Intermittent/Premature Shutdown of Ruud Gas FHA Furnace


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Old 02-07-13, 03:12 PM
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Intermittent/Premature Shutdown of Ruud Gas FHA Furnace

Hello All,

I've benefited many times from reading various DIY threads but this is my first post. I'll try to provide enough info without going overboard. I have a Ruud forced hot air (natural gas) furnace model UGDG-15EARJR that has developed a difficult-to-diagnose problem. Briefly, the gas sometimes shuts off after the indoor blower starts but before the thermostat has stopped calling for heat. Shutoff is typically anywhere from 15 to 90 seconds after IBM startup. The ignition sequence starts again and most times, but not always, it will run to successful completion on the second try. I've not had it fail enough times in a row (four I think) to lock out on me yet.

The troubleshooting guide in the users manual suggests checking all the ground connections and cleaning or replacing the flame sensor. I have checked as many ground connections as I can find with my meter and all continuity seems ok. The flame sensor looked ok but I replaced it anyway. The current now measures 3.2 microamps when the flame is on so that seems ok. The hot surface ignitor is referenced as a possible culprit and although it was glowing every time and the flame lights without problem, I replaced it since it wasn't too expensive. No effect.

The filter is electronic and was just cleaned so I don't think airflow restriction is the problem. Just to be sure, I took the filter completely out and the problem still occurred.

The final suggestion in the user manual is to replace the circuit board. Since that runs around $200 I thought I would ask if anyone else has a suggestion. I really would hate to got to the trouble and expense of replacing the board and find out that wasn't the problem. The board has been replaced once about twelve years ago.

The last clue I can offer is that the problem seems to go away for awhile if the furnace doesn't run for a period of time. We have recently been away for two separate 5 day periods and turned the thermostats (there are three) down so that the system probably didn't come on while we were gone. After returning and re-heating the house, we went for multiple days up to a week or so before the problem started to show up.

Thanks in advance for any ideas.
 
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Old 02-07-13, 04:53 PM
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Does your furnace use PVC flues? If so, then flush the condensate trap with hot water. Also make sure all the hoses are free and clear of obstructions. If your furnace uses a metal flue then remove the hose from the pressure switch to the draft motor and make sure the draft motor hose connection is free and clear of debris.
 
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Old 02-08-13, 06:36 AM
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Thanks for the quick response. The furnace has a metal flue (connected to a sidewall power venter) so I'll check to make sure the pressure switch to draft motor hose is clear. There is also an A/C unit connected to the system which does have a separate condensate trap. Can I assume that this trap would not be engaged when the furnace is in heat mode? I will clean it anyway but won't do so immediately if it wouldn't matter.
 
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Old 02-08-13, 03:36 PM
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I'd use a multimeter to check and see if the pressure switch or limit switch is opening and shutting off the burners. The odds are pretty good that one or the other is shutting off the burners.

Check on that and let us know what you find.

In short, you want to identify WHY the furnace is shutting off. Then you can figure out what to do about it.
 
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Old 02-11-13, 08:13 AM
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Thanks again for the suggestions. Sorry for my slow response. I decided not to fool around with a furnace that is working (most of the time) during a major blizzard! In addition, since my first post the problem seems to have virtually disappeared. I wish I could say it was in response to something that I did. I will wait until it starts happening again, try these ideas and let you know.
 
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Old 02-28-13, 01:22 PM
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Hello all,

Back with a bit more info about this annoying problem. As I mentioned earlier, I chose not to do anything in advance of a huge blizzard and also because it suddenly stopped happening. A day or so after my last post I noticed I had forgotten to put the upper access door back on. Since it was still working perfectly I decided to leave it off for a while. I ended up leaving it off until yesterday (almost 3 weeks) and did not notice the problem once.

After replacing the door (and doing nothing else) the problem returned within hours. So I'm thinking it must be related to air flow somehow.

Today I pulled off the tube connecting the draft motor to the pressure switch. It was clean as a whistle inside. I poked into both motor and switch openings with a small wire and didn't feel any obstruction. Then I hooked my meter to the switch terminals to test for resistance. By sucking slightly on the tube I was able to confirm the switch was opening and closing without sticking at all. When open it read 0.L and when closed it read zero. Then I hooked everything back up (except for the leads to the switch) and tested resistance when the draft motor kicked on. It went to zero immediately and stayed there.

The flue is not very long (metal to power side wall venter outside) and I'm confident there is no obstruction in it. Gases exit smoothly to the outside.

After hooking the pressure switch leads up, I watched it go properly through one cycle with the door off.

So, I'm back to wondering what I'm missing. Any other things I should try?
 
 

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