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I have a Nordyne KG7SA, about 2009 vintage, which has begun acting strangely. We started noticing that the thermostat would drop several degrees below the set temperature before engaging. More recently I started noticing it seemed to cycle very quickly several times in a row, then just quit. Then on Christmas Eve it stopped working altogether.

I was able to get it working again by turning it off and back on again, but it would only run for ten minutes or so before starting the short-cycle routing again, and shutting down.

After doing some Googling I came opon the thought that the filter might be too dense (it was one of those 3M "allergenic" filters). So I removed it and the furnace began working normally again.

Today (12/26) I picked up a new, lighter filter and the quick recycling started happening again after a few rounds of heating. I had the presense this time to observe the status code - alternating blinking red and green - which I'm led to believe indicate a one-hour lockout mode. This seems to be consistent with the delayed cycling we'd observed in the past. (Side note - why are these status code legends so hard to find online?) When the filter is off and the unit is operating normally the status lights are solid red and green.

So in my most uneducated estimation it seems my furnace is having some sort of air flow issue.

One other point I should add - my dehumidifier filter is in dire need of replacement. It's a GeneralAir 1099LHS and I'm having a hard time finding new filters for it. Could this be contributing to the air flow issue I'm experiencing? If I set it to summer mode and bypass it (in the short term, anyway) might this alleviate the issue until I can find a replacement?

My main questions are, do I seem to be on the right track in my diagnosis? What would be some logical next steps to troubleshoot and isolate the issue? What might be some low-cost things to try in an attempt to solve the problem?

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I'd have to say to get an HVAC technician in for a complete cleaning, overall inspection, adjustment and, of course, solving the problem...keep using the better filters, they're worth it to you and to the system.

Yes, it could very well be an air-flow problem and the filter removal may have partially addressed the problem. However, if you have cooling on top, then the filter removal, possibly, revealed the problem's deeper and you have a dirty A-coil. This, is what the air always blows through regardless of heating or cooling.

And yes, it could be your humidifier if a majority of air flows through it with your setup...uncommon for what I've seen. However, your blower fan or even burners could be clogged-up or you may have a circuit board, transformer, sequencer, limiter or even igniter that's failing.

Sorry, HVAC is just a bit too complicated anymore and even the codes thrown by them are commonly just a symptom and not the cause. If the unit's never been serviced since 2009, then it's definitely due and may have been wasting money for a couple of years.

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  • I don't think you'll regret it and they really aren't expensive and are only ever a worthy investment that can pay for themselves.
    – Iggy
    Dec 27, 2017 at 21:21
  • I've contacted a technician and have scheduled the inspection. In the short term it turned out that cleaning out the flame sensor, which the tech walked me through over the phone, seems to have alleviated the short-cycling. Thanks for the advice!
    – Derek Gusoff
    Dec 28, 2017 at 14:25
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    Beautiful! Let him replace it, if determined to be a little too far gone. They're cheap and cleaning can be temporary. If he had you lightly sand or steel wool it, then it may only have 1 more cleaning left. Hopefully, that's it for the problems and he can have a fully successful cleaning and adjustment. I'm glad something easy could get you comfort, but I'm sorry the code turned out to be one of those false reads.
    – Iggy
    Dec 29, 2017 at 1:14
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Get a digital meat thermometer. Stick it in a duct close to the furnace and record the temperature. Stick the thermometer in the return air. Subtract the return air from the supply air, that will give you your delta T or temperature rise. Look on the nameplate inside the furnace, it will have the recommended temperature rise. Always a range such as 30-60. Your delta T should be within this range. If it is way over it is probably tripping the limit switch. Try this test with the filter in and out. Often times the furnace and or ducts were never sized right in the first place and any filter will put it over the edge. The humidifier pad generally will not effect air flow but it may cause other problems such as leaks.

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  • Turned out the filter was a red herring, I just had a dirty flame sensor. Thanks for the tips though.
    – Derek Gusoff
    Jan 8, 2018 at 14:44

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