Heat can kill living contaminants that cold water leaves behind
One clear reason hot water cleans better is that high temperatures can kill organisms that cause allergies or disease. In bedding, hot water washing is used to reduce house dust mite populations and the allergens they produce [4]. The abstract does not give the exact temperature threshold, but it identifies water temperature as a key factor in reducing mites and allergens.
A similar principle appears in drinking-water systems. In a multi-family house, hot water was kept Legionella-free by ultrafiltration, while unfiltered cold water allowed contamination [1]. That study attributes the better contamination control to filtration rather than to heat alone, so it does not prove that hot water by itself cleans better. Still, it is consistent with the idea that hot-water systems are managed to limit microbial growth.
Warmer water can make chemical cleaning more effective
Heat can also improve how well cleaning chemicals work. In a study of nanofiltration membranes, chemical cleaning at 10 °C restored 95% of the membrane's original flow performance, but at 5 °C it restored only 74% [5]. That is a large difference from just a 5 °C change, and it shows that temperature can materially change cleaning outcomes even in a narrow range.
The same study found that physical cleaning with water alone was insufficient to remove the fouling layer at any temperature tested [5]. So warmth helped the chemical cleaner, but warm water by itself did not do the job. This is a useful distinction: hot water is not a universal solvent, but it can make other cleaning agents work better.
Cold water can match hot water when a disinfectant is added
The advantage of hot water is not automatic. In an abattoir, a cold-water disinfectant containing peracetic acid and peroctanoic acid was tested against the standard practice of decontaminating knives with 82 °C hot water [2]. The cold-water treatment reduced E. coli and S. aureus on contaminated knives by roughly 4 to 6 log units, depending on whether fat or blood was present, and it was judged an adequate alternative to hot water [2].
That finding comes with a practical condition: pre-rinsing is recommended, especially in dirty process steps with high blood and fat contamination [2]. So cold water plus a disinfectant can work well, but it may need extra steps that hot water does not. The comparison is about decontamination of knives, not about laundry or household cleaning, so it does not show that cold water is generally as good as hot water.
Hot water can be too aggressive for delicate surfaces
Hot water is not always the better choice. In a study of cleaning natural stone, high-pressure hot water and high-pressure cold water both had a significantly stronger abrasive effect than steam cleaning when the nozzle was too close to the surface [3]. The study compared cold water under high pressure, hot water under high pressure, and hot-water steam; it did not isolate temperature alone as the cause of abrasion.
Steam cleaning had the lowest cleaning performance on biologically weathered samples, but the high temperatures of steam also permanently eliminated much of the biological matter on and under the surface in a field test [3]. That suggests a trade-off: heat can help remove living growth, but aggressive methods can also damage the material being cleaned. The right choice depends on what is being cleaned and how much surface loss is acceptable.
Sources used in this answer
Installation of an ultrafiltration plant in a multi-family house to reduce the hot-water temperature and energy demand: A case study in Germany
In a multi-family house, filtered hot water remained Legionella-free while unfiltered cold water allowed contamination, but the better contamination control was attributed to ultrafiltration rather than to heat alone.
Knife Decontamination by Cold Water Treatment Supplemented with InspexxTM 210—A Validation Study in an Abattoir
A cold-water disinfectant was found to be an adequate alternative to 82 °C hot water for decontaminating knives in an abattoir, though pre-rinsing is recommended for dirty steps with blood and fat.
Quantification of surface changes and volume losses of selected rock types due to different cleaning processes
High-pressure hot water and high-pressure cold water both had stronger abrasive effects than steam cleaning on natural stone when the nozzle was too close, and steam cleaning had the lowest cleaning performance on biologically weathered samples.
The role of water temperature and laundry procedures in reducing house dust mite populations and allergen content of bedding
Hot water washing is used to reduce house dust mite populations and allergen content in bedding, while many people prefer warm or cold water.
Effect of temperature on organic fouling and cleaning efficiency of nanofiltration membranes for loch water treatment
Chemical cleaning of nanofiltration membranes restored 95% of original permeance at 10 °C but only 74% at 5 °C, while physical cleaning with water alone was insufficient at any temperature tested.