Fan coil isolation valves control the supply and return water from the buildings risers into and out of your fan coil. Over time these valves become: rusted, corroded, seized and or fall off. When this happens your HVAC unit becomes at risk for floods. If your coil began to leak water the islaotion valves would need to be shut off to stop the leak. If those valves didn't work the riser valves that control the water to everyone above and below you would need to be turned off. If those didn't work then the entire building would need to be turned off. In fan coils over 20 years of age these valves are closer to failing then they are to working.