Im re-reading your post & it realy sounds like you have a central air/heating unit outside your house. If so, that unit cools by using freon (freons outlawed, R-22 or something now) If your control box/thermostat says "change water" I would say that it means the condensate water hold is overflowing. Theres usualy a tube, plastic hose that the water drips out of, away from the unit. This hose may be plugged & the water is backing up setting off the sensor. I had a portable AC unit that when this happened sensors turned off the compressor automaticaly, so it would blow, but not cold air. These central air units are not real repair freindly, you may have to call a "professional" to open it up & unplug a piece of hose.