My furnace will run great for most of its cycle, but when it has almost reached the thermostat setting, I will hear it shut off, run the blower, then re-light. It will sometimes finally reach temperature, but other times it will continue to cycle until I shut it off.
I had the installer look at it in Nov. 2016 and they told me "cannot find anything wrong." Then in Feb. 2017 they told me "unit has issues with gas valve" and replaced the modulating gas valve. I continued to have issues, so in April 2017, they replaced the flame sensor.
The next year, I continued to have issues. At one point, I finally heard what sounded like water in the inducer motor. I pulled the drain lines and cleaned the condensate trap. That fixed it for a while, but then the issue started happening again.
The problem started again this past December. I had a different HVAC tech come look at it, and he replaced the entire condensate trap at the beginning of January. He also told me the furnace was tilted away from the trap and that made it hard to drain water and also that the inducer was going bad (it's extremely loud) due to the water that had backed up.
The furnace ran great for about a month but is now having the same issue. The error codes are always either "low flame sense" or "lost flame sense."
I have spent about $900 so far, and while I don't mind paying to have something repaired, I feel like I've been chasing the same problem for two years now. Any help is greatly appreciated!
Model: Rheem RGGD-09EZCMS
Best Answer
I'd definitely correct the slope regardless, you may be able to use a pry bar and shims if you're carful. The gas valve can only deliver what it's given. Gas line sizing is a pressure/distance/capacity/diameter relationship. @7"water column (normal delivery pressure, at least where I live) a 120mbtu furnace with 50' of gas pipe between the meter and gas valve will require a 3/4" black iron gas line. 50' of 1/2" will only deliver 95mbtus. You have to allow for anything else like a water heater or stove. Tankless water heaters are usually 150-200mbtus.
It's hard to say what the problem is without seeing it, but it seems like you've explored other potential problems already. It could be a combination of issues too.
P.S. Pipe size is measured by the internal diameter. 1/2" pipe is actually about 3/4" outside diameter.