Our water furnace was installed in 1994, (ATV045A110C2T). We have had no problem with it until Dec. 1, 2008 when a run capacitor was replaced, (the circuit breaker was tripping). We then went on vacation for about 3 weeks. When we returned the fault light and the Aux. light on the thermostat were on. We turned off both breakers for the furnace since there had been several power outages, then turned them back on to reset them. The aux. light turned off, but the fault light indicating low pressure came back on and has remained on. The technician came back to re-check the settings and has indicated that the compressor needs to be replaced. We still have warm air coming thru the vents, and no indication that the aux. heat in on, at least at the thermostat. Do we really have a problem with the compressor? If the backup system is actually doing the heating, why isn't the aux. light coming on when it's in use? If I up the temp. from 63 (where I usually keep it) to 70 the aux. light comes on at the thermostat. I know that one problem may look like something else when your are trying to diagnose something. I'm having trouble understanding what is happening. Thanks for any enlightenment you can give me.