Most home drinking water filters are installed on cold water lines, treating only the cold water tap. Understanding whether PFAS concentrations differ between hot and cold water, and whether temperature affects filtration performance, helps households make complete decisions about which water sources to filter.
PFAS in Hot Water Heater Systems
Water heaters do not remove or add PFAS. Hot water PFAS concentrations reflect the same concentrations as cold source water, unless the hot water heater introduces its own contamination — which is not characteristic of standard water heaters. Using hot tap water for cooking (to shorten filling time) rather than cold does not increase or decrease PFAS compared to cold tap water from the same source.
Temperature Effects on Carbon Filter Performance
PFAS adsorption onto activated carbon is somewhat temperature-dependent. Higher temperatures reduce adsorption slightly because higher thermal energy allows PFAS molecules to desorb more readily from carbon surfaces. Standard carbon filters are designed and certified for use with cold water — treating hot water through carbon filters is generally not recommended both because hot water decreases adsorption efficiency slightly and because hot water can accelerate degradation of filter housing materials not designed for hot water use.
Practical Guidance
For cooking applications where hot water is used — making pasta, boiling soups, making tea — the appropriate approach is to draw cold filtered water from the filtered tap, then heat it in a kettle or pot rather than using hot tap water. This ensures PFAS removal occurs before heating rather than attempting to filter already-hot water through components not designed for high temperatures.