We have a new (MAY09) Lennox G71MPP-60D-135 furnace which has been problematic, but likely because of poor installation. The latest problem is that the furnace shuts down after about 15-20 minutes of use (the error message is E-225: High Pressure Switch Failed Open). The technician (the third of three who have come out to examine system problems) says that the fault is likely caused because the input and exhaust piping is too small (2" rather than 3"; the piping was not replaced when the old furnace was). Rather than shell out another $1000 (or more), is this the likely reason for the system to shut itself down?