RUUD Furnace cutting off

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Old 12-08-19, 10:17 AM
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RUUD Furnace cutting off

I have a Ruud gas furnace that stopped working at some point last night. I checked it out this morning and found the igniter to be cracked. I removed the igniter and lit the furnace manually with a match. It runs for several minutes, but then cuts off. Apparently the igniter isn't the only issue. I am sitting by the furnace watching it and the flame abruptly cuts out. The inducer motor runs for a bit, then cuts off. A few moments later, it turns back on a attempts to light. I cleaned the flame sensor and that didn't make a difference. I also noticed a large crack on the inducer housing. I find it odd that there are two issues that abruptly popped up at the same time though. What else should I check? Model is UGLJ-10EAMER.
 
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Old 12-08-19, 05:33 PM
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After a few hours of troubleshooting, I began to see a pattern, although I wasn't able to pinpoint the exact issue. I can light the furnace manually with a lighter, it runs for about 10 minutes, then the burners cut off, the blower continues to run, the inducer fan turns off, blower off, inducer on, gas valve on, then attempts to reignite (which I continued to do manually). Runs again for about 10 minutes then repeats this pattern. One time it cut back off after less than a minute, but the majority of the time it remained on for at least 10 minutes. I cleaned the flame sensor and this had no effect. I then observed the thermostat display going blank, which coincided with the burners shutting off. The t-stat would come back on after about a minute and send a call for heat. I replaced the t-stat to rule it out, and replacing it did not solve the issue. So, apparently something is malfunctioning in the furnace, whether it be a sensor or the control board, which is shutting power off to the t-stat. I've never observed a furnace shutting supply voltage off to a t-stat, so I'm wondering if it's a control board issue. Is it common for a furnace to cut power to the t-stat only to power it back on a minute later? Doesn't sound logical.
 
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Old 12-08-19, 05:56 PM
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I then observed the thermostat display going blank, which coincided with the burners shutting off.
That means a high limit sensor is opening. There should be a sight glass in the blower door where you can look thru and see a fault code blinking. The fault codes will be listed on the back of the blower door.
 
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Old 12-08-19, 09:30 PM
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I have the cover off and I see two LEDs on the control board. A green one for OK and an amber one for Flame. Both are lit. I don't see any other LEDs. This unit is 25+ years old.

What are the most likely things that would cause the high limit switch to open? I suppose I could replace the switch and hope that does it. If that doesn't resolve the issue, sounds like maybe the furnace needs cleaning perhaps?
 

Last edited by mossman; 12-08-19 at 09:53 PM.
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Old 12-09-19, 08:26 AM
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In addition to the crack in the draft inducer fan housing, there is a white crusty substance around the housing where it mates to the furnace. I read this is caused by nitric acid from the exhaust gases reacting with the zinc in the galvanized metal. If this is accurate, evidently I have a leak in the gasket around the housing. If so, sounds like the entire fan assembly should be replaced? I believe the plastic ones are known to be problematic and they now make them with metal housings.
 
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Old 12-09-19, 10:09 AM
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Is the blower starting and running correctly?
It could be the furnace is dirty, or overfired. Or low on airflow
 
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Old 12-09-19, 10:27 AM
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Is the blower starting and running correctly?
It could be the furnace is dirty, or overfired. Or low on airflow
Yes, the blower appears to be running correctly. Everything seems fine for 10 minutes or so, then the gas valve closes, the inducer fan stops a bit after, the blower continues to run for a minute or so, blower turns off, inducer turns back on, gas valve opens, flame re-ignites, unit runs for 10+ minutes again, and so on.
 
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Old 12-09-19, 10:28 AM
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What air filter is being used?
Check gas pressure, airflow and for dirt buildup.
 
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Old 12-09-19, 11:00 AM
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What air filter is being used?
Check gas pressure, airflow and for dirt buildup.
Just a standard 1" thick pleated filter. Changed it out yesterday (old one wasn't too bad). All other gas appliances are working fine and I'm getting nice steady blue flames. Don't see any dirt/dust buildup and air flow is strong.
 
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Old 12-12-19, 10:00 AM
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Could a hairline crack and/or leaky gasket around the inducer assembly cause the furnace to run hot and trip the high limit switch? Or is ot more likely the limit switch itself is bad? There's some dust buildup on the blower wheel, but not enough to cause a significant air flow restriction. Air flow is strong.
 
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Old 12-24-19, 12:02 PM
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I used my home warranty to have the inducer assembly replaced (because it made financial sense), but the technician didn't fix the underlying issue, which I still suspect is the high limit sensor/switch. The heat is working for now (because I replaced the igniter), so it's not ultra high priority, but I'll eventually need to figure out what the cause is. He said the evap coil could be clogged and need cleaning, but the air velocity coming out of the vents is pretty strong so I don't really buy that. I'll check nevertheless.
 
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Old 12-24-19, 12:19 PM
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What was wrong with the inducer that required it to be replaced?
Tread carefully, home warranties are notorious for substandard, poor quality work.
 
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