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

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Old 02-14-19, 06:45 AM
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Furnace continously shutting down due to lost or low flame sense (Rheem)

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
 
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Old 02-14-19, 08:22 AM
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If the codes reflect a flame sensing problem, playing with the condensate trap isn’t going to help the situation.
While I’m not very familiar with that furnace, you mention it’s a modulating unit. The inlet and manifold gas pressure should have been set during commissioning. If it’s not set correctly the gas pressure will be too low during low fire and the flame will drop out. If supply pressure is dropping during high demand a similar problem will occur.
 
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Old 02-14-19, 08:32 AM
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Thanks for your reply. I was playing with the drain because, each time that I cleared the drain (and after it was replaced), the furnace seemed to run okay for about a month following. I don't have anything to check pressure, so I will have a tech do that.
 
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Old 02-14-19, 08:48 AM
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You probably just reset the error code after clearing the drain, by turning the furnace off and on.
 
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Old 02-14-19, 11:21 AM
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A problem with the flame sensor won't usually allow the furnace to run for a while and then fault. A flame sense problem is not always a sensor rod problem. Many times thru a burner problem the rod is not completely engulfed in the flame. Low gas pressure could also cause that.

Rheem manual
 
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Old 02-19-19, 11:14 AM
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The Short Answer: Change the Filter

I had previously replaced my filter on Jan. 2. However, after lots of experimenting, I ran the furnace without a filter for an entire day without any issues. I then put in a new filter and have not had any issues since.

My furnace may have other issues, but that seemed to have fixed this problem.
 
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Old 02-19-19, 06:28 PM
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The wrong filter should have given you a high heat fault..... not a flame loss or flame sensor fault.
I'll be watching for your return.
 
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Old 02-19-19, 07:21 PM
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Oh, well, then in that case I will be back when the problem returns! The problem happened to go away the same day I replaced the filter, but that must have just been coincidence.
 
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