HVAC Fan Runs 24/7, Please Add Opinions. Thanks

Hi gang,
A friend called. The HVAC fan runs all the time in her 1 story home built in 1979. She has a digital stat above her return duct in the hall, the only return, 1700 square feet or so.
The problen started when she used the heater for the first time last season a few weeks ago, Houston, Texas is hot most of the time. :-)
I have no specifics: model, make, electric or gas, original unit...
She has a digital stat. It is set to Auto..
I am not a pro HVAC man, I am a locksmith, I make lost car keys. That said:
I think the possible cause IS NOT the thermostat (she wants to buy this first), but a relay AND/OR a thermal limit switch located by the heater output on a PCB or on the side of the attic unit some place, (max limit switch with a double pole [winter/summer push/pull switch depending on the year of production and ajustable temp settings?). I think the heater switch is effecting the AC too because the heater around here isn't used much and once it shorts it shorts and you know.., (now she says the AC fan stays on too), it shorted last year. (Would this be correct?).
She can cut the AC or heat off or on at the stat, the fan stays on hi BTW, not low which I understand happens up north on purpose to allow the basement air to go upstairs and vise/versa to lower the bill, not that complicated here. We don't have basements being Houston.
That said, Possible causes;
(1) Bad limit switch? (2) Bad fuse or relay on PCB controlling limit switch? (3) Bad relay from thromastat controling fan? (Note: style of heater unknown) (4)Rat bit the stat wires shorting the green control wire to the feed wire? Naww...)
Am I missing something, Thanks. Tomorrow is Sunday. Thought I could help her out while not doing nothing else.
Ed
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put the thermostat to Auto not On
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We could guess for hours, the only real way of correcting the problem is to go there check the settings. I remember when I moved into my last house with a digital stat, took me a couple of weeks with the manual to get it working the way I wanted.
Any of the above are POSSIBLE, I would WAG doubtful. Go on line get the manual for the stat, then set all of the variables for her. or call someone on Monday. Personally I would never attempt working with locks.
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On Sat, 25 Feb 2006 21:44:45 -0600, "Glen Cooper"

Same year as mine. I thought being above a duct was not the best location, but that's not the problem now. I'm sure it works well enough there.

I have read your whole post, and I think the problem is that she has keys in her vents. But don't tell her that. She'll expect you to be able to fix that.

You don't say if it is putting out heat or not.
1) IF it is, is putting out too much heat, since it runs all the time.
2) Or are the ducts blowing room temperature air some of the time.
1) If the former, it might be the thermostat or it might be something else.
2) If the latter, the only problem is that the fan is on all the time. From what you say below, and from what you didn't say, I think that this is the problem.

Can you use a volt-ohm meter?

Do you mean even though it's cold, when she puts it on AC, the fan blows, even though the compressor outside doesn't go on?
I suppose it could be the thermostat's fan auto/on swtich, although it is usually a simple switch that is not likely to short. The manual or prior posts will show what colors represent the fan/auto/on switch. You can take the thermostat off the wall, put it on auto, disconnect one of the two wires from the thermostat, and the meter should show an open circuit at the two screws on the thermostat that connect to those wires, the fan wires.. When you move the switch to ON, the meter should show close to zero ohms. If it does both of those things, especially the first, the thermostat is good, wrt this issue..

Would what be correct? A few lines up you refer to "heater swtich". Do you mean fan relay or fan swtich?

I suppose.

Doesn't the limit switich affect the relay, rather than the other way around. A fuse doesn't sound likely.

Yes but what do you mean "from thermostat". YOu mean at the furnace, right? If it's bad (always closed for example) the thermostat would not make it open.

I was going to suggest that you check the wires to the thermostat from the furance when the therm is disconnected, but because of ignorance, I don't know what would make those show that they are connected and what wouldn't.

Remove NOPSAM to email me. Please let me know if you have posted also.
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http://www.misterfix-it.com/Solutions/5Air.html
http://yourhome.honeywell.com/Consumer/Cultures/en-US/Support/Thermostat+Wizard/
http://toad.net/~jsmeenen/wiring.html
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Go to the furnace, shut the power off and disconnect the green wire from the thermostat (the green wire connects to the G terminal and runs the blower). Then turn the power back on. If the blower still runs, the problem is not the thermostat or thermostat cable. If you are not familiar with electrical testing, you are done there.
If you can use a meter, check the fan relay. Use a clamp on amp meter to determine which wire is powering the blower motor and go from there. If the blower motor has multiple speed taps, usually the black wire is high speed, blue is medium speed and red is low speed. If the color code is differemt, it will usually be on the motor or the furnace schematic.
If you can't test it, you are just guessing. Guessing with a gas furnace is not a good idea. In that case, the best bet is to hire a company to test it, but all techs are not created equal. Ask if their techs are NATE certified. For more information on this, go to www.natex.org
Good luck
Stretch
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Not trying to question your knowledge there Stretch, but:
What if (and I have seen it happen many times) the wires got crossed somewhere in the wall? Someone was hanging a picture and drove a nail through the wire. Wire is run in the attic someplace and they set something on or against it and pinched them together. Someone had the cover/door off the furnace and the pinched the wire putting the cover back on.
What you describe could maybe eliminate the thermostat but not necessarily the thermostat wire....
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Dr.
What if?
It is possible but not likely that they used a color other than Green for the blower control wire. Even the old Lennox systems used Green, and they had the weirdest color code out there.
The nail through the wires is possiblity, but then disconnecting the green wire AT THE FURNACE would stop the blower, which would show the thermostat OR thermostat cable is at fault, as I said in my post.
Same thing for pinched together.
I said to disconnect it at the FURNACE so the thermostat cable would be included in the test.
Stretch
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Ain't Lennox wiring great? I like the old V and VR terminals. or York's 66 and 60 ones.

O.K. I guess the way I read it, it sounded like you said to disconnect it from the thermostat:

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You should listen to your friend (girl). She is right. It is the thermostat. Its almost always the thermostat. If you suggest this and you are right, you will most likely get laid as that is what you are looking for................to be the hero to her. Bubba
On Sat, 25 Feb 2006 21:44:45 -0600, "Glen Cooper"

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Hello again gang. Great group I must say.
I disconnected the green fan wire at borh ends with the power off, checked it for a short to the other wires. The fan cut off with the power on and the safety switch jumped. I replaced the thremostat, and bingo!
Thanks again
Ed
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On Sun, 26 Feb 2006 18:20:52 -0600, "Glen Cooper"

gas line a little to let it breathe. Make sure you replace the thermostat again. It takes twice to get acclimated to your system. Bubba
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wrote:

Thanks for the tips.
Ed
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wrote:

Blonde hair, green eyes, about 5'8", about 37C/25/37, six figure income, ..., interesting night I must say. Thanks again Bubba for all your help! :-)
Ed
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