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Gas furnace short cycles / trips limit unless filter is out or panel is open


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01-03-18, 09:03 AM   #1  
Gas furnace short cycles / trips limit unless filter is out or panel is open

Hi all.
I've been fighting a short cycling furnace for the past few weeks, and would appreciate any ideas.

This is the furnace:
8 years old
Natural Gas
Nordyne GL1RA 108C - 16B
Pretty basic furnace, nothing highly efficient.

These are the symptoms:
Thermostat calles for heat, furnace fires up.
Blower starts (albeit is with a slight laboring whopp ..... whopp ..... whopp .. whoop .. whopppppp)
After about 6 minutes of closer run-time, a relay clicks once or twice on the control board, the red indicator light flashes (hard to read the code, since the flashing is irregular), and the burners cut out.
Blower continues to blow a little bit longer, and eventually stops.
Furnace starts up again, and so on, until finally the set temperature is reached.

The only thing that will prevent the furnace from short-cycling / going into limit is
a_ taking out the filter
b_ Opening the filter access panel and leaving it open for additional air supply
c_ Opening the blower panel and having it open for additional air supply

What I've done so far:
1_ Made sure all registers are open.
2_ Made sure all returns are un-blocked.
3_ Tried every filter known to man, even some cheap spun fiberglass filters, to lessen air flow restrictions.
4_ Jumped the thermostat wires to make sure the thermostat (nest) wasn't the issue.
5_ Replaced the blower's capacitor (I was convinced that this was the culprit, since the microfarads were out of spec. Alaska new cap did not improve the situation)
6_Checked the coils, and they are all clean.
7_Cleaned motor and blower.

What I haven't done so far:
i_Thoroughly cleaned the air return ducts with a vacuum.
ii_Cleaned the flame sensor (did that a year ago, and figured the burners would cut out much sooner if that as the issue)

Where I am right now:
Since it seems like it's an air flow issue, I'm leaning towards the motor that needs to be replaced. But I would like to pin that down and confirm before I order a new one. Any thoughts on how to diagnose a motor that is under-performing and hence contributing to overheating?

Any other ideas or suggestions are much appreciated as well.
Thanks so much!

- olaf


Last edited by olaflikesbikes; 01-03-18 at 09:30 AM.
 
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PJmax's Avatar
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01-03-18, 09:19 AM   #2  
If the blower motor is making noise..... that is most likely the problem.
The only other thing left is a dirty A/C evaporator coil if you have central A/C.


~ Pete ~

 
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01-03-18, 09:30 AM   #3  
Thanks Pete. Appreciate it.
To be clear though, the whopp whopp whopp is just for about 2-3 seconds during start-up. Any way to diagnose a bad motor to know for sure?

And, forgot to include in OP, I did check on the coils, and they were clean.

 
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01-03-18, 09:58 AM   #4  
Yes.... you said you checked the coils. Did you check both sides ? The dust will be on the incoming side of the coil while the backside of the coil where the air comes out will be clean.

It sounds like your furnace is suffering from the typically undersized return ductwork.


~ Pete ~

 
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01-03-18, 10:06 AM   #5  
Short term fix - try running the fan at a higher speed. Looks like yours is factory set to medium-high so change to high - maybe by turning on the fan setting at thermostat.

 
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01-03-18, 10:30 AM   #6  
Pete -
My coils are in a triangular shape, with left and right angled sides.
Like these:
Nordyne 921951 COIL ASSY at Controls Central

Can you clarify how I would clean the "incoming side"? Both left and right exterior sides are clean right now.

Thanks.

 
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01-03-18, 10:52 AM   #7  
AStuff -

Interesting. I did not know I could bump it up.
Any more specific thoughts on how to change the fan speed?
I have a nest thermostat. If I run the fan by itself with the setting on the thermostat, the furnace would not be activated, so the heat wouldn't be an issue.

Thanks!

 
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01-03-18, 11:27 AM   #8  
AStuff -

Nevermind. I think I found how to do it. On the control board. Will try tonight. Thanks.

 
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01-03-18, 06:14 PM   #9  
Update:

1_Set the fan speed for Heat to High. No luck. Still short cycling.

2_ Measured the voltage on the limit switch. When the control board throws the code and cuts the gas, the control board side of the switch cuts out, but just for a few seconds. Meanwhile, there are some relay clicking noises. Then it goes right back up. This is odd, isn't it? It was my understanding that the control board side would always continue to supply 24 V, and that the switch then cuts the connection to the blower.

Another thing I noticed here was that during blower operation, the control board only supplied about 22 V, but I read 25 V on the blower side. After the control board cut the power for a few seconds, it went up to 25 V.

Lastly, here is a video of the light flashes. It's unclear to me if this is is supposed to be read as 4, or if its a repetition of 1. 1 Would be limit, 4 would be "Ignition Failure (Check Ground)".

https://www.youtube.com/watch?v=vc5ZvbWlWbI

Any help is much appreciated! Thanks!

- olaf

 
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01-03-18, 07:34 PM   #10  
Very hard to clean the inside of those coils but that's where the dust would be.

The yellow LED is the flame sense strength which shows good.
I couldn't get a code from the red light. It looks like the board lost power before it was done.

When you changed to high speed... did it sound like the blower was running faster?
That should have taken care of the overheating.

Where were you measuring the 24v ?
There will be 24vac to the limit switch and then 24vac back to the board. If the limit switch is opening... the 24vac to the board should stay gone until the switch cools. It sounds like that switch is jittering.

You can do a heat rise check to see if the furnace is indeed overheating. Measure the incoming return air and the outgoing heated air. Subtracting them will give you the heat rise. The acceptable rise will be listed on the ID tag neat the burner.


~ Pete ~

 
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01-03-18, 09:43 PM   #11  
Couldn't really tell if the blower was spinning any faster.

I measured the switch voltage on both sides on the switch terminals. Am I understanding you correct when you're saying that the control board side DOES cut out the power when the limit switch goes into limit. So measuring 0 VAC for a few seconds on that side on the switch would be correct?

I'll see if I can get some temperature readings on the outgoing air.

Also, why would to board loose power in the event of the switch going into limit?

Would it make sense to jump the limit switch for a few minutes to see if that will avoid the short cycling? AS a troubleshooting method, not a permanent fix, of course.

Thanks,
- olaf

 
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01-04-18, 08:15 AM   #12  
It doesn't happen very often but a limit switch can wear out. I have a very simple Coleman NG furnace and it kept limiting out so I bought a new limit switch and that fixed it. Just make sure you get one the same temperature.

 
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01-04-18, 08:38 AM   #13  
Baldwin -
Thanks. Were you able to diagnose the limit switch as being faulty before you replaced it? Or was it jut a hunch?
Any idea on how to troubleshoot / pinpoint it?

Thanks.

- olaf

 
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01-04-18, 09:33 AM   #14  
Put a voltmeter directly across the switch. One probe on each terminal. There should be NO voltage measured. If the meter reads voltage.... the switch is open or bad.


~ Pete ~

 
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01-04-18, 09:39 AM   #15  
Pete -

You mean during normal operation? Furnace blowing?
Or during the short shut-down?

During normal operation the switch would measure 24 VAC, no?
And in the few seconds it cuts out (at least observed on my unit), it would read 0 VAC.

That's what I measured yesterday. Except that on the control board lead it measured only 22 VAC on the switch terminal during normal operation.

Please confirm.

Thanks!

 
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01-04-18, 09:46 AM   #16  
Just a hunch after I tried everything else. Your furnace a way above my pay grade, Pete knows much more than I do.

I don't remember now but the one for my furnace was pretty cheap, yours might be more.

 
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01-04-18, 10:08 AM   #17  
During a call for heat............................................

A limit switch is always closed or shorted in the normal state. When tripped it should be open for more than a few seconds. If it opens for only a few seconds and then re-closes.... change it.


~ Pete ~

 
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01-04-18, 10:25 AM   #18  
Got it.

Any change on the limit switch readings (across the terminals) when the main blower kicks on?

Thanks.

 
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01-04-18, 12:48 PM   #19  
What you are watching for is voltage across the switch. If everything is normal.... there is 0v across the switch. The blower turning on should not affect it.

Usually once a high limit switch opens..... it stays open for at least several minutes depending on how fast the system cools down.


~ Pete ~

 
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01-04-18, 01:34 PM   #20  
Got it. Thanks!

Based on that, I'll jump the switch tonight to see if the control board still cuts out the gas burners after 3-5 minutes. Will report back.

 
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01-11-18, 09:08 PM   #21  
Hi all.

I was traveling for a week, and as I came back, the furnace was out, throwing a code 4, which is "Ignition Failure." Here's what's happening now:

Ignitor glows.
Relay on board clicks.
I read 18 VAC on the gas valve.
No flame.
Audible thump on gas valve, 0 VAC.

I know I should be reading 24 VAC on the valve, correct?
Control board?
Gas Valve?

Any thoughts are much appreciated. Cant really wrap my head around how this might be connected to the furnace short-cycling earlier, unless the filter was removed. One step a t at time now I guess, need to get heat back into the house...

Thanks much!


Last edited by olaflikesbikes; 01-11-18 at 09:47 PM. Reason: typo
 
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01-11-18, 09:23 PM   #22  
You should be measuring 24vac on the gas valve.


~ Pete ~

 
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01-11-18, 09:52 PM   #23  
Yes, typo. Of course, should be 24. Measured again, just getting shy of 17.5 VAC.
But, through some vigorous tapping, I was able to coax the valve to fire the burners as it was getting power. The flames would kick off as soon as I stopped tapping, but once they stayed on for about a minute, until I heard the board relay clicking and the gas turned of.

Sounds like the gas valve, but the low voltage is odd no? Thoughts?

Thanks mucho!

 
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01-12-18, 07:12 AM   #24  
SOLVED!

After reading just 18 VAC on the gas valve, which oddly appears to be enough to coax the valve into opening for a short period of time through some vigorous tapping on it, I tried to identify where the power drop occurred.

Starting at the transformer, I read 27 VAC. Moved on to measuring all the safeties, and noticed a pretty significant drop in Voltage occurring across the high limit switch. Jumped the high limit switch, and Bingo. Somehow the high limit safety seemed to have dropped the voltage enough to create unstable conditions for the rest of the furnace. New one is on the way.

Thanks for helping!

 
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01-12-18, 11:20 AM   #25  
This probably is not the problem, but make sure that the blower is spinning the right way and that the blower wheel was put on the right way. It does not happen often but I have seen the wheel installed wrong and/or spinning backward. Also make sure that nothing is in the blower wheel, paper etc.

 
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01-13-18, 09:41 PM   #26  
Thanks, I found the culprit in being a faulty high limit switched.

 
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01-17-18, 03:31 PM   #27  
FOLLOW UP:

Installed the new limit switch, and bingo. Works like a charm.
I did however notice a lot of heat coming from the exterior sheet metal of the plenum. The part the the exhaust fan attaches to, just above the burners. I put a temperature probe right on the metal, and measured 200 degrees. Limit switch did not go into limit.

Can it be normal for the metal to get this hot?

Thanks!

 
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01-17-18, 03:37 PM   #28  
Can it be normal for the metal to get this hot?
It does get hot.

What was the temperature of the limit switch ?
If it's close to what you've measured.... that limit switch will be active again.


~ Pete ~

 
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01-18-18, 08:37 AM   #29  
Temperature limit on the switch is 170.

Thanks,

 
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