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
(4)Rat bit the stat wires shorting the green control wire to the feed wire?
Am I missing something, Thanks. Tomorrow is Sunday. Thought I could help
her out while not doing nothing else.
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.
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
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
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?
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
Remove NOPSAM to email me. Please let
me know if you have posted also.
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
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
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....
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.
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.
On Sat, 25 Feb 2006 21:44:45 -0600, "Glen Cooper"
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!
Great job. Make sure you leave all the safetys jumped and crack the
gas line a little to let it breathe. Make sure you replace the
thermostat again. It takes twice to get acclimated to your system.
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