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Old 12-02-2011, 08:03 PM   #1
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Furnace Cycle Problem


I have an old Carrier 58GSC that seems to be cycling too often. The sequence is something like the following:

1. Heat is called for by the stat.
2. Igniter fires and lights the pilot.
3. Pilot burns for approx. 1 min.
4. Main burner fires up and burns for another 1 min.
5. Blower comes on, with main burner staying lit, and runs for 2-3 min.
6. Pilot and main burner turn off, fan keeps running for approx. 2 min.
7. Igniter fires, lights the pilot, and the cycle repeats.

This doesn't seem right to me. Shouldn't the main burner stay lit with the fan running for a longer period of time? Any troubleshooting help would be greatly appreciated.

Thanks,
Dave

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Old 12-02-2011, 09:34 PM   #2
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Furnace Cycle Problem


filters clean? sounds like it is cycling on the high temp limit control from poor airflow. dirty AC coil can cause that problem too.

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Old 12-02-2011, 09:38 PM   #3
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Furnace Cycle Problem


has any thing been changed on the system..
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Old 12-02-2011, 10:39 PM   #4
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Furnace Cycle Problem


Bad 3 wire pilot. You can get one online around 80.00 bucks
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Old 12-02-2011, 10:39 PM   #5
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Furnace Cycle Problem


Yeah, I just changed the filter. I'm out in CA, and we're just turning the heater on. The new filter is the only thing that's changed on the system. Is there something I can test for the limit control? A dirty AC coil?

I did notice today, one time I had the door off the bottom of the heater, so airflow was probably nil, and the unit just kept running with the main burner and blower -- it didn't shut the gas off like it usually does. Does that mean anything?

Thanks,
Dave
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Old 12-02-2011, 10:48 PM   #6
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Furnace Cycle Problem


Tator,
Are you sure? I tried running the furnace manually by first shorting green/yellow on the spark generator side of the pigtail that connects to the 3-wire pilot. That gave me spark, and once the pilot was live, I moved the jumper to white/yellow, and the main burner fired. I left the jumper across white/yellow, but the gas shut off anyway -- both main burner and pilot went off. It essentially didn't change anything. Wouldn't this have achieved the same thing that the 3-wire sensor is supposed to do? I mean, $80 to fix the thing would be way cool, but if possible, I'd like to be sure.

Thanks,
Dave
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Old 12-03-2011, 08:19 AM   #7
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Furnace Cycle Problem


3 wire pilot it is a sensor for the flame of pilot and burners. Its also for ground for the system. If that 3 wire pilot has a break in the green wire. It will stop everything to work. I don't know how fast it will come for you. But I do have a old one that still works.
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Old 12-03-2011, 08:27 AM   #8
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Furnace Cycle Problem


"I did notice today, one time I had the door off the bottom of the heater, so airflow was probably nil, and the unit just kept running with the main burner and blower -- it didn't shut the gas off like it usually does. Does that mean anything?"

If this is a new problem with an old system, I'm only guessing that their is something clogging/restricting airflow through your system and a limit switch is shutting it down to prevent overheating. Does system have air conditioning? A-coil or similar may be clogged with dirt.
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Old 12-03-2011, 09:17 AM   #9
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Furnace Cycle Problem


Quote:
Originally Posted by dave54 View Post
Yeah, I just changed the filter. I'm out in CA, and we're just turning the heater on. The new filter is the only thing that's changed on the system. Is there something I can test for the limit control? A dirty AC coil?

I did notice today, one time I had the door off the bottom of the heater, so airflow was probably nil, and the unit just kept running with the main burner and blower -- it didn't shut the gas off like it usually does. Does that mean anything?

Thanks,
Dave
That means it is a high limit issue......it is starving for return air.....has furniture been moved around and now a return air grille is being blocked?
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Old 12-03-2011, 12:48 PM   #10
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Furnace Cycle Problem


Thanks Guys,
The air return is in my ceiling, so it's entirely unobstructed. So then, what you're telling me is that the airflow exiting the furnace is obstructed? I have one switch, which my owners manual calls a "Blocked Vent System Shutoff Switch." Is that a the "limit switch" you're talking about? And I do have A/C, so the evap coils are above the furnace, but the box looks sealed. Is this where I need to look for obstruction and try cleaning?

Tator, didn't my manual shorting of the wires that connect to the 3-wire pilot eliminate that as the potential problem?

Any help is appreciated.

Thanks,
Dave
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Old 12-03-2011, 12:51 PM   #11
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Furnace Cycle Problem


limit switch sits above the burners and usually has a rating stamped on it like L170 degF or higher. put a volt meter across it while running, if it opens then U will get 24 volt reading, closed gives U 0 volts.
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Old 12-03-2011, 03:28 PM   #12
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Furnace Cycle Problem


Well, it appear that I have more than one problem. Today, the unit started running for even briefer periods of time before the main burner shut off. I suspected the 3-wire pilot, so I manually jumpered the white and yellow wires which brought the unit back to its previous behavior of running with main burner and blower for 2-3 minutes. While it was running, I hung a voltmeter on the limit switch, and sure enough, right when the burner shut down, the voltage when to 27v. Once the voltage dropped back to zero again, the blower turned off and the system tried to restart again.

So, I'm going to order a new 3-wire pilot, but I don't think that's going to fix my short cycling. Is there anyway to test the limit switch. And where/how do I look for airflow obstructions?

Thanks,
Dave
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Old 12-03-2011, 03:38 PM   #13
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Furnace Cycle Problem


Quote:
Originally Posted by dave54 View Post
Well, it appear that I have more than one problem. Today, the unit started running for even briefer periods of time before the main burner shut off. I suspected the 3-wire pilot, so I manually jumpered the white and yellow wires which brought the unit back to its previous behavior of running with main burner and blower for 2-3 minutes. While it was running, I hung a voltmeter on the limit switch, and sure enough, right when the burner shut down, the voltage when to 27v. Once the voltage dropped back to zero again, the blower turned off and the system tried to restart again.

So, I'm going to order a new 3-wire pilot, but I don't think that's going to fix my short cycling. Is there anyway to test the limit switch. And where/how do I look for airflow obstructions?

Thanks,
Dave
test the tempature rise .......the data plate on the furnace will tell you what the correct tempature rise should be.
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Old 12-03-2011, 04:04 PM   #14
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Furnace Cycle Problem


And how do I test the temperature rise? The plate says that the "Design max. output temp" is 200 degrees, and the switch is labeled "L200-40F."

Thanks,
Dave
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Old 12-03-2011, 04:23 PM   #15
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Furnace Cycle Problem


it should have an allowable temp rise spec like 35-65F on that plate too. stick a metal cooking thermometer about 1-2 ft downstream from the plenum and subtract the return temp or house temp. needs to run for 5-10 mins to be accurate. your limit control has been cycled so many times it is now weak and inaccurate. get a new one and then do the test.

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