Water Heater shut down during unoccupied periods

RobertK

Is there the protentional for Legionellae if a water heater (maintaining 140f) is shut down, left full for weeks at a time, becomes room temp and then is heated again for occupancy?

EBEBRATT-Ed

when i go on vacation i shut the city water off and the water heater as well in case the WH lets go while i am gone

Larry Weingarten

Hi, Legionella grows best at body temperatures. 140 F kills it and it's dormant at room temperatures, so sitting in an unheated tank shouldn't be a real problem. You don't say if it's a gas or electric heater. If gas and if you leave the pilot on, it's a different story as the tank will stay warmer.

A different concern would be odor caused by sitting stagnant for weeks. You may get a rotten egg odor from a sulfate-reducing bacteria that likes the taste of hydrogen produced by the action of the anode rod. Those are usually considered to be non-toxic bacteria.

Still to be on the safe side, I'd let the tank reheat completely before using any hot water.

Yours, Larry

RobertK

Thanks to you both. The tank in question is an indirect, heated from a gas boiler, which is also down when the home is Unocc. It's in a mechanical room with no exterior walls, so the ambient is never below 70f. This is a guest house and a commercial water tank. Unsure of the Anode rod, but assuming there is one. The thought was to reheat the tank to 140f prior to Occ return. Was unaware of the potential odor. Thanks.