be a serious Legionella risk. If the top of the tank is 130-140F, the bottom of the tank will definitely be below 122F due to stratification-right in their favorite growing temperature sweet spot. The bacteria also need a still place to breed-sediments on the bottom of the tank provide a perfect breeding sponge maintaining that perfect temperature and sheltering the bacteria from being flushed out with the next water draw. Even a gas heater warmed from the bottom by the burner will often retain areas of lower temperature that won't kill off the colony. A tankless system that does not allow any water to stagnate at a warm temp never gives the bugs a foothold. The water is either starting out hot enough or turbulent enough to kill/prevent growth when the system is in use, and quickly cools below their sweet spot once shut off and no longer turbulent. That's why a properly designed "year round flow through" open loop system is important-water can't stick around long enough to get stagnant. How about the water sitting in the DHW plumbing leg that serves the back part of the house you seldom use? An open loop radiant system prevents the water from ever sitting around like that. EVERY time ANY hot water is used ANYWHERE in the house, fresh water is drawn through the radiant loops to replace the water that was used.
It's interesting that Canada requires the 140F water heater setting, but apparently prevents open loop systems that by nature are less risky than unused portions of the DHW system. Open loop systems are ok by our national building codes and most local codes.
I read some reports from the CDC that said improperly maintained chiller equipment is the largest source of legionella infections, followed by tank style water heaters. One of the next largest sources? Windshield washer fluid in vehicles! Since only northern climate washer fluid (the stuff that says "good to -20F") has enough methanol in it to kill the bugs, most people are driving with low or no methanol washer fluid. Add warm summer temperatures, a cozy engine compartment that retains heat, and a way to atomize the bacteria laden fluid (the washer nozzles) into a respirable mist, usually in front of the fresh air inlets at the base of the windshield, it's no surprise. Truckers have a 4x risk of legionella infection compared to the general public since they put on so many miles.
I always recommend putting a capful of chlorine bleach in your reservoir 1-2x per year to prevent this. I see lots of cars with washer nozzles and pumps plugged up with growth in the reservoir. I wonder how many have legionella? I always throw some bleach in there and give them some contact time before removing them to clean them out.
Craig
The MMT
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