Thermostat yellow wire optional?

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I'm having some odd fan cycling problem on my Carrier furnace. It has behaved oddly ever since it was installed. At the furnace controller card no yellow wire is attached. The voltage on the Green wire is varying and when it gets down to 18VAC the blower fan kicks off. The red wire is a solid 27VAC.
I'm still using an old round Honeywell thermostat and I'm guessing the mercury switch in the thermostat has gone bad. Is that a good guess? When I get a new thermostat should I add a yellow wire?
Also, is the wiring diagram below essentially accurate as a typical example?
http://xtronics.com/reference/thermostat_wires.html
Thanks!
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To the best of my knowledge, Honeywell has sold over 27 billion yo-yo thermostats since their inception, and none have ever gone bad. Should you add a yellow wire? Why, what for and what would you connect it to? Thermostat cables come with a variety of colors, so don't necessarily expect colors to have definitive meanings. Your best bet is to contact a HVAC professional and let him check it out

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RBM wrote:

You right about having the system checked out by his service company.
In the business you try to stay with the same code which I learned over 20 years ago:
RED = Power= Hot (24volt) GREEN=Go=Blower WHITE=Snow=Heat YELLOW=Sun=Air Conditioning or BLUE=Cool=Air Conditioning
--
Moe Jones
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That's a qute rhyme Moe Jo....
Zyp

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Those old round thermostats are only for heat, with 2 contacts (R & W), no?
You need a FAN ON switch on the thermostat to activate the green.
The furnace fan will turn itself on when heating, but not when cooling.
-Pat
Davej wrote:

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They make a heat-cool sub base for it to accommodate a variety of applications

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I think my problem may be that the switch contacts in the base plate have gone bad. I cleaned them with alcohol but they may have lost their plating. Also I did find appropriate schematic info on the Honeywell website (CT87B thermostat looks like it). Thanks for the replies.
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If you think the switch contacts may be bad, bypass them. No need to buy a test replacement thermostat. Either bypass the switch with a separate wire**, or disconnect the wires and touch them together***.
**Everyone should have 10 or even 20 jumper wires with alligator clips on the end. You can make special ones, like 6 feet long, and you can buy them in bags of 10 of 5 different colors from radio shack. You may be able to clip the alligator clips to the screws on the back of the thermostat, or to the wires.
***It's only 24 volts and you won't even feel it. Or you can probably use even a single alligator clip, paper clip, wire nut, or twist them together a little bit. It's only for testing.
Or disconnect one wire and mount it under the screw that goes to the other wire.
P&M, reply by post.
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Yes I've tried jumpering, and once I found the thermostat schematic the situation became a lot less mysterious. I'm going to try the green wire attached to the (O)range thermostat terminal -- and let the blower fan run when I have the AC on.
http://yourhome.honeywell.com/Consumer/Cultures/en-US/Products/Thermostats/Non-Programmable/Non-Digital/Do-It-Yourself/Round/Default.htm
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wrote:

My house came with AC and a round Honeywell thermostat. I only changed it because I wanted an automatic setback for work and sleeping.

It has a switch like that too. It's in a box in the basement, and I may connect it to the burglar alarm so that if the furnace goes out while I'm out of town, the monitoring company will know.

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Davej wrote:

That 18V is a problem. Spec calls for minimum 20V. You have bad poor connection somewhere or thermostat wet contaq ct is resistive.
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You're right. Except, Is it possible for a mercury switch to be resistive?
Could his 24 volt transformer be failing? Maybe not since he has 27 volts most of the time. But I like this story, so I'll tell it again anyhow. I moved into this house in late May, and had 4 ffriends from NYC for July 4 weekend. At noon on Saturday, the AC failed. At 6PM the water failed. And at 6PM Sunday, the electricity failed. And the house was almost new!
But we had a good time, and after they left, I checked the oil furnace and the 24 volt transformer that powered the AC control system also had failed to zero. They wanted 150 for a whole control unit, but sold me a transformer for 20. Too big for the space so I mounted it a foot away. STill working fine 24 years later.
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mm wrote:

How wet switch makes connection? Mercury bubble bridges two electrodes. Those metal parts can get you know what. Easy thing to prove that is measure across the switch with meter tilting the bulb back and forth.
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I suspected the mercury switch at first but the problem turned out to be the mechanical ON/AUTO switch in the thermostat base.
I guess connecting the yellow wire to the furnace would increase the blower speed but I think I prefer running the blower continuously at a lower speed. I don't want the compressor and blower fan to turn off simultaneously and I don't know if connecting the yellow wire would activate a delay.
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I've got no experience with heat pumps. But the furnace and central air diagram is not wired the way I would have drawn the diagram.
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Christopher A. Young
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This is what I believe it should show...
http://home.att.net/~galt_57/thermostat.bmp
wrote:

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That is very definitely not how I've ever seen a stat wired. If you wired a unit like that, it would not work very well for cooling. Actually, it would not cool the house at all.
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Christopher A. Young
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Ok Stormy;
His diagram actually is sound. The only problem I see is that with the [newer] computer boards on some furnaces require that the "Y" terminal be connected to the furance computer board as well. Some newer furnace boards run the "continuously fan" at a slower speed than with the air conditioning ["G" terminal]. The computer recgonizes the demand for air conditioning ["Y" terminal] and runs the blower at highest speed.
Stormy, go back and check the wiring again. You will see it is correct. 2-wire condenser - 4 wire thermostat.
Zyp

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Other than the Y signal to the furnace, the rest of it looked OK. The lower fan speed won't do as good a job of pushing cold air up hill. Would work fine if the air handler was in the attic. Where I am, furnaces are typically in cellars.
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Christopher A. Young
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Up hill????
The reason for the "higher" air speed is that cool air is dense [heaver] than heated air. Take a look at a pscyometric chart and plot... you'll see the cooled air carries more weight per foot than heated air, thus needs more torque.
Actually, you can run air conditioning down to 200 cfm per ton Stormy, but the results are dismal. You will remove a considerable amount of moisture from the air but will pay for it in performance. 25 years ago, 325 cfm per ton was the norm and 400 cfm per ton on heat pumps. Now the norm is 400 cfm per ton on air conditioning and 450 cfm per ton on heat pumps. Why the difference? Performance.
Zyp

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