Trane/Kenmore Furnace problem

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Old 11-08-05, 07:35 AM
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Trane/Kenmore Furnace problem

About 2 years ago I purchased a 90+ Kenmore/Trane Furnace from Sears model number T9MPV075F12A2. On at least 10 occasions the unit has refused to light. When I check the light inside it is flashing 8 flashes which indicates the High Pressure Blower switch is stuck closed. If I go down and remove the rubber hose that connects from the switch to the blower and slightly blow/suck in the hose it comes right back on and works fine for a period of time (no telling how long). Everytime the Sears tech comes after one of these episodes and checks, the furnace is running correctly and he does absolutely nothing but put it in his computer as OK, which has come back to haunt me as they will not replace the furnace under the Lemon Law as they have not had to replace any parts. I checked the length of the exhaust/intake plastic pipes and they are about 32 feet long from the furnace to the outside. The intake tube has approximately a combination of 8 45 and 90 degree bends in it and according to the literature this is just over the recommended length for the run. Could this be my problem? I have enough air exchange in the basement to cut the intake pipe off and let it pull from the basement and reduce the length down to about 5 feet. Would this be a good start to alleviate the problem? Sears is absolutely useless in helping me to solve the problem. ANY suggestions would be appreciated.
 
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Old 11-08-05, 02:58 PM
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I would say you need a new pressure switch.

I am assuming you are talking about a negative pressure switch the is normally open, and closes when the combustion fan comes on.

Sometimes they stick open, and sometimes they stick closed.

I would start with that if you are trying to fix it your self.

If it is still under warranty I would call them next time it sticks before unsticking it yourself so it will be not working when they get there.
 
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Old 11-08-05, 06:41 PM
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Vent Length

I agree with Shank on the warranty part. Often, 75,000 btu furnaces with long runs need to be increased to 3" pipe instead of the 2" normally used. Also many furnaces are CRITICAL on the venting. If not installed exactly as specified, there are problems. The installer should have left the installation instructions. If they did, read them carefully, especially the parts about venting. If not installed per the manufacturer's instructions, contact Sears & show them where it was not. Do not accept "that doesn't matter" or "that is standard practice" as an answer. The people who write these manuals are not idiots nor do they put things in there just to make life difficult for the installer.
 
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Old 11-09-05, 05:07 AM
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Trane/Kenmore Furnace

The manual reads "the maximum number of bends is 5 if the pipe is 40' in length, for every bend over 5 reduce the length by 5' for each additional elbow used. Feet of pipe is whichever pipe run is the longest, either inlet or outlet side". My question then becomes,"can I safely assume that if I cut the inlet pipe to less than 10' and leave the exhaust at the current length of 32', this will alleviate the problem with the switch"? I currently have disconnected the rubber hose that runs from the switch to the blower motor and the furnace has been working correctly since. As far as going to Sears for repair--FORGET it. The technicians they use in this area are dumber than a stump. I have not been able to get any satisfaction or an intelligent answer from any one of them. None of them specialize in any one product, they are all generalists.
 
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Old 11-09-05, 07:22 PM
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Disconnected hose

If you disconnected the hose, it should not work at all. On most furnaces, that switch has to start out in one position & then switch to the other. Without the hose attached it could not cycle as it should. Something else is wrong here. I am begining to think you have a contol board problem as well as a venting issue.
 
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Old 11-10-05, 04:55 PM
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The problem MAY be with the pressure switch but MAY be with ventor back pressure.

When you suck on the pressure switch and this then makes the furnace run...but then quits after a bit...you need to do this test so you can tell IF it is the pressure switch. You have to suck in the tube and then pinch off the rubber real tight. If the diaphram is still good on the pressure switch, it will hold air and the furnace wil continue to run. But if the furnace goes out, even after repeat tests of this, then I'd say you have a bad diaphram on your pressure switch. You can be sure of this also by jumper wire bypasing the pressure switch. If it then continues to run..always..you have found your problem

Another test to make is a back pressure test. First go outside with the furnace ventor motor running and feel how much pressure you feel on your hand at the outside vent. Then, disconnect the exhaust piping from the ventor exhaust motor and feel the exhaust presure and compare (note that you can't do this too long as CO will be discharging). IF you feel a great disparity, you may want to try snaking out the exhaust pipe as there could be an object or creature in it. And if there isn't? Maybe you are right then about all the bends.
 

Last edited by mattison; 11-14-05 at 08:25 AM.
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Old 11-11-05, 04:28 PM
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[QUOTE=Bonehead]The problem MAY be with the pressure switch but MAY be with ventor back pressure.

When you suck on the pressure switch and this then makes the furnace run...but then quits after a bit...you need to do this test so you can tell IF it is the pressure switch. You have to suck in the tube and then pinch off the rubber real tight. If the diaphram is still good on the pressure switch, it will hold air and the furnace wil continue to run. But if the furnace goes out, even after repeat tests of this, then I'd say you have a bad diaphram on your pressure switch. You can be sure of this also by jumper wire bypasing the pressure switch. If it then continues to run..always..you have found your problem

When I suck/blow on the rubber hose that is connected to the blower sensor switch, the furnace comes on and works OK for anywhere from a month to 6 weeks, sometimes longer. I can never tell how long it will continue to work. I trie to run a jumper across trhe terminals of the sensor and got a message on the green light of 9 flashes. I then disconnected the wires from the sensor and tried a jumper there as well and got 9 flashes again. I disconnected the rubber hose from the sensor and the system worked OK. A quote from "Grady" said that the unit should not work with the hose disconnected. "Grady" also said that ther should be no more than 24VDC across the sensor and I have 35VDC. Today I went down and cut the inlet PE to a length of about 10' and reinstalled the rubber hose on to the sensor and it appears to be OK as it has been working since I performed this function. I am now drawing combustion air from the basement rather than from outside and there is only 2 elbows to contend with now as opposed to the 7 I had before with a run of 33'.
 
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Old 11-11-05, 06:32 PM
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Timkins

Please look closer at my reply regarding voltage. I said 24 volts AC not DC. I don't know what kind of readings you would get if you are reading on the DC scale.
If you want to go back to outside air, try increasing the pipe size to 3" as soon as it gets out of the heater cabinet. 3" will carry more than twice as much air as 2" with everything else being equal.
 
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Old 11-12-05, 05:25 AM
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Originally Posted by Grady
Please look closer at my reply regarding voltage. I said 24 volts AC not DC. I don't know what kind of readings you would get if you are reading on the DC scale. If you want to go back to outside air, try increasing the pipe size to 3" as soon as it gets out of the heater cabinet. 3" will carry more than twice as much air as 2" with everything else being equal.
My apologies to "Grady" for misquoting him, I was trying to toggle between my reply and the previous replies and everytime I toggled back I lost what I had been typing and could not remember what had been written. I think that is called "old age".
 
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Old 11-12-05, 08:40 AM
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Originally Posted by Bonehead
You have to suck in the tube and then pinch off the rubber real tight. If the diaphram is still good on the pressure switch, it will hold air and the furnace wil continue to run. But if the furnace goes out, even after repeat tests of this, then I'd say you have a bad diaphram on your pressure switch.
This test doesn't work all the time. Some of the pressure switches actually have a bleed hole in them by design that will prevent this from working.
 
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Old 11-13-05, 10:52 AM
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Originally Posted by shank
This test doesn't work all the time. Some of the pressure switches actually have a bleed hole in them by design that will prevent this from working.
Interesting. Is the bleed hole teeny tiny compared to the like 1/4 inch tubing port that is in the switch?

I guess the best test thewn would be to bypass the switch (jumper) and if the furnace works, then it just has to be that switch.
 
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Old 11-14-05, 05:16 AM
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Originally Posted by Bonehead
Interesting. Is the bleed hole teeny tiny compared to the like 1/4 inch tubing port that is in the switch?

I guess the best test thewn would be to bypass the switch (jumper) and if the furnace works, then it just has to be that switch.

I tried to bypass the switch with a jumper and the furnace did not work at all. It started to flash 9 times indicating a problem. I tried jumpering with the wires attached to the sensor and detached from the sensor and it did not work either way.
 
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Old 11-14-05, 06:54 AM
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Originally Posted by Bonehead
Interesting. Is the bleed hole teeny tiny compared to the like 1/4 inch tubing port that is in the switch?

I guess the best test thewn would be to bypass the switch (jumper) and if the furnace works, then it just has to be that switch.
It is a tiny hole.

And by bypassing the switch you only determine that the switch is what is keeping it from coming on, not that the switch is bad.

Timkins, is there anyway you couldpost a pic of the inside of you burner compartment and highlight what they are calling the High Pressure Blower switch somewhere and link us to it?
 
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Old 11-14-05, 09:46 AM
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Originally Posted by shank
It is a tiny hole.


Timkins, is there anyway you couldpost a pic of the inside of you burner compartment and highlight what they are calling the High Pressure Blower switch somewhere and link us to it?
This site does not allow any attachments to be sent in. I tried a private
e-mail, but that does not allow attachments either. I have been trying unsuccessfully to put a photo on MSN but that site is down temporarily. If you want to send me an e-mail with your address I can send you an attachment. I can be reached at [email protected]
 
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Old 11-14-05, 10:58 AM
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Picture seen here

I still say it is the switch, but I still don't know how it was working with the hose off.
 
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Old 11-14-05, 03:05 PM
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Originally Posted by timkins
About 2 years ago I purchased a 90+ Kenmore/Trane Furnace from Sears model number T9MPV075F12A2. On at least 10 occasions the unit has refused to light. When I check the light inside it is flashing 8 flashes which indicates the High Pressure Blower switch is stuck closed. If I go down and remove the rubber hose that connects from the switch to the blower and slightly blow/suck in the hose it comes right back on and works fine for a period of time (no telling how long). Everytime the Sears tech comes after one of these episodes and checks, the furnace is running correctly and he does absolutely nothing but put it in his computer as OK, which has come back to haunt me as they will not replace the furnace under the Lemon Law as they have not had to replace any parts. I checked the length of the exhaust/intake plastic pipes and they are about 32 feet long from the furnace to the outside. The intake tube has approximately a combination of 8 45 and 90 degree bends in it and according to the literature this is just over the recommended length for the run. Could this be my problem? I have enough air exchange in the basement to cut the intake pipe off and let it pull from the basement and reduce the length down to about 5 feet. Would this be a good start to alleviate the problem? Sears is absolutely useless in helping me to solve the problem. ANY suggestions would be appreciated.
Is this actually called a "high pressure blower switch" ?? I'm not sure what that does? Is it for the ventor or the house air circulating blower motor?

Indicates it is stuck "closed"? Remember that the terminology for "closed" in electrical = current is going through it. So, when testing switches and you see "NO" (normally open), or "NC" stamped in the metal by the terminals, you then know how you should go about testing.

So before we go any further, you need to be specific about the two issues I address here, so that we can help you more.

Do you know what switch they are talking about for sure? Can you describe it?
 
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Old 11-15-05, 05:10 AM
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Originally Posted by Bonehead
Is this actually called a "high pressure blower switch" ?? I'm not sure what that does? Is it for the ventor or the house air circulating blower motor?

Indicates it is stuck "closed"? Remember that the terminology for "closed" in electrical = current is going through it. So, when testing switches and you see "NO" (normally open), or "NC" stamped in the metal by the terminals, you then know how you should go about testing.

So before we go any further, you need to be specific about the two issues I address here, so that we can help you more.

Do you know what switch they are talking about for sure? Can you describe it?
When the unit fails to start and I check it, the indicator light on the unit is flashing a sequence of 8 flashes. On the literature I received from the manufacturer it states that 8 flashes indicate, 1. "High Pressure closed when should be open. (i,e,, when call for heat begins).
2.(Combustion blower is not energized until pressure switches open)
Under the check /Repair column the manual states for the first indication that 1. "Pressure switches stuck closed (system will wait for pressure switch to open). And for the second;2. "Pressure switch miswired or jumpered". I hope that is clear.
 
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