Drinking hot tap water is generally not recommended. The infrastructure and treatment of water differ significantly once it enters a home’s plumbing, depending on whether it is routed for immediate use as cold water or sent to a heating appliance. The process of heating and storing water can introduce both chemical and biological changes that make the hot water supply less suitable for drinking, cooking, or preparing infant formula.
Accelerated Leaching from Plumbing Materials
The elevated temperature of hot water accelerates the rate at which various metals and minerals can dissolve from the internal plumbing components. This chemical process, known as leaching, increases the concentration of substances in the water compared to the cold water supply. The risk is particularly relevant in older homes that may contain lead-based solder used to join copper pipes, which was common before its ban in 1986. Hot water causes lead to dissolve more readily from these materials, as well as from brass fixtures and fittings that may contain trace amounts of the metal.
Copper can also leach into the water, and while less toxic than lead, high levels can still cause health issues like gastrointestinal distress. Furthermore, the high heat in the water heater can dissolve sediment and protective lining materials from the tank, introducing them into the hot water stream. Using only cold water for drinking and cooking minimizes exposure to these leached materials.
Bacterial Growth in Storage Tanks
Hot water systems, particularly those that utilize a storage tank, create an environment where waterborne bacteria can multiply if the temperature is not properly maintained. Water temperatures between 20°C (68°F) and 45°C (113°F) are ideal for the rapid growth of pathogens such as Legionella species.
The design of a storage tank can also contribute to this biological risk through stagnation and temperature stratification. Water that sits unused in the tank or in certain sections of the plumbing allows bacteria to form biofilms, which are protective layers that harbor the microorganisms. To prevent this, health guidelines often recommend storing hot water at 60°C (140°F) or higher, as temperatures above this point will kill the bacteria. However, a lower temperature setting, often used to prevent scalding, increases the risk of bacterial development within the tank.
Even when a water heater is set to a high temperature, colder zones can exist, particularly at the bottom of the tank, where sediment collects and provides a nutrient source for bacteria. This localized temperature variation can still allow for pathogen survival and growth, which is a risk not present in the constantly moving and treated cold water supply. The aerosolization of this contaminated water, such as through a showerhead, is the primary route of exposure for respiratory illnesses like Legionnaires’ disease.
Distinctions Between Hot and Cold Water Supply
The fundamental difference between the two water sources lies in their intended function and the path they take within a residence. Cold water is delivered directly from the public water main or well supply and is designed for potable use, meaning it is safe for drinking and food preparation. This water remains fresh because it is constantly renewed from the source and does not pass through any heating or storage device within the home.
Hot water is utility water, meant for washing, cleaning, and bathing. The water is diverted to a water heater, where it is held, heated, and distributed through a separate set of pipes. This infrastructure, involving storage and heating, fundamentally changes its quality profile compared to the cold water supply.