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Old 01-16-2010, 09:13 PM   #1
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American Standard Furnace Mystery


I have a 5 year old American Standard furnace model # CDC120A960A3 that has been giving me fits for the past few months. The furnace will sometimes start up normally, and burn without any problems for an entire day. Some days, during a cycle, all of the burners will have a flickering to them, as if the control board is killing the gas, though the flicker is only a fraction of a second and the flame remains lit. The flickering itself is random in its pattern. Sometimes it will flicker once during a cycle. Sometimes it will flicker over and over back to back to where the furnace shuts down the flame and completely restarts. The board LED does not give any error readouts. It continues blinking, indicating regular call for heat. I have had an American Standard tech out to check it out, and of course, the furnace wouldn't do it the entire 30 mins he had it on. On the work order sheet, he says that he checked gas pressure, inducer motor, flue outlet for freeze-up, heat exchanger, condensate drains, blower, ignitor, and flame sensor. He indicated that the valve pressure crept down a bit during the test time, but that it was within normal operational boundaries. I am at a complete loss as to what the problem may be, and if I need to call another tech out, I would like to have maybe narrowed the problem a bit. Sorry for the long post, but this is the first time I have every asked for help via a forum.

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Old 01-16-2010, 09:18 PM   #2
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American Standard Furnace Mystery


Natural gas, or LP?

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Old 01-16-2010, 09:23 PM   #3
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Natural gas
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Old 01-16-2010, 09:29 PM   #4
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American Standard Furnace Mystery


Almost sounds like moisture in the gas. Or a moving restriction in the gas valve or manifold.
Along with possible loose wire connection on the board or gas valve.
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Old 01-16-2010, 09:39 PM   #5
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American Standard Furnace Mystery


Quote:
Originally Posted by David6 View Post
Some days, during a cycle, all of the burners will have a flickering to them, as if the control board is killing the gas, though the flicker is only a fraction of a second and the flame remains lit. The flickering itself is random in its pattern. Sometimes it will flicker once during a cycle. Sometimes it will flicker over and over back to back to where the furnace shuts down the flame and completely restarts. The board LED does not give any error readouts.
If I post a schematic can you wire up a DPDT relay to do the function in the title? Parts cost would be a few bucks.

You'd hook it up to the gas valve and if the symptom shows it would sound an alarm if the gas valve voltage wavered. If the power comes back the alarm continues to sound.

If the gas valve voltage stayed constant through the symptom you hook it up to other points that are supposed to have 24v on them until you find a point that drops out when the symptom is present.

If you build two, you could find the problem twice as fast.

It would be good, though, if you can somehow induce the problem. If you can do that, all you need is several LEDs that monitor critical voltages. Even less bucks for parts.
That's how I found an intermittent cranking problem in my car.

Last edited by Yoyizit; 01-16-2010 at 09:43 PM.
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Old 01-16-2010, 09:46 PM   #6
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American Standard Furnace Mystery


Sure, it's worth a shot, but as to getting it to produce the symptoms, it seems to be somewhat random, though I feel that it seems to do it the most in the morning when bringing it from the overnight temp to daily temp. It happened a few times this morning, but yesterday, not at all. Thanks for the input/help.
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Old 01-16-2010, 09:59 PM   #7
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American Standard Furnace Mystery


Part number 45-640

Mfg. - GUARDIAN
Mfg.# - 1390VS-2C-24A
1-1/16" x 13/16" x 1-3/8"
13 amp @ 30 VDC/120 VAC
Coil resistance: 160 ohm, 24 vac, less than 3.6 VA

It's $1.75 but shipping is extra.

You need to remove the plastic relay case or find a similar relay with no case on it.

Using only one section [SPST] from the two sections,
run a wire from the normally open [NO] contact to one coil terminal.
The 24vac goes into the common terminal of that section and the other 24 vac lead goes into the other coil terminal.
This relay may want a socket, so it may not like soldering temps. You can use push-on connectors or just wrap the wire around the terminals several times. If you do solder the terminals, be quick.

You'll probably need clip leads; Hosfelt may have them.

To latch it to the "armed" position, with 24vac applied and using a toothpick, push the relay armature down until the normally open contact is passing current to the coil.
Now, when power drops out, the armature will release and stay released, so you will know what voltage dropped out.

Last edited by Yoyizit; 01-16-2010 at 10:11 PM.
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Old 01-17-2010, 08:22 AM   #8
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American Standard Furnace Mystery


If the pressure is flickering/pulsating there is a VERY good possibility the gas co regulator AND or meter is mechanically failing. I had one where it took me 2 trips and a few dozen cycles of the unit B4 the problem showed up. Kept losing gas pressure intermittently. Gas co changed the meter and regulator and problem went away. The meter and regulator are mechanical devices, can wear out/fail like anything else. I would NICELY talk to your utility co about it, yell at them and you won't get good results. Show them the report from the tech, they are responsible to supply you the proper pressure and amount of gas. Make sure the meter and regulator are not covered in snow or ice and that the screen on the regulator is not covered in spider webs/debris or snow.

Last edited by yuri; 01-17-2010 at 08:48 AM.
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Old 01-17-2010, 10:50 AM   #9
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Yes, have the regulator checked.
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Old 01-18-2010, 02:23 PM   #10
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Thanks for the suggestions. I had the luck to actually capture the failure happening on video. If you're curious to see it, I uploaded it online http://www.megaupload.com/?d=LKFQLEOA Around 6:45 is when the complete flicker/shutdown happens. I had another friend in the HVAC field take a look at the video, and he thinks the issue is most likely the control board, as the circulation motor immediately tries to start-up again after the flames go out. The circulation motor runs for around 5 seconds after the flame goes out during a normal cycle, and he feels that something is killing the motor, probably only for a fraction of a second, causing the flames to flicker/shut off as a safety. Any opinions? There is a bit of browning on the control board around some resistors near the top of the board, but nothing I would really call a burn. I would love to hear your guys' take on this theory.
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Old 01-21-2010, 06:32 AM   #11
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American Standard Furnace Mystery


Well, I was able to rule out the control board. Another guy I know happened to have a board that I could switch out to test. The symptoms showed up immediately. I'm going to give the gas company a call today and see what they say.
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Old 01-21-2010, 10:39 AM   #12
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Update: Gas company said I was being shorted under full load, so they changed the meter and regulator. Now we wait and see . . .
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Old 01-21-2010, 05:59 PM   #13
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INTERESTING,
Thanks for the follow up, lots of posters don't.
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Old 01-21-2010, 08:15 PM   #14
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Well, unfortunately, that doesn't appear to have been the problem. Still getting the flickering, with the occasional restart. May have to go the electrical route suggested by a previous poster.
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Old 01-21-2010, 08:45 PM   #15
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American Standard Furnace Mystery


I am going to say it may not be the gas pressure. In the video every time the burners shut down I see a large amount of water gushing down the clear drain hose on the left of the furnace. the pressure switch hose is also connected near the same hight as the drain. I belive the water is not draining properly durring operation and flooding enough to quickly block the pressure switch shutting down the burners. The induser shuts down the water drains and starts over again. But then again it is hard to determon with out being there.

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