Heat exchangers are designed to transfer the heat from one fluid to another without mixing the two together.
A heat exchanger is a number of flat plates pressed together with a tube on one side where the warm water from the heater travels through and heats the plates. On the other side of the heat exchanger is another tube where the heating fluid enters and picks up the heat from the plates and then travels out to the heating system. This allows you to add glycol to the radiant system without it mixing with your domestic water.
A Radiantec system that uses a heat exchanger is called an Indirect System.
You can learn more about it here.