Four years and countless calls to technicians, no one knows what's wrong with our furnace. Can you figure it out?
Some background information: We moved into our home four years ago (2021). Nice, new HVAC system installed in 2019 that had worked fine for the previous owners.
In the sale of the home, the installation company had to replace the intake and exhaust due to not following city code. The furnace began throwing error code 31 (pressure switch) the first time we really had to use furnace that winter. It didn't always trip, but many nights we woke up to 55 degrees unless we had electric heat on. The furnace also began giving error code 33 (high limit). We had the install company out countless times over the first two years for both error codes, but they never had a solution. They always told us the PVC was fine, but that was the only thing that had changed. They finally said the furnace was oversized and gave us a brand new furnace (Bryant 80,000 BTU to Payne 60,000 BTU) to fix both error codes 31 and 33.
With the new furnace over the past two years, the furnace continues to give error code 31 (and less frequently error code 33). Not as often as the old furnace, and mostly not when the crew is there. They usually tell us it's fine, and just watch it (then it goes out a half hour after they leave).
For the new furnace, they have replaced the pressure switch, added a new control board, changed the condensate drain, cleaned the trap, adjusted the gas pressure, we hired an electrician to completely rewire the furnace, they also added a hose to the exhaust that directly drains into the condensate drain, and many more attempts at a solution. We have no faith in them, but we’ve used the same company so far because they haven’t charged us for the service calls or our new furnace.
Our theory has always been the exhaust/intake. The first four years they said the reroute was fine (many people said this), but in our measurements, the exhaust/intake were out of specs with many things. They finally took the exhaust off this year and > a gallon of water came out. These pictures are after three fixes to it, and then they completely re-did it. They shortened the run (after fixing it, it has 30 feet of total run), took out all of the short 90 degree elbows originally there (now with elbow calculations about 80 feet total, previously >140 feet), and changed to 3 inch PVC (was two inch). These are now technically within specs. It is in an unconditioned space, and with three inch, it should only be 10 feet uninsulated. And the slope is still wrong (even after the three “fixes”)!
With the longest run across the garage being (>12 feet) with only 1.5 inches drop toward the furnace. That’s a slope of 0.13 at that long section. The manual states the slope should be >0.25 inch per foot. We have been told by many technicians over the past four years that the slope doesn’t matter.
They say with the improved airflow the slope doesn’t matter. I tell them it’s about the drainage, and they say it’s draining fine. But obviously it’s not when it keeps throwing error code 31 (and sometimes error code 33, not sure if this is two separate problems. They also only installed with one return in the whole house, but again, the previous owners had no issues with the old oversized furnace).
What am I missing? I feel like I’m going crazy talking to the technicians!! They have never been able to explain why the numbers I give them don’t matter. Am I wrong? If I am wrong, what has been causing all this trouble?
TL; DR: Does the slope of the exhaust really not matter in Error Code 31? If not, what has been plaguing us for four years?
EDIT: Thank you everyone for all of your advice and support. This thread really helped us gather our thoughts and come up with a game plan.
Fire current HVAC company
Adjust slope (luckily on the long run, we do have enough space above garage door to drop it an additional three inches by adding 45's; no idea why they didn't do this on the most recent installation as we specifically asked them to do this and it will make the slope in specs).
Insulate the exhaust with at least 1/2 inch insulation
If it still throws and error, break regulations and terminate exhaust above garage door? Breaking code makes me nervous, and we would have to do it ourselves, but at this point it might be worth it to have a working furnace.
If that doesn't work, back to square one with a different company.