I have a GeoLogix Model: HTV048B1C01JLK by Comfort-Air and have been experiencing a code 4 fault (Water coil low temperature FP1). This is a ground source system located in Vermont with a 400 ft. standing column well normally running around 47°. The heat pump is located in an insulated cabinet in my garage and this morning it was about 50° in the cabinet. The fault only occurs when the system has been idle overnight or during the day when the system is in Energy Star mode. This morning at 7:30AM the house was still 70° and the system had not run since 7:00PM. I set the thermostat to 72° and monitored the water temperatures. When the system started the temperatures were; Incoming: 39°, Outgoing: 34°, Difference: 5°. After the system ran for a minute or so the temperatures started to increase reflecting the colder water in the line being replaced by the warmer water from the well. After about two minutes there was an increase in sound coming from the system and the compressor shut down with a code 4 fault. I restarted the system and everything is running fine. Temperatures; Incoming: 44.4°, Outgoing: 39.2°, Difference: 5.2°. This fault does not occur after the system is restarted and only occurs when the system has been idle. Has anyone experienced this type of situation or have ideas on how to remedy this problem. The contractor is aware of the situation, however he hasn’t come up with any solution.