American Standard Freedom 80 problem


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Old 01-20-09, 07:25 AM
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American Standard Freedom 80 problem

Hello.

Furnace:
American Standard Freedom 80 two-stage, ~14yrs old, down flow
Model#:ADD100R948BO
Serial#:J19501099
Stage1: 65,000BTU
Stage2: 100,000BTU

Home:
Single story on a crawl space, ~1500sq.ft.

Problem:
After ~15-20min. of running in stage 2 the whole system will shut down and will give a fault code of 3, which is the pressure switch. During the time that the system is shut down the thermostat continues to say that the system is still running. The system will stay shut down for ~10min. then it will start back up and will keep repeating the same process.

Stage 2 is only needed when the temp inside the house is < 2*F than the target temp. The system has ran as much as 24hrs straight in stage 1 with no issues. Stage 1 alone can maintain the house temp until it gets below 5-10*F outside. Once stage 2 starts it will go into this on/off cycle after only 15-20min.

The circuit board has been replaced that worked for 45min. now it is back to doing the same thing. Both intake filters are new, none of the vents are blocked in any way. Not sure of any other service more than 2yrs ago.

I odd thing is that there are two intake points for the furnace. One is the cold air return in the hallway that goes up through the attic and down into the furnace. The other intake point is right on the side of the furnace itself, this seems really odd to me.

Any help would be great!
Thanks.
-Scott
 
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Old 01-20-09, 08:37 AM
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Good description of your system and write up of the problem! That makes providing some help to you much more efficient.


According to the diagnostic code of the limit switch opening, the furnace is overheating, usually because of an air circulation problem, but a bad limit switch or a bad ignition control module would be unlikely possibilities.

Poor air circulation is the most common reason for the limit switch to open, because not enough air is circulating to strip[ away all the heat the furnace produces, therefore the furnaces gets hotter and hotter until the limit switch opens and shuts off the burner. Continued operation of the fan then gradually cools the furnace until the limit switch closes and the burners reignite, starting the cycle again.

That tends to happen when the weather outside is coldest, just as you describe. That's because the furnace burners run long enough to start this cycle.

So--- do you hear the fan motor speed up to a faster speed when the furnace enters the high input 2nd stage? You should.


Where is the furnace located? Manufacturers usually recommend against taking circulating air from the vicinity of the furnace because it can cause problems with supplying combustion air for the furnace to use. Also, by having an opening so close to the furnace you very likely get most return air from that point, with the more remote return air vent supplying little air.

Was this furnace a replacement for an earlier furnace or installed when the house was built?

Do you have central air conditioning? A plugged evaporator coil in the airstream of the furnace could cause this problem. You should inspect the evaporator coil from the side facing the fan motor.

At the same time, inspect the duct work for any sign of plugging by debris, which could include sound deadening insulation which might have pulled loose from the ductwork near the furnace and been sucked into a blocking position.

You can double check that the limit switch is actually the problem with an AC voltmeter, and double check that the limit switch is operating properly by drilling a small hole in the warm air plenum, inserting a suitable thermometer and watching at what temperature the limit switch opens. The temperature at which the limit opens is commonly stamped on the limit switch itself, often in the for L185 which would indicate a limit switch that opens at 185 degrees Fahrenheit.

Anyway, those would be a few things to check.
 
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Old 01-20-09, 01:52 PM
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I am a little confused by your response because you are referring to these switches as being temperature related.

The way I understand this, the fault code is flashing 3 times which says there is a problem with the pressure switches. These pressure switches are actually vacuum switches that get a signal from the intake side of the inducer draft motor. The stage 1 switch has to see at least .17inhg and the stage 2 switch has to see at least .50inhg for the furnace to continue to operate.

To clarify when the system shuts down, nothing is operating, the fan does not continue to run.

Yes, it is very easy to tell when it jumps to stage 2.

The furnace is located in the center of the house in a large closet.

The house was built in 1959 so, yes this was a replacement furnace. The house also has central air.

Also, these switches as you said do have these temp. ratings on them, both are L162. So I guess these pressure switches also have temp ratings?

Thanks.
-Scott
 
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Old 01-20-09, 04:05 PM
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To find out if it is the pressure switch (as opposed to something else, whether it be the stat, a limit switch, a smart valve, a ?...) causing the blink code, rather than something else on the 24 volt safety circuit, test the pressure switch terminals. If you put one voltmeter (set to 120 volts+) test probe on the incoming wire/terminal and the other voltmeter probe to good furnace metal or ground and that has 24 volts, but the outgoing terminal/wire does NOT, then it indeed has to be pressure related (which could be due to the switch itself or a number of things that cause the venting to be weaker than it should be.

Obviously you have to quickly conduct the test right when the furnace shuts down.

I once had smart gas valve and the blink code said it was the pressure switch. No such thing. Turns out the smart valve was faulty! When the smart valve malfunctioned, then so did the entire 24 volt circuit. Replacing the smart valve fixed it. Never had to touch the pressure switch. I don't stake my life on those blink codes. And that is not the only erroneous blink code I have gotten.
 
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Old 01-20-09, 06:24 PM
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Oops---

Hello Hayden---

Your question to me is well taken. Apparently I didn't read your post very well and I did indeed read it as a limit switch problem rather than a pressure switch problem.

Sorry.


Never mind.
 
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Old 01-21-09, 10:50 AM
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I found out that it is not the pressure switches directly, it is my inducer draft motor overheating and shutting itself down. Once it does this, the pressure switches no longer have the vacuum and that is why I was getting a error code of 3.

Now, I realized that if I keep the top cover off of the side of the furnace that the motor will not overheat and will continue to run. I'm wondering if I can just keep the cover off so it stays cool or is this a sign that the motor is on its way out the door?

The trouble with this situation is that I do not know how long it has been doing this. The previous owners had this 2 stage furnace installed sometime between 94'-95' but they had the thermostat set up to use only the first stage. I know this because when I replaced the old thermostat the W2 wire was sitting there not attached to anything.
 
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Old 01-21-09, 12:46 PM
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Sounds like you did a good job of diagnosing your own problem, Hayden.

Sounds like a failing motor, it shouldn't require special handling to function reliably. I would imagine you will find the problem will get worse and will quit working altogether at some unexpected time.

I'd replace the inducer motor and be done with it.

Then again--- what do I know? I couldn't accurately tell the difference between the limit switch and the pressure switch....
 
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Old 01-21-09, 04:03 PM
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Originally Posted by SHayden83
Now, I realized that if I keep the top cover off of the side of the furnace that the motor will not overheat and will continue to run. I'm wondering if I can just keep the cover off so it stays cool or is this a sign that the motor is on its way out the door?
How hot does it get in your furnace closet? Put a thermometer in there and tell us. You could tape it on the furnace cover right in front of where the inducer is.

Then for another test, you could put it right inside the furnace next to the inducer, with the furnace cover on.

Then for the final test, you could put it next to the inducer with the furnace cover left off.
 
 

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