Upon arrival, we noticed the system was off due to trouble code on the furnace. The system was off due to an open pressure switch. After we reset the system we noticed a gurgling sound in the flue pipe due to a build up of condensation. Once we detached the flue pipe from the unit we noticed that the furnace operated normally. The flue pipe coming from the furnace doesn't have the proper pitch result in condensation build in the flue pipe. We must return and make proper modifications to ensure the system will operate to specs. Homeowners have been briefed on the current status and assured we will return the next day to make modifications.