Armstrong air ultra V tech 80 wont complete start up


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Old 11-17-10, 06:39 PM
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Armstrong air ultra V tech 80 wont complete start up

The unit will start the fire up procedure with the exhaust fan kicking in the pressure switch proves the burner ignites the unit will then run for about 10-20 seconds and the gas valve will shut down. The gas valve is showing a code of three flashes which the manual says is the pressure switch is open when it should be closed. I replaced the pressure switch and the flame rod and the problem still occurs i have jumped the high limit switch to see if that is the problem but the issue presists i'm stumped. any help would be greatly appreciated
 
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Old 11-17-10, 06:45 PM
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Also i have pulled the venting apart and everything looks clear and the fan is spinning without any interference. I have taken the air tube off the pressure switch and made sure its clear and have made sure the nipple on the fan is clear everything seems to be in proper operating order, its seems like its shuting the gas valve off just before the main blower should start up.
 
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Old 11-17-10, 06:45 PM
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95% of the time when a pressure switch opens it's because of a problem in the furnace or venting system, not with the pressure switch.

Unfortunately, people have replaced mountains of perfectly good pressure switches.

Unfortunately, there are many possible problems that can cause the pressure switch to open, many of them subtle and hard to identify.

Frankly, this kind of problem is the kind of thing a good repairman should diagnose. A good repairman has a manometer with him and uses it to test the condition of the furnace rather than changing out parts like pressure switches and such.
 
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Old 11-17-10, 06:52 PM
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the thing that has me stumped is the fact the burner ignites and runs, if it was a pressure switch issue shouldn't the burner not ignite?
 
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Old 11-17-10, 07:05 PM
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Originally Posted by showtime84
the thing that has me stumped is the fact the burner ignites and runs, if it was a pressure switch issue shouldn't the burner not ignite?
That's correct. What is probably happening is that the pressure switch is closing to light the burners but not staying closed reliably. When the pressure changes slightly, the pressure switch opens and the burners shut off.

So you have the venting system partially plugged up or defective and failing so it wont operate reliably.

I reach that conclusion because the diagnostic code you cite occurs when the pressure switch opens, which would shut off the burners.

You can and should verify that by using an AC voltmeter to measure the AC voltage at the pressure switch. Very likely what you will find is that the 24 VAC voltage at the pressure switch goes to zero when the burners shut off, indicating that the pressure switch has opened.

A good repairman would use a manometer to measure the pressure being applied to the pressure switch and would likely find that pressure marginal --- not negative enough to reliably keep the pressure switch closed.

When the problem is correctly diagnosed, the repairman would again measure the negative pressure and verify that it is now more than needed to keep the pressure switch closed.

Of course if the negative pressure were where it should be and the pressure switch didn't close, that would prove the pressure switch is defective and needed to be replaced.
 

Last edited by SeattlePioneer; 11-17-10 at 07:59 PM.
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Old 11-17-10, 07:45 PM
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Did you see into the vent pipe clear through to the cap?
How many feet of vent pipe do we have, 3 floors up?
How old is this furnace?

A manometer would be the right tool for this.
 
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Old 11-17-10, 08:06 PM
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The pipe runs up threw two floors and out the roof. I just jumped the pressure switch to see if i could bypass it and it still shut off after the burner had run for about 30 seconds. The furnace is 2003. I'd say roughly 25 to 30 feet of vent pipe
 
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Old 11-17-10, 10:12 PM
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When my home furnace cuts off short of a full cycle it has been:
1) flue blocked by chimney debris CO heat sensor shut furnace down.
2) blower motor overheated and shut down due to fouled blower wheel. The wheel spun free but did not blow any air, so it spun too fast at the same time did not cool the motor with moving air.
 
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Old 11-17-10, 10:27 PM
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In post #1 you said "the gas valve" was showing a three flash diagnostic code. Usually the diagnostic code is signaled by the circuit board or control module.

Is the diagnostic code really being displayed on the gas valve?

Secondly, was the three flash diagnostic code indicating the pressure switch was opening occurring when you bypassed the pressure switch and the burners still shut off?

Do you have a multimeter that allows you to measure AC voltages? How many wires are there to the pressure switch ---two or three would be typical.

Please post the model number of the furnace which can be obtained on the rating plate in the furnace burner compartment.
 
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Old 11-18-10, 06:31 AM
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The model number is G1N80AR050D12A-1A. The gas valve has the led attach to the top of it, with three wire harnesses coming to it. The pressure switch has two wires and one air tube. After jumping the pressure switch it still read 3 flashes.
 
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Old 11-18-10, 07:08 AM
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Unfortunately I haven't been able to locate information for your furnace based on the model number you provided.

What's the make and model number of the gas valve?
 
 

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