Furnace continously shutting down due to lost or low flame sense (Rheem)

Question

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

Answer 1

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.

Answer 2

A few key points to consider. The venting (intake and exhaust) must be installed properly and terminated properly. If you don't have the intake piped to outside you may have dirt accumulating on your flame sensor. Also check gas pressure. If your gas line is undersized, at full modulation your fuel to air ratio will be low causing a flame out. Make sure your condensate drain is at least 1/2 inch and VENTED after the trap. Most furnaces have a internal trap so there should be a tee, not a 90 coming off of the condensate. The tee should be open to the atmosphere, not connected to the air conditioner or humidifier drain. And yes they should be installed level or SLIGHTLY forward to allow drainage from the secondary heat exchanger. Also not having the intake piped to outside leads to build up of gunk in the condensate collection system.