The safety of drinking water from a bathroom tap depends almost entirely on the home’s plumbing configuration. While the public water supply is the same clean source for the entire house, internal factors create major distinctions in quality between the kitchen and bathroom, and between cold and hot water. These differences determine if the water is fresh from the main supply or has been held in a potentially contaminated storage system.
Household Water Systems and Storage
Residential water systems generally use one of two methods: direct feed or indirect storage feed. In a direct feed system, common in many modern homes, cold water is supplied straight from the public water main to every cold tap. This setup ensures the water at the bathroom sink is fresh, high-pressure, and has not been stored internally.
The indirect storage feed system is frequently found in older homes. With this configuration, mains water first fills a cold water cistern, often located in an attic or loft space. This cistern then feeds the cold water taps in the bathrooms and supplies water to the hot water cylinder. The kitchen cold tap is typically the only fixture connected directly to the high-pressure main supply.
The physical difference in delivery is a major distinction regarding water safety. Water coming from a storage cistern is decoupled from the immediate freshness and pressure of the public supply. Because the bathroom cold tap is often fed by this cistern, the water is under lower, gravity-fed pressure compared to the kitchen tap.
The Risks Associated with Stored Water
Drinking cold water held in a storage cistern introduces several potential hazards not present in a direct-feed system. Even if the water entering the tank is initially safe, the container is susceptible to environmental contamination. Storage tanks, particularly those in attics, can be exposed to dust, debris, insects, or small rodents if the lid is not tightly fitted or properly sealed.
Water quality may also suffer from stagnation if the water is not regularly used and replenished. Stagnant conditions encourage the growth of bacteria, including Legionella, which thrives in water not maintained below 20°C. Furthermore, the material of the tank itself, especially in older installations, may degrade over time, potentially leaching contaminants into the water. For these reasons, the kitchen cold tap is usually the only guaranteed direct-feed source and the safest option for cold drinking water in homes with an indirect system.
Why Hot Water is Never Potable
Regardless of the home’s plumbing system, hot water from any tap should never be used for drinking or cooking. Heating water significantly increases the rate at which heavy metals, such as copper or lead solder, can leach from plumbing materials into the water supply. Water heaters and hot water lines accumulate these metals, which then become concentrated in the water sitting in the pipes.
A serious health concern is the potential for bacterial proliferation in the warm water environment. Thermophilic bacteria, notably Legionella species, thrive in the warm, stagnant conditions often found in hot water heaters and distribution systems. These bacteria grow optimally between 32°C and 40°C, and they can colonize pipe surfaces by forming a protective layer called biofilm.
While the risk of infection is primarily through inhaling contaminated water aerosols, such as those created by a shower, ingesting water with high concentrations of the bacteria is still a risk. To manage this risk, many systems are designed to store water at temperatures above 60°C, which is hot enough to kill most pathogens. However, the water temperature often drops below this level in the pipes leading to the tap, creating a warm-water zone where bacteria can still multiply.