Correct size for furnace flue pipe?
I have a new construction house with a Bryant Evolution system. Friday morning we woke up to find the system was shut down and a message of, "SYSTEM MALFUNCTION CALL TECHNICIAN." The installer was out of town for the holiday so I called a local company that I have used before with 24/7 service. The technician comes over, runs a diagnostic, goes into the attic and then goes outside to look at the flue pipe on the roof.
He says that the flue pipe coming out of the roof is not venting correctly thus causing an air pressure build-up which caused the system to shut down. He also said we were probably getting a build up of carbon monoxide (runs on propane). He said that the flue pipe is correctly sized inside the attic (6") but on top of the roof it is connected to a 4" dryer vent pipe and we need a flue pipe that is 6" and made for venting the furnace. He also said this was a city code issue as well.
So we decided to change the flue pipe and are just waiting for Monday to call a roofer because the HVAC technician said they don't cut holes in roofs due to liability issues.
In the meantime, the original installer calls this morning. I tell him what happened and what the other company's technician said and he says that's bull. He says that the specs of the system calls for a 4" flue pipe and that the problem is probably a "low pressure switch" needs to be adjusted. The other HVAC company says that switching out the flue pipe not including cutting the hole and installing a roof jack will cost $300.
Any thoughts or advice?