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Intermittent reduction of temperature set-points and periodic shutdowns of water heaters have been proposed to reduce energy consumption in buildings. However, the consequences of such measures on the occurrence and proliferation of <i>Legionella pneumophila</i> (<i>Lp</i>) in hot water systems have not been documented. The impact of single and repeated heat shocks was investigated using an environmental strain of <i>L. pneumophila</i> and a reference strain of <i>V. vermiformis</i>. Heat shocks at temperatures ranging from 50 °C to 70 °C were applied for 1 h and 4 h in water and water heaters loose deposits (sludge). The regrowth potential of heat-treated culturable <i>L. pneumophila</i> in presence of <i>V. vermiformis</i> in water heaters sludges was evaluated. A 2.5-log loss of culturability of <i>L. pneumophila</i> was observed in simulated drinking water at 60 °C while a 4-log reduction was reached in water heaters loose deposits. Persistence of <i>Lp</i> after 4 h at 55 °C was shown and the presence of <i>V. vermiformis</i> in water heater’s loose deposits resulted in a drastic amplification (5-log). Results show that thermal inactivation by heat shock is only efficient at elevated temperatures (50 °C) in both water and loose deposits. The few remaining organisms can rapidly proliferate during storage at lower temperature in the presence of hosts.