In a mains pressure hot water system there is no intermediate cold water storage tank. Cold water is fed directly from the mains to the points of use. The rising main is connected directly to the calorifier, water heater or plate heat exchanger. Since the water in the system will expand due to heating, an expansion vessel and a safety temperature and pressure relief valve are required. Hot water distribution from pressurised systems can be used in both recirculation and non-recirculation systems. The latter is commonly found in houses with combination heating and hot water (combi) boilers.