Hello Experts,

I am having problems with my furnace short cycling. When I look at the control board I have no flashing LED's indicating a fault. The furnace is just over 3 years old. I usually keep the house at 71 degrees, at night goes down to 69. In the morning when the programmable thermostat is calling for 71 the furnace will come on but it will cycle non stop before it gets to 71. I remember when it was first installed it would come on, run on low for about 3 minutes and if the temp wasn't reached the blower would ramp up and the furnace wouldn't stop until it was at the programmed temp. I even tried cranking the heat up to like 75 and still cycles on and off.

During that time it was pretty mild outside -2 Celcius.

The stat clicks, inducer comes on, igniter comes on, flames starts, flame sensor LED comes on, blower starts and then after about 3-4 minutes the flames stops and the blower keeps running then about 2 or 3 minutes flames comes back on. This keeps repeating non stop.

There is a new clean filter installed, the blower fan blades are nice and clean, no vents are covered or blocked. The exhaust and intake PVC pipe aren't plugged up.


Here's what I tried so far.

New thermostat Honeywell programmable 5-2...no fix
Bypassed the stat by adding a jumper at the furnace between W and R...no fix
I tested all pressure switches and I have 24+ VAC on all terminals to ground.
I tested all limit switches and I have 24+ VAC on all terminals to ground.
I checked resistance from the beginning of the limit switch loop to the end I get around 60 ohms total when it runs.
I cleaned up the condenser drain pipes....no fix


I'm starting to think I have a bad control board.

Oh and by the way the other night after I poked around in there it started running normally for 2 cycles. It started on low then ramped up for about 10 minutes and got the house to temp....then it didn't run for a good hour or so.

Later that night it started short cycling again.

I would like to eliminate the common stuff before I call a tech in.

Cheers

 

Find and clean out the drain trap.

 

I'm guessing this is what you're talking about.



If that's it I cleaned that out too.

 

yep. Make sure all connecting drain lines are also cleared out.

 

Here's a youtube video I took about an hour ago. Look what it did at 1:32, it's never done that before, that I've noticed anyway. Since then it's done twice.

Looks like a flaky relay on the board or the board itself.

http://www.youtube.com/watch?v=Q-VZpPmnhkI&feature=youtu.be Checked all the drain lines and they are clear.

 

Just thought I'd bump this thread.

 

It says "this video is private."

Does the circuit board give any error code when it fails?

The board is unlikely to be bad.

 

Sorry about that I made the video public now.

I get absolutely no error codes at all....both LED's stay lit and according to the furnace panel it means normal operation.

 

Here's an update on this.

Tech came in today and it turns out to be a design flaw in these furnaces. The 2 ports on the condenser box get blocked with water intermittently. He said there was a tech bulletin about this problem from Nordyne and the solution is to put a small tube inside the port that extends inside a little bit just to prevent the water from sliding down the wall and plugging the hole. Apparently Nordyne has redesigned the condenser box on the newer models to prevent this problem.

He also told me that you will not get an error code on the board from this problem.

Furnace no longer short cycles and now ramps up to full speed.

Just and FYI.

You can close this thread.

 

Thank you.
My Nordyne furnace has this problem and it developed before it was 2 year old. The company that installed it showed up about 5 times over the past 3 months and yesterday replaced the PC board and the thermostat. Then they told me they can not do anymore. I have to call the factory and try to fix it myself. And I had a 5 year labor warranty with the 10 year labor warranty!
At least your description gives me hope that a fix is in sight. Now can I do it myself or do I have to hire another company and a lawyer?
Thanks again.

 

the issue started surfacing 4 years after installing fg7tc 80 unit.
ofcourse happens on the coldest of the days. the condensation drains clogs up and the low pressure switches malfunction!!!!!!! - parts and warranty covered call to replace low pressure switch

now 6 year after install i have the same issue again - i hope they come out and do the technical bulletin to fix it for good.