Carrier gas furnace short cycling

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  #1  
Old 12-01-08, 09:21 PM
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Carrier gas furnace short cycling

I started a thread some time back, but had to leave it for a while: life got in the way.

Our Carrier gas furnace, model 58ssc, is short cycling. The LED blink code indicates "limit or flame rollout switch is open", and the decoder gives these tips:
- defective blower motor or start capacitor (I don't think this is the case; the blower seems to run just fine)
- loose blower wheel (mine spins but has no side-to-side movement, seems normal)
- inadequate combustion air supply
Flame Roll-Out Switch or fuse link
(that's how it's written, looks almost like an error)
- dirty filter or restricted duct system (filters are clean, ducts are open and unblocked)
- defective switch or connections
- Open Flame Roll-Out Switch or fuse link. Manual reset or replace (I pressed the manual reset, but I don't think it required a reset.)

How do I check for "inadequate combustion air supply" or "defective switch or connections"?

In a different forum I read about scale in the heat exchanger could indicate a problem, or a cracked heat exchanger allowing air in at the wrong place, and not allowing the air flow to cool the furnace causing the high limit to open. How do I know if I have a scale problem? When my condenser tube was blocked earlier in the summer I removed a knot of some kind of junk, looked almost like tree leaves. Is that how scale would present itself?

Thanks-
Dave O
 
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Old 12-02-08, 08:20 AM
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Originally Posted by CycleZen View Post
I started a thread some time back, but had to leave it for a while: life got in the way.

Our Carrier gas furnace, model 58ssc, is short cycling. The LED blink code indicates "limit or flame rollout switch is open", and the decoder gives these tips:
- defective blower motor or start capacitor (I don't think this is the case; the blower seems to run just fine)
- loose blower wheel (mine spins but has no side-to-side movement, seems normal)
- inadequate combustion air supply
Flame Roll-Out Switch or fuse link
(that's how it's written, looks almost like an error)
- dirty filter or restricted duct system (filters are clean, ducts are open and unblocked)
- defective switch or connections
- Open Flame Roll-Out Switch or fuse link. Manual reset or replace (I pressed the manual reset, but I don't think it required a reset.)

How do I check for "inadequate combustion air supply" or "defective switch or connections"?

In a different forum I read about scale in the heat exchanger could indicate a problem, or a cracked heat exchanger allowing air in at the wrong place, and not allowing the air flow to cool the furnace causing the high limit to open. How do I know if I have a scale problem? When my condenser tube was blocked earlier in the summer I removed a knot of some kind of junk, looked almost like tree leaves. Is that how scale would present itself?

Thanks-
Dave O
Hi Dave. I was about to ask about you and your (ongoing?) poor furnace problems, and I see you gave explanation.

Test your entire 24 volt safety circuit pathway from one switch to the next for 24 volts leaving the outgoing side of each switch. Like pressure switch, roll out limit, spill switch if you have one, etc. We really need to know exactly what is doing the cutting out here, and not jump to any conclusion yet.
 
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Old 12-02-08, 02:34 PM
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Thanks, ecman51`

Hi, and thanks for your (ongoing) replies. By the way I figured out (I think) why that fuse blew: I was testing for 24 volts DC, not AC, from the red to white poles.

Originally Posted by ecman51` View Post
Test your entire 24 volt safety circuit pathway from one switch to the next for 24 volts leaving the outgoing side of each switch.
I'll sure give that a shot. I noticed the high limit and the rollout are connected, for instance, so I get the idea of checking for 24 volts from one to the next. Can you recommend a place to start, or what are the bases I need to tag?

Thanks,
Dave O
 
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Old 12-02-08, 04:56 PM
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There is more than one way to volt or ohms test. But to erase all doubt as to what you are seeing on the meter, and which makes obvious visual sense, put your red probe on a safety switch terminal and put your black probe on any clean unpainted metal of the furnace, or other known ground of the furnace (i.e. green wire hookup point). It should read 24 volts. Do every 24 vac switch this way. Set your meter to at least a 120 volt setting or more in case you have some 120 volt limit. You may want to start, in your case, by testing the limit while it is running, and right when the furnace goes out, to see if ONLY the outgoing wire no longer passes current.

Now if that does indeed happen, then we have to figure out why, it is limiting.

But if both the incoming and outgoing wire loses current, then it is not the limit switch, but somewhere else in the circuit.
 
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Old 12-02-08, 11:28 PM
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High Limit

The high limit's left hand blade connector had 24v while running and after the short cycle occurred. The right-side connector lost 24v- dropped to zero- at the same time as the flames went off.

Another fact: last night when I was fussing with the voltage readings on the red and white poles on the control board, the furnace did not short cycle when the lower access panel was off.

I'll make sure the cold air returns are open and clear (that'll have to wait until morning). This time instead of a visual I'll get a shopvac after them.

A question: this furnace also has a Honeywell electronic air cleaner, which uses those aluminum mesh air filters. I've been vacuuming the filters every month, and whenever I think about it I go over them with soapy water and a scrub brush. Do those filters wear out, or have a useable life that I may have past?

Thanks!
Dave O
 
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Old 12-03-08, 04:24 PM
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Is your furnace in a basement that is cooler than upstairs cold air return? If yes, then your solve may not be from the fact you think you discovered a restriction. It may be because you now are passing colder basement air over the exchanger, which is just enough to keep it over the hump- running.

Do you have an AC coil in the plenum?, that may have gotten dirty from not changing filters regularly enough? The metal screen can't 'wear out'. But you might see what happens if you leave them out.
 
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Old 12-06-08, 02:46 PM
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Checked the AC

I took off the front access panel to get to the AC coil in the plenum, and couldn't figure out how to get in from the side. I was able to shine a light through from one side to the other, and it looked clean and clear. Then I removed the squirrel cage blower and shined a light up from the bottom, and saw that the AC coils were clean and clear. The squirrel cage is clean too.

I removed the electronic air cleaners to clean them, and cleaned the mesh filters too. To test your idea I left the filters and air cleaners out of the system and I'm running it now: still short cycling.

What can I try next?

Thanks,
Dave O
 
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