Upon arriving I discussed with customer what was going on. The system was gurgling water and was blowing cold air in the mornings. I found out that the furnace had been cooling in heat and made changes to the dip switch to the Thermostat to allow furnace to run the heat pump or gas heat. Next I found a way to shim flue up to get better slope due to the low spot in the flue length. I used a 2x4 to shim the flue up in the center. Talked with customer and let the, know what was done and if they had further concerns to contact us.