Air Conditioning/Thermostat Selection

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I have a puzzling problem. When I select "Cool" on my thermostat, the central fan does not come on, and so, while the air conditioner outside runs, it quickly freezes, as there is no air movement. On the other hand, if I select the heat-cool switch to "Off" and turn the fan "On" the fan works fine. But if while the fan is running, I move the Heat-Cool switch to "Cool" the fan shuts off. I have replace the thermostat, and have the same problem.
Any help will be greatly appreciated, as the outdoor temperature is now 90F.
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POST the make and model # of thermostat

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The first was an AprilAire 8363, the new one a Honeywell T87N
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POST the make and model of the Air conditioner

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randallbrink wrote:

Is your thermostat such that you can access the wires connected to it while it is operational? you ought to slap a meter on there and figure out if the issue is with the thermostat not commanding the fan to be on or if it's in the furnace. If your furnace is like mine, the fan is controlled at different speeds for each of heat, cool, and manual fan on - so at least in my case, if I were having your problem I'd be digging into the control board for the furnace not the thermostat. If however you have a single speed fan, it would likely be the thermostat. Which kind of setup do you have?
good luck
nate
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randallbrink wrote:

Hi, Model of outside unit, furnace inside and your thermostat? Do you have a multimeter? How many wires are on the 'stat. When you are calling for cooling, you should see 24V AC between terminals on the 'stat R and G and again between R and Y(since outside fan runs you must see 24V AC here) If you don't see 24V AC between R and G 'stat is bad.
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I'll do this test. However, the same problem has re-occured after replacing the thermostat. I can, however, access the wiring with the system in operation.
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It sounds like a wiring problem somewhere. You already confirmed the fan and compressor are working, which is good. You just need to confirm if the wiring is correct. If it is, it could be a defective circuit board on your A/C unit.
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Furnace is Coleman 8.T and the AC unit is a Lennox HS18-411-5P.
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On Fri, 16 May 2008 18:23:24 -0700 (PDT), randallbrink

Did this just suddenly happen to an existing installation, or was anything changed in the installation before the problem began, e.g. wiring, equipment, supporting electrical wiring, etc.
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If ( R ) on the indoor unit is connected to ( G ) on the thermostat and ( G ) on the indoor unit is connected to ( R ) thermostat then with the fan on it would connect G to R and with the cooling on R to ( Y ) with ( G ) OFF
Check the wire colour on the indoor Unit R - Y - W - G and make sure these are connected to the right thermostat terminals

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By "Indoor Unit" I assume you mean the furnace blower?
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randallbrink wrote:

yes, that is correct (or air handler if you have a heat pump)
nate
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replace "roosters" with "cox" to reply.
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All of the wiring seems to be correct in accordance with the installation instructions. And still, when I turn the "Heat-Cool" switch "Off" and move the fan switch from "Auto" to "On" the fan comes on. If I put the fan switch on "Auto" and turn the AC to "Cool" the AC fan comes on and cold air enters the building, but wothout the fan, it does not move and the AC unit soon freezes up.
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randallbrink wrote:

Hi, Sounds like you have bad 'stat/
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..
Well, I've already installed a new thermostat, and still have the same symptoms. Odds are not good that two new thermostats would be faulty.
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1- Check the Wire link R to Ro is inserted
2- Check fan setting switch set to F
3-Put the wire going to G on the thermostat to the R Terminal on the thermostat so the R terminal has two wires R and G ( The fan should run without ) changing the thermostat switch )

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.com> wrote in message

Will do. Thanks!
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Cycling the system
Make up a jumper wire one foot long - bare a 1/4 inch at each end. 1 Bridge R to G Fan should run 2 Bridge R to Y cooling should start after timer delay 3 Bridge R to W Heating 24 V is a low voltage however care should be taken R is the 24V Active which is switched by the Stat to Outputs G + Y ( Cooling ) G + W ( Heating ) G Fan

Will do. Thanks!
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OK, will try it.
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