Goodman Flame Rollout Sensors Shutting Down Furnace

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Old 10-17-17, 08:50 PM
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Goodman Flame Rollout Sensors Shutting Down Furnace

My propane-fired Goodman GMS950904 CXA keeps tripping out due to the flame rollout sensors. They are the manual reset click-disc type so it's easy to tell when this happens. Also the diagnostic light tells me the same thing. With the furnace running I can place my finger right next to the sensors and can hold it there. The air temperature there is no where close to the 200F limit of the switches. However, the metal cage (lots of holes and cutouts) on which the sensors are mounted that supports the burner tubes, igniter, flame rod etc. gets quite hot--apparently hot enough to cause them to trip. That cage is in turn mounted to a metal plate directly in front of the heat exchanger assembly. So heat from there is the causing the rollout sensors to overheat.

There isn't what I would call flame rollout per se from the burner tubes. There's a little curling up of the flames. After the furnace runs for maybe 15 minutes one of the two rollout sensors gets hot enough and shuts things down. Both of them have done this. I've replaced both and the problem persists. I'm wondering if the primary limit switch is supposed to shut the gas valve before the rollout sensors get too hot, and then open up the gas valve when the primary limit switch cools below its lower limit.

The gas manifold pressure is 10" water column. New furnace filter. I blasted out the flue pipe with my big backpack leaf blower--nothing came out. The intake pipe has a coarse plastic grid on it and I can feel cool air flowing into the combustion chamber area so it seems clear. I've opened all the registers in the house. Secondary heat exchanger is clean as is the AC coil on top.

So should this furnace be cycling the gas valve and open and closed? I also replaced the primary limit control. So if the gas valve should be turning on and off is there anything other than the control board that could be malfunctioning?
 
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Old 10-17-17, 09:18 PM
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It sounds like the furnace is over firing. Checking the gas pressure is the first thing I would have recommended. The high limit's job is to shut the burner down only if the heat exchanger gets too hot. It's the job of the rollout sensors to trip if the heating area is too hot.

I'm not really sure where to go from here. Possibly one of the other pros can offer a recommendation.

In further researching I found your thread from last year. I was going to merge them because as I mentioned in the old thread.... when a new one is started..... all the same questions get asked again.

Link to the old thread.... goodman-flame-rollout-limit-circuit-open.
 

Last edited by PJmax; 10-17-17 at 09:45 PM.
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Old 10-18-17, 06:40 AM
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New Problem!!

So now the burner will not start. I get one flash on the LED which is lockout due to excessive number of retries (3 total). If I power off the furnace for a while and then turn it back on it immediately goes into lockout without the igniter ever having turned on. What the what??

I agree it sounds like overfiring--things are just getting too hot. I should mention this happens with the furnace door off which should make the area in front of the heat exchanger cooler than normal. And yet it trips out every time it runs. Also, this problem has gradually been getting worse with time over a period of several months (excluding summer vacation for the furnace). Does that suggest anything?

Let me go back to how the furnace should be operating. Normally, should the primary limit not shut off the gas
during a run cycle? In other words, on a call for heat should the flame not shut down until the room thermostat tells it to? Or is normal for the flame to cycle on and off?

Sorry about my post. Instead of adding on to my earlier post I thought I'd condense information previously contained in several exchanges to save people the trouble of reading through a series of posts to extract the relevant information.
 
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Old 10-18-17, 05:19 PM
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The flame should not cycle. It should be on until the call for heat ends.
 
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Old 10-20-17, 08:12 AM
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New Information

So I swapped out the furnace control board with another board from a similar furnace in the attic. The basement furnace still trips out due to the rollout sensors. So that knocks out the control board as a possible cause.

I've been thinking about how this problem has gradually but steadily gotten worse over the last several months. What could do this? I'm thinking maybe cracks or holes in the heat exchanger getting bigger allowing the sheet metal box around the heat exchanger to run hotter? I also thought about soot building up inside the burner tubes. It's to hard to quantify this but I'd say the flames are maybe 40% yellow and 60% blue. Is that acceptable for propane? But I think soot would just impair the heat transfer from inside the tubes to the circulated air flowing over them, so the heat exchanger box would run cooler. Comments?
 
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Old 10-20-17, 09:45 AM
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Back to the basics: Have you measured the temp rise (delta-T) of the air going through the furnace? In addition to over firing if you don't have enough air flow things will overheat. Check on blocked registers or pet hair getting past the filters.
 
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Old 10-20-17, 11:12 AM
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See if the flame pattern changes when the blower comes on.
 
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Old 10-20-17, 08:07 PM
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Astuff: I like the basics. No, I haven't measured the temp rise through the furnace; I will do that. All registers are open. The fins on the secondary heat exchanger and A coil are clean. We do have a cat that is an ambulatory fur factory. But I just changed the furnace filter.

Muggle: I did check that but forgot to include that bit of information in my write up. I don't notice any disturbance of the flames when the blower motor turns on.
 
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Old 10-22-17, 08:11 AM
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Temp Rise Data

OK, so I measured the temp rise through the furnace and got 50 degrees F. The name plate says the allowable range is 30 to 60 degrees F. So I sure don't see a smoking gun there, unfortunately. Any other suggestions?
 
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Old 10-22-17, 09:30 AM
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Other thread mentioned that you are using propane. Related to over firing: Can you verify that a LP conversion kit was installed? There might be a sticker saying that it was done.
 
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Old 10-22-17, 11:36 AM
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Yes, there is a sticker on the gas valve that says LP. The gas valve has been adjusted to 10" water column. I don't know how to ensure the gas orifices have been properly changed out. If you can tell me how to do that I'd be happy to check them. They've been in the furnace since the install.
 
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Old 10-23-17, 11:20 AM
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Could This Be It???

So I decided to measure the single capacitor connected to the blower motor. The rating on the cap is 20 uF +/- 5%, or 19 to 21 uF. My cap measures 14.5 uF. I'm wondering if this thing has been degrading over time and is now causing the blower to run slower, pull less heat out of the furnace and thus cause an overheat condition. I just did a quick search and read that fans will slow as their capacitors lose their capacitance with age. I think it's off to the supply house for a new cap.
 
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Old 10-23-17, 08:52 PM
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It's possible that the blower could be running a bit slower but that will usually cause the temperature rise to increase.
 
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Old 10-25-17, 04:52 PM
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OK, So Another Thing It's Not

I replaced the out-of-spec run cap for the blower motor but I still have the same old flame rollout senor failure.

I'm really scraping the bottom of the barrel here. I have a furnace tech coming tomorrow with a snake camera. At this point I don't know what else to check but the heat exchanger.
 
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Old 10-25-17, 08:13 PM
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Check the orifices too and make sure they are the propane ones. I'm not sure how you'd know unless there was something stamped on them. The tech may recognize the propane ones.
 
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Old 10-26-17, 12:31 PM
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And The Problem Is......

.....a cracked heat exchanger. A combustion analyzer read 1200 ppm CO in the flue gas. But the cause of the failed heat exchanger is that the furnace was never properly converted to LP gas. The gas pressure was changed from 3.5" water column to 10" but the gas orifices were NEVER changed. So the furnace has been overfired since day one. I truly hate when people do half-assed work.

Thanks to all for your suggestions.
 
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Old 10-26-17, 02:56 PM
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Surprised it didn’t soot up with the wrong orifices
 
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Old 11-15-17, 07:06 PM
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Are you getting the heat exchanger changed under warranty?

would be a shame to scrap the whole furnace.
 
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Old 11-18-17, 08:41 AM
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Sorry to jump in on this convo, but I am having somewhat of the same problem. My Goodman furnace goes out on flame rollout unless I keep the front cover off the unit. If the unit is running, and i put the cover back on, it will reach setpoint but then it wont start up again when it calls for heat. If I leave the front cover off, it will run fine all winter. Does this have something to do with the intake? The vent pipe goes to the outside, but there is no pipe for the intake side, it is just open to the basement. Any help would be greatly appreciated so I can get this thing to run with cover on and no more exposed flames. Thank you.
 
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Old 11-18-17, 09:32 AM
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Welcome to the forums.

I will be moving this to it's own thread.

You posted in a thread with a furnace running on propane.... are you on propane?
You need to check or have the flue checked for obstructions.
The next step would be to have the gas pressure checked at the valve.

You should
 
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Old 11-18-17, 02:24 PM
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Sorry for not clarifying - yes I am running propane. It is a Goodman GMSS961005CNAA Furnace. As I said, everything runs fine as long as I have the cover off which makes me think it has something to do with airflow. Exhaust flue is clear.
 
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Old 11-18-17, 03:40 PM
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It sounds like more of an overfiring problem..... the same problem the OP had. He had the wrong orifices installed. I still recommend having the gas pressure checked.
 
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