Armstrong Ultra 80SX Intermittent short cycle

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Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Sun Oct 30, 2011 12:27 pm

Hello Everyone,
Looking for a little advice, we have an Armstrong Ultra 80SX Gas Furnace (model# GUJ125D14-1B) with a digital thermostat. Had a problem with it short cycling last year cleaned the sensor assy. still had an issue so replaced assy., Furnace ran fine most of the winter. Fired it up this morning and again it started doing it again, cleaned sensor and now it runs fine for a good 10 minutes or so and then starts acting up. The blower doesn't shut off but the flame goes out and then fires back up. What would cause it to cycle after it gets hot.

Have jumped the pressure switch same thing runs strong for a solid 10 minutes and then short cycles. Have checked every connection and ground no change.

Any thoughts or is it time to call in THE MAN. Trying not to go the man route.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Freon » Mon Oct 31, 2011 1:21 pm

It seems that the high temperature limit safety is tripping thereby preventing your heat exchanger from overheating. I would check to make sure all registers (air out) are open and you can feel air flow. Then check all returns (Air in) and be sure there's nothing blocking them. Lastly check all filters. If you do not have adequate air flow through the furnace, the high limit will shut off the flame while the blower continues, cooling the heat exchanger down to where the burner reignites. So your ducting is the first place to start.

If the ducting seems good, do you feel a good flow of air from the supply registers? Has anyone worked on the motor? It is possible the run capacitor may be sick causing the motor to not produce the horsepower it should. Not likely, but a possibility.

Lastly the upper limit sensor may be defective. Determining that involves measuring the air temperature before and after the heat exchanger. The furnace will list an allowable temperature rise across the heat exchanger and also should list the break (switch opens) temperature for the high temperature sensor. If the high temperature limit seems to be working then next stop is the control board.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Tue Nov 29, 2011 3:03 pm

Thanks for the quick response, and sorry for the delay in responding.
Ended up in calling an HVAC tech. for a service call. Deemed it not a pressure switch nor flame sensor. He looked over things and measured the intake temp and output temp was reading well over 70 degrees difference. The fan was already blowing on the high setting with good volume, so he measured the gas pressure which was reading a 4 and reduced it to 2 1/2. That did drop the temp to a 58 degree difference which is what the furnace is rated for. It ran the rest of the day normally.
The next day it started to short cycle again, made the call. He came out and checked over things decided to run a new ground to the furnace stating hes seen this problem in the past as well. Furnace ran fine, stated the next thing would be a control board and a pressure switch ( because the pressure switch is an inexpensive part ).
Well needless to say it started short cycling again, decided to start throwing parts at it my self at this point. Have replaced the pressure switch (no change), now have replaced the control board and again no change.
So new ground wire, new pressure switch, new control board. Flame sensor is relatively new and has been cleaned.
Furnace will fir up when cold and run for a good 10 minutes and then start short cycling (10 to 15 sec intervals).
All vents are open, filter clean and have even run without the filter (no change)

Any thoughts, suggestions, recommendations greatly appreciated.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby heatseeker » Tue Nov 29, 2011 3:10 pm

have you considered the possibility of a loose wire? Look around at all the connections and see what you can find. Also I haven't heard if you have eliminated the high temp switch.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby heatseeker » Tue Nov 29, 2011 3:11 pm

You should have bought a new furnace with all those parts. Did you check the inducer fan motor performance?
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Tue Nov 29, 2011 3:20 pm

Figure I'll just slowly build a new one. Checked every connection, wire nut, ground I could find. Inducer fan comes on when called and actually removed it, housing was clean and gasket in good condition,also tube to pressure switch. As far as the high limit switch, when the gas pressure was reduced that brought the temp difference back to within range. I guess could it be a bad limit switch and could I just jump it out for a test?

Again Thanks.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby heatseeker » Tue Nov 29, 2011 3:53 pm

When in doubt jump it out.
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Tue Nov 29, 2011 5:09 pm

I might need some help on this one, I've got leads running to a 1/2 inch dia. by 3/4 tall cylinder which is mounted to the plate directly above the venturi tubes, it seems to have a small reset on top of it and then one lead (from it) goes to a sensor mounted in the fire box. The fire box sensor is mounted between the burners and the inducer fan. I assume this is the high temp sensor, send a corrective shock over if mistaken. BTW whats that other sensor with the small button switch on top?

Thanks again
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Tue Nov 29, 2011 5:26 pm

One more question, have one of those infrared thermometers. The exhaust gas from the vent pipe reads around 230 or so. It rises 2 inches above the furnace makes a 45 degree turn and travels 3 feet to the chimney. Just 6 inches from the elbow the temp drops to 130 in a matter of and inch or so. Sounds as if its clear inside but can remove to inspect.
Thoughts?
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- Armstrong Ultra 80SX Intermittent short cycle

Postby Gunny1966 » Tue Nov 29, 2011 5:42 pm

Alright jumped the limit switch, (looked on ebay to verify it was the right switch). Furnace ran for a solid 15 minutes and then started to cycle, every 30 to 60 seconds sometime shorter sometime longer. It cycled maybe 5 times and is now running solid. If any tech lives in the Columbus Ohio Area let me know and of course any suggestions greatly appreciated.
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