Heat exchangers transfer the heat from one fluid to another without mixing the two together.
A heat exchanger is a number of flat stainless-steel plates pressed together. There is 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. This fluid then travels out to the heating system and warms the floors. This separation allows you to add glycol to the radiant system without it mixing with your domestic water. Also, some local building codes require this separation.
A Radiantec system using a heat exchanger is called an Indirect System. This system allows you to use one water heater for both domestic purposes and heating. The two fluids do not mix because the heat exchanger keeps them separate. To learn more, please follow this link: http://radiantec.wpengine.com/systems-sources/indirect-system.php