Arrived to inspect rtu15 System is a multistage(two separate compressors) gas furnace Multistage air conditioning and multistage gas furnace First stage cooling fully is functional, refrigerant pressures are appropriate with indoor and outdoor conditions Second stage cooling is low on refrigerant, system is operating below 32 degrees, system is approximately 2lbs low of r-410a possible causes of low refrigerant , refrigerant could be lost by previous repair and services or by possible leaks in the system System thermostat is a legacy style mercury switch, which offers limited functionality and precision Some surface rust from exhaust heat exchanger(photos attached)