I need to manually turn on my a/c from the wires behind therm stat


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Old 08-28-18, 06:24 AM
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I need to manually turn on my a/c from the wires behind therm stat

My thermostat was removed by a curious family member and I need to temporarily turn on the a/c before a new one comes it going to be 100' today. It's a Baystat 239 with the old mercury switches.
My question is if you want to turn the AC on do you connect the 24V red wire to the cool terminal or do you connect it to cool-O, fan-G and compressor-Y all at once? Is there harm is the three things don't turn on all at the same time (in the time it takes me to screw on jumper wires so a minute, starting with fan then cool then compressor?)

One time I blindly tried to jump the wires with a volt meter and it resulted in blowing the fuse which was not easy to change so I don't want to do that again.

Its just a regular system not heat pump or anything like that. Only care about ac

X2 - Black - Emergency Heat
O - Orange - Cool (Change Over)
G - Green - Fan
Y - Yellow - Compressor
T - Tan (Brown) - Thermistor (Outside)
R - Red - 24 VAC Hot
W - White - Aux Heat
U - No Conn. - Jumped to W below terminals
F - No Conn.
B - Blue - 24VAC Common
 
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Old 08-28-18, 08:12 AM
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For cooling only, you need to connect the 24VAC (Red) to the Y (compressor) and G (fan) terminals.
 
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Old 08-28-18, 01:46 PM
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The wiring you described is for a heat pump. But you say you don’t have a heat pump.
Which do you have?
 
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Old 09-03-18, 02:33 AM
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I think the thermostat has the extra wires for the heat pump but the attic unit doesn't have one.

So just tie red to Y compressor and G fan? I'm guessing the cool wire is for a heat pump option? Thanks this helps out a lot!

What is the blue 24VAC common for if you just connect the 24VAC hot to the thing you are trying to run?
Or is that for powering an electronic thermostats internal screen back light and functions?
 
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Old 09-03-18, 06:07 AM
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What’s the model number of the outdoor unit?
You wouldn’t have a heat pump thermostat, and extra conductors if you didn’t have one.
 
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Old 09-03-18, 08:24 AM
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I cant see the outdoor unit its too hard to get to. I know this doesnt have a heat pump.

I found that pic on the internet so maybe its not right.

I hooked up red green yellow left it on for 10 minutes and hot air came out. Its like 100'F out today!
 
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Old 09-03-18, 08:49 AM
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How do you know you don’t have a heat pump?
Hooking up those wires and getting hot air means your reversing valve in your heat pump is in heating mode.
It’s very difficult giving you advice when you have no idea what equipment you have.
 
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Old 09-03-18, 10:41 AM
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As Roughneck mentioned, if you have a heat pump, it's possible that it's in heat rather than cool mode. If the Orange wire is connected to something on the air handler, try connecting it to the Red wire in addition to the Yellow and Green. In other words, connect the R, Y, G, O wires together and see if it goes into cool mode. The Orange wire is likely the reversing valve (assuming you have a heat pump).
 
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Old 09-03-18, 10:49 AM
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On a Rheem or Ruud product...... powering the O line gives you heat.
On every other brand.... no power on O gives you cooling mode.
 
 

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