Warm Water Is Not the Active Ingredient: Energy Saved by Cold-Water Handwashing at Equal Modelled Removal

This study examines a practical question in everyday hygiene by explicit arithmetic from stated assumptions. Modelled removal at twenty seconds was 79.6% at 15 C and 87.5% at 40 C, a difference of 7.9 percentage points across the whole practical range, and 83.7% at 25 C. Heating the wash water to 35 C cost 71.9 kWh and USD 10.79 per household per year at the stated use, for that difference. Warm water would need to extend the average wash by 6 seconds, through comfort alone, to match a full-duration cool wash, and a cool wash extended by just 3 seconds at no energy cost surpassed the warm short wash in the model. The proposed arrangement is as follows. No device is proposed. The deliverable is a practice rule with its boundary: wash cold or cool, for the full derived duration, with soap, and reserve warm water for comfort where a washer, a child or a patient, would otherwise cut the wash short. The rule is stated with the behavioural exception inside it, because a rule that ignores why people prefer warmth will be quietly disobeyed. All results are derived from the accompanying simulation script, in which every assumption appears as a named, replaceable value. SIMULATED, NOT MEASURED.