To fill a heating installation with a closed expansion tank with water, we use devices we call automatic fillers. The automatic filler is a device that allows water to pass from the high pressure side (water supply network) to the low pressure side (heating network) until the low pressure side reaches the limit we have set in the filler. In addition to the initial filling, the automatic filling constantly replenishes the water leaving the installation, either through the safety valve (when it opens), or through the automatic vents, or from micro-leaks in the network.