Most Aquatherm Eco boiler stoves are available as CS models. CS stands for Closed System and the CS models are designed to go on closed, or pressurised, systems. Inside the boiler of each CS Aquatherm there is a heat exchange coil fitted. A thermostat in the boiler detects the temperature of the water. If the temperature of the water in the boiler goes over 95ºC the thermostat sends a signal to a valve which floods the coil with mains pressure cold water. This cold water takes the heat from the boiler and is then flushed safely to waste. This stops the boiler from overheating and, because it is an entirely mechanical process, will still work in a power cut. Aquatherm Eco CS boiler stoves take the headache out of installing a boiler stove to a pressurised system. We still recommend fitting a heat leak rad as well just so you have double backup, but it is not strictly speaking necessary.