Why would cold water be introduced into underfloor heating system?
As longneck mentions this is a pressure reducing valve and this connection of the domestic cold water to your heating system is called the system make-up. It used to fill the system and to maintain the heating system's pressure. The knob on the top can be turned to adjust the system's pressure and should be done while the system is static.

However, a pressure reducing valve is not a cross connection device and should not be considered as one. While by its nature it creates a lower pressure on the heating side then that of the domestic side (therefore not allowing the water to back feed due to the pressure difference), if there is a drop of pressure on the domestic side (the required significance would depend on the domestic system pressure) or if the safety mechanisms on the heating side fail (such as the boiler fails to cut off and the pressure relieve valve fails to open) the heating side can result in a higher system pressure then that of the domestic side. This valve will not prevent back-flow in such scenarios. For true cross connection prevention you would need a double check valve or a reduced pressure back-flow assembly.

One last note: if you shut off the system make-up be sure to periodically check that the heating system maintains its set pressure. If the system drops too low you could cause damage to your heating source if it doesn't have built-in low water cut-off.