Since I moved into this new house, for past two winters the furnace is not working okay. Carrier Payne PG95S installed in 2013 by original owners of the house who sold it to a renovator in 2015 and he flipped house to me in 2017. Initially, I noticed it ran for 3-5 mins and stopped. I called an HVAC Tech who saw code 31. Could not find any reason other than suspecting that vent pipe sizing is inadequate. He checked vent pipe CO reading 5 ppm, gas manifold pressure 35 wc, condensate & pressure hoses checked, took pressure switch readings, checked power at nest thermostat okay, refastened trap. Since basement ceiling was previously finished by the renovator owner before I moved in, the vent pipe is not visible other than viewing it partially through a wall opening at the top of the furnace room. On the second visit, the Tech replaced pressure switch and condensate trap. He adjusted vent pipe slope of the vent pipe in the furnace room. However, that did not resolve the issue, furnace errored out but ran up to 15 mins rather than 5 mins previously to error 31 (pressure switch open). He mentioned since vent pipe is the cusp of specs, let's try replacing it. Another tech from the same company visited, found water stuck in inducer motor, found no blockage in motor and drain, thought vent pipe is long running. I took the second opinion before going ahead with the vent pipe replacement. This company tech found code 31 (pressure switch open), runs for 5 mins after reset. Pulled vent pipe from inducer motor, found water accumulated, drained it on the floor. This tech found CO readings were too high 30 ppm, so replaced heat exchanger. They checked/adjusted slope of vent pipe as well. The furnace did not error on code 31 (pressure switch open). Rather it errored on code 33 high limit switch open, furnace overheating. Air temp behind sensor was 110 F, supply air 140 F and high limit switch rated for 180 F. They replaced high limit switch thinking the premature failure of the switch but it did not resolve. They checked the neighboring house for ductwork and airflow and found one of the walls that were brought down by renovator owner had a return. So they opened a return on that duct from the basement and increased the furnace fan speed. They also replaced the Nest thermostat (found that fan function was not working okay). Thereafter C0 readings were 9 ppm, 71 degrees F flue termination. The furnace worked okay last winter. This fall/winter, it showed high limit issue again. It high limits in 15-20 mins. The tech thinks it is air flow issue this time as well. We kept bottom furnace door open. The furnace ran okay for few days. However, it then started showing error code 31 pressure switch open. The pressure switch remains closed for a min, then opens up. The cycle continues for three times and then only the blower keeps running. Gas/burner does not come on until I reset the furnace. I got the third opinion. This company HVAC tech did pressure test, found 0.7 inches on supply vent (said its higher than usual, but I have air conditioner coil as well, AC coils are clean). Applied black tape to the back wall of the heat exchanger, took Thermal camera measurement, 100 F on the outside edges of the back plate and 180 F at the center where high limit switch is. He thinks that heat exchanger is loose (found couple of screws missing in the back plate that hold up the restrictor plate that forces air to go through center rather than sides of the heat exchanger). However, upon hearing that code 31 pressure switch error is also showing up, he is thinking if this furnace must be replaced. Thinking the draft inducer motor is defective and not able to push out the flu quickly, I replaced the motor with a second-hand motor from the same year as the furnace (2013). The new motor was too costly. However, the issue is not resolved. If I keep the bottom furnace door closed it still shows code 33 high limit error code as previously in 15 mins. With the door closed, it will show code 31 pressure switch open. It takes a bit longer than 5 mins previously to show this error. I will greatly appreciate any thoughts in troubleshooting this. Next, I am thinking of replacing the white PVC vent flue pipe with an aluminum duct pipe, terminating through the basement window to test if this is an inadequate vent pipe issue.

I’m very handy I used to be a residential carpenter and am now an instrumentation and Controls tech and an electrical journeyman apprentice. I am also overly cautious when I’ve never done something before and am a little worried about doing it myself, but I just got a ballpark estimate from and HVAC guy and it was a lot more than I want to pay! I’ve only added duct runs from an existing trunk line, but I’m hoping with a little instruction and or direction I could do it myself. Can anyone give me any information or direction to help me decide if it’s worth doing it myself? Currently there is no system, it’s all electric and expensive so I definitely need to do something!