help with wiring problem Honeywell 7400


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Old 05-19-13, 07:19 PM
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help with wiring problem Honeywell 7400

Having a wiring problem installing a new Honeywell 7400 Thermostat. Furnace is Carrier 58GR100-2 Series 211. On my furnace block the C is connected to yellow wire, Y is empty, G is connected to the green, the W is connected to white, the R is empty and RC is connected to red wire.
The old thermostat was a White-Rodgers Mercury. only numbers on inside were 8550 E6 which was working but wanted to upgrade to auto heat/cool.
The issue seems to be the yellow wire. Old White-Rodgers thermostat had yellow connected to Y and system ran OK. Installed new Honeywell 7400 thermostat set to COOL and FAN ON with the yellow wire connected to Y but fan and compressor would not come on. Moved yellow wire to the C connection, fan came on but compressor did not start. Honeywell tech could not offer a fix. Any help will be appreciated.
 
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Old 05-19-13, 07:28 PM
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Did you write down the old terminal = color combination before removing the old stat?

Post this information for better assistance.


A picture of the wire connections at your furnace would also help us help you.
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Last edited by Houston204; 05-19-13 at 07:46 PM.
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Old 05-19-13, 08:54 PM
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photos of wiring

I worked on that thing for hours and when the inside temp hit 88 I put the old one back on and it worked to cool down the house. I put everything back as it was and the photo shows the original hookup. hope that helps.
 
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Old 05-19-13, 09:59 PM
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I don't see the 2 wires to your outdoor unit at the furnace.


Did you connect Yellow to the Y terminal on the left (also labeled O/B)?
Is option 170 set to 1?

Did you turn off power to the furnace before replacing the stat?
 
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Old 05-20-13, 03:52 AM
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You need to find the outdoor unit connection. Follow the pipes from the coil on top of the furnace (or below) and locate the cable that follows the pipes outside. Follow that cable back towards the furnace and find the splice. Houston's wire schematic is dead on.
 
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Old 05-20-13, 05:59 AM
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the yellow wire problem

Before I disconnected the old thermostat, I marked the old wires as G, RC, W and Y. The old thermostat wiring shown in the top two photos works as it has for years. When I tried setting up the new thermostat. I set 170 to 1.
My problem is with the new thermostat install. The connection of the cooling or yellow wire is the problem. The new thermostat has only two slots open, the C and Y. With only one yellow wire, putting it on the Y does nothing while connecting to C will turn on the fan but not the compressor hence the problem. I looked on a wiring diagram that said that C was a 24 Vac Common and Y was a cooling call. Since the fan will not work unless I connect to C, what will happen if I use a jumper to connect the C and Y on the new thermostat? I don't want to fry the new thermostat.
 
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Old 05-20-13, 07:04 AM
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Do as I said and find the splice. Houston's diagram is the only correct way in which the system will operate with the wires you have present at the thermostat.
 
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Old 05-20-13, 10:03 AM
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finding the splice

I think I found what you were asking for. Sorry for the misunderstanding of the importance. I can only blame it on a 78 year old mind. That among other important body parts doesn't improve with age. Name:  splice.jpg
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Old 05-20-13, 04:24 PM
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ok, now it makes sense and it should work just fine as wired

At the new thermostat:

Red = R & Rc
Green = G
White = W
Yellow = Y
Blue = not used

Remember there may be a 5 minute delay built into the thermostat for cooling mode to protect the compressor from short cycling.
 
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Old 05-21-13, 05:52 PM
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Thanks

I wanted to thank hvactechfw for all your help and patients dealing with me. The 5 minute delay was the final thing that made the difference. I had it wired correctly but when it didn't turn in right away I mistakenly thought I had it wrong. A little older but a bit wiser. Again many thanks to you and this awesome site.
 
 

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