tepstar furnaceshortcycling?

This is a Tempstar ntv6050fb. It is about 12 years old. On a cold day or when the house has been allowed to cool down, and then the thermostat is turned up, the furnace starts up and runs properly for several minutes, then the gas shuts off, and the eductor fan goes to high, and the furnace relights. There is virtually no delay.
The fact that it relights right away - and then runs fine for some time (not sure if it does it a second time after 5 or more minutes - usually gets up to temperature fairly quickly and then shuts down normally) leads me to believe it is not likely cycling on limit, or flame sensor??
I have not cleaned thesensor. Filter is kept clean and the problem has happened even when the filter had just been washed (washable electrostatic)
Would this be "normal" behaviour if the furnace had run 12 minutes on low fire and then went to high fire?? Does the gas shut off (momentarily) when the gas valve switches from low to high??
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Make that TEMPSTAR
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On 1/12/2016 1:02 AM, snipped-for-privacy@snyder.on.ca wrote:

Still says tepstart in the subject line.
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On 1/12/2016 12:49 AM, snipped-for-privacy@snyder.on.ca wrote:

The two things come to mind, is over heating (not enough air flow). This is unlikely with a clean filter. Might check for dirty blower wheel, or return air vents closed, or under sized return air.
Second thing to check is the flame sensor rod might be dirty. Some times if you lightly polish it with steel wool or emery cloth, it's good for a while.
I don't know about the switch from low to high.
- . Christopher A. Young learn more about Jesus . www.lds.org . .
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On Tue, 12 Jan 2016 15:37:55 -0500, Stormin Mormon

Only one problem with that (overheat)scenario. The system doesn't shut down and wait to cool off.
I have not cleaned the flame rod, but the flame sensor problem should be more consistent in it's behavior? My strong suspicion is that it is a normal situation when the system over-runs the 12 minute low fire period and switches to high fire - but I don't KNOW. I DO know it is not cycling on the high limit because the plenum is not uncomfortable to the touch when it does it..
It only does it when the furnace has to run longer than normal - extremely cold days, or pulling temp up from a deep set-back.
It uses a Honeywell SmartValvr

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snipped-for-privacy@snyder.on.ca wrote:

Is it 2 stage unit?
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wrote:

Yes, it is a 2 stage unit. That's why I asked : "Would this be "normal" behaviour if the furnace had run 12 minutes on low fire and then went to high fire?? Does the gas shut off (momentarily) when the gas valve switches from> low to high??"
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snipped-for-privacy@snyder.on.ca wrote:

Ours is Carrier 2 stage. It just ramps up when needed, no hesitation from 1st to 2nd stage. 2nd stage only comes on when it is pretty cold outside. Specially when it comes on in the morning from sleep period.
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wrote:

That's how mine behaves. Just that the "ramp-up" involves the flame shutting off, the combustion chamber purging, and then relighting - I assume on high.
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snipped-for-privacy@snyder.on.ca wrote:

Sounds alright then. Check W2 lead to see it is enabled when it happens.
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snipped-for-privacy@snyder.on.ca wrote:

Gas supply? No trouble code?
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wrote:

No trouble code
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On 1/13/2016 3:52 PM, snipped-for-privacy@snyder.on.ca wrote:

One time I was working on a furnace that relit frequently. Really had me confused. I called in another tech, who looked it all over. He said it had a bad gas valve. I bought another gas valve and replaced, which cured the problem. Might be the case, here.
- . Christopher A. Young learn more about Jesus . www.lds.org . .
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On Sat, 16 Jan 2016 08:14:25 -0500, Stormin Mormon

I'd agree with you if it was an every-day occurrence - but it ONLY does it when called to run for extended periods. - - -
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