The Maquet Heater Unit 35 (HU35) is widely used to maintain patient body temperature during extracorporeal life support. Water is used as a medium for heat transfer though it also provides a medium for the growth of pathogens. Thus, the use of a heating unit presents a risk for transmission of water-borne pathogens in critically ill patients. Recently, a Mycobacterium chimaera outbreak in cardiac surgery has been linked to the production of bioaerosols by heater-cooler devices. Consequently, manufacturers have revised cleaning recommendations, with significant impact on staff, budget and environment. Heterotrophic plate counts (HPC) and non-tuberculous mycobacterium growth were assessed following three disinfection processes over a 16-month period. It was found that water quality was acceptable in HU35s when disinfecting with a lower concentration of Chloramine-T than currently recommended, provided exposure of the device to potential pathogens was minimised by the use of a 0.2 μm water filter.