Goodman GMT115-5b Furnace - Inconsistant problems

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Old 01-02-18, 06:40 PM
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Angry Goodman GMT115-5b Furnace - Inconsistant problems

Hello,

I have a Goodman GMT115-5b furnace that has been having issues. It is super cold in NY right now and we are on day 5 of no constant heat! Thank goodness for electric heaters! =) ...due to the holidays, many companies were booked or not open. Any help would be greatly appreciated!!!

It started with the furnace working properly and then all of a sudden, it stopped staying lit and would shut off, going through this cycle several times (ignites, blue flames, then when orange glow goes away, everything shuts off). We replaced the flame sensor and still had the problem. Then, it worked fine the next day for a few hours, and then randomly it wouldn't ignite. Over the past four days we've had it work for a few hours, then not work for a few hours, then randomly work again.... very inconsistent.

With everything said and done, just to cover our bases.... we replaced the flame sensor, the ignitor, the filter, and the control board. Still having the same problems. I should also mention that when the problem began, it worked during the day, but then shut off as it got later in the day/night as it got colder. We also have a nest thermostat that warned us that the furnace was having difficulty because it was shutting off prematurely, but it only happened a few times in the past several months, but now it's daily. When we try to look at the status light for any clues to the problem, we are unsuccessful. It has a solid light when it is going through a cycle, but then the light turns off when the unit shuts down. It never blinks to give an error code.

Thanks in advance for any help! Right now it works for about 2 hours, doesn't work for about 6-9 hours, then works again for about another 2 hours each day.... 4 hours of heat per day just isn't cutting it! lol
 
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Old 01-02-18, 07:40 PM
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I'm not a furnace pro, but I've had my share of fights with Goodmans.

What exactly is it doing during all the hours that it's not working? In one place you said it would shut down after igniting, but in another sentence you said it wouldn't ignite. Does it lock itself out for an hour after 3 tries, or is it constantly going through cycle attempts without rest the whole time?

When it "shuts down", does the venter blower / draft inducer shut off at the exact same time that everything else shuts off?
 
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Old 01-02-18, 07:43 PM
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Welcome to the forums.

What I would do is use a meter to monitor the R to C voltage at the control board. If that 24vac is lost when the problem occurs then it sounds like you may have a thermal limit switch opening.

Manual... uploads/1/0/6/6/10667336/g.pdf
 
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Old 01-03-18, 03:56 AM
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Thank you for your responses!is going through cycle attempts

Core,
It is inconsistent. Sometimes it shuts off because it won't ignite, sometimes it shuts off after it's ignited and the blue flames are running for a few seconds, sometimes it functions perfectly. Most of the time it goes through cycle attempts without resetting, so we shut it down manually. However, keeping with the inconsistency, I do believe that there were a few times where it locked itself out after 3 attempts. Once it shuts down, the blower stops as well.


Pjmax,
I will use a meter when I get home from work and test it out!



Thank you again for your responses!!!
 
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Old 01-03-18, 11:28 AM
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In a lot of ways it does sound like the thermal cutout that Pjmax was getting at. (However that would not cause lockouts and I wouldn't expect it to cool off quickly enough for an immediate cycle attempt again.) From the board 9-pin connector there are 3 red wires. One red wire goes to your transformer; ignore him. Follow the other 2 red wires from that connector and they will lead right to the thermal cutout in question down there. This is not the same limit switch(es) that are on the purple wire chain -- the one you're interested in is on the red wires. As a test you could jumper this switch, but only momentarily while you're standing right there to see if everything lights fine. Do not leave it running that way for any length of time!

Use a thermometer to see if it's really getting up to the cutoff temp. (In just a few seconds, you say?) If it happens to be a bad sensor it's a cheap part, $10-12. But assume it's not just bad and investigate the cause/temperature.

(Edited to remove a bunch of stuff because I had the physical location of the sensor wrong until I looked at my own)
 

Last edited by core; 01-03-18 at 12:18 PM.
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Old 01-15-18, 11:39 AM
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Sorry for the delay... We usually aren't "throw parts at it" people, lol, but we definitely have been this time! It worked well for several days, then two nights ago and last night it stopped working at night, but then was okay during the daytime. We have now replaced the limit switch and pressure switch, in addition to the already replaced flame sensor, ignitor, and switch board. Still having inconsistent problems. We are now having someone come in later this week.
 
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Old 01-15-18, 01:26 PM
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On some control boards..... if the high limit opens three times during the same call for heat.... it can go to lockout on the fourth attempt.

I've been thinking about the fault light going out. Usually if it's a limit problem the fault LED still works although the 24vac out via the R terminal is shut down. You really need to monitor the 24vac to that board to see if it's failing.
 
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