You have to be careful when the cooling coil is downstream from the heating coil in the winter even when units are not in freeze. If you have chiller pumps running and a chiller not running and you build heat in the chilled water system because a chilled water valve is also open,or just from pump friction , you can actually build enough heat to blow the rupture disk out of a chiller. I've seen it happen on large centrifugal chillers and it cost a fortune to replace the refrigerant, not to mention the environmental issues with the release.