I replaced my old mercury thermostat last fall with a honeywell rth230b digital model and furnace worked fine all winter, now that I am trying to use my air conditioner its like the thermostat does not recognize cool. I have 4 wires two red and two white, one of the Red wires I have plugged into RH and a jumber between RH and RC, then a white in W and a white in Y and the other red in G. I can turn the fan on but the cannot get the actual air conditioner to turn on. Do I need to move the red from G to RC and discard the jumper?

Thanks much.

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Originally Posted by harlan_husker

I replaced my old mercury thermostat last fall with a honeywell rth230b digital model and furnace worked fine all winter, now that I am trying to use my air conditioner its like the thermostat does not recognize cool. I have 4 wires two red and two white, one of the Red wires I have plugged into RH and a jumber between RH and RC, then a white in W and a white in Y and the other red in G. I can turn the fan on but the cannot get the actual air conditioner to turn on. Do I need to move the red from G to RC and discard the jumper?

Thanks much.

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Since the wires came off a mercury stat I assume the new digital works off batteries, meaning there is no 24v common wire . You should be able to switch the wires to check their positions. You still need the jumper. If that doesn't work then you have something wrong with the outdoor unit. Let me know.

Thermostat wire Furnace
RH-RC-----red-----R
W---------white---W
G---------red-----G
Y---------white---Y You also need a pair wires running from Y and com at the furnace out to the coil of the compressor relay. Make sure the breaker is on and the outside disconnect is and no blown fuses.

Maybe Red is common, Green is for fan, Yellow for cool, and white for heat.
When you set thermostat to cool, the Red connects to Green for the Fan to Run, the Red also connects to Yellow to go outside to the compressor contactor. With 2 reds and 2 whites you you will have to identify, unless you identified when you removed old unit.
Many Central Air Air Handlers(inside unit)has a 240 volt to 24 volt transformer for control.
With Thermostat set to cool, you should have 24 volts at Contactor Coil.

If it's possible, you may want to consider changing your thermostat wires to one five conductor cable. It's much easier to make the connections when you have five different colors. The connections labman mentioned should work, you just need to make sure you match the correct whites & reds at both ends.

Take the cover off the thermostat and expose the four wires and remove them from the screw terminals on the thermostat.
Start with one of the red wires, touch it to the other red wire. If that make the blower run that will mean that one of the red wires is your hot wire. Now pick one of the red wires and touch the white wire, if the fire comes on you now know that the red wire you used is the hot wire, so connect it to the RH terminal (leave the jumper). Also since the fire came on using the white wire that means it controls the gas valve so connect it to the W terminal. Now you have already used the other red wire when you touched it to the red hot wire and the blower came on so that means that the red wire is the blower control so connect it to the G terminal. That leaves the odd white wire which has to be the AC so connect it to the Y terminal. If everything is OK out at the AC unit and at the furnace the AC should run. If you have a problem let people here know.

Having read your post, I thought it was obvious the heat was working. I hope you had the good sense to ignore the instructions to mess with the pair of wires connected to RH and W at the thermostat. It is just a matter of connecting the other 2 in place of the yellow and green wires.

You have two sets of 18-2 thermostat cables, each with a white and red wire. You need to trace the wires back to the indoor unit and figure out which goes to which terminal. R is for 24V hot to the thermostat. W is your heat circuit. G is for the blower. Y operates your air conditioning contactor at the outside unit. If you want you can jumper each of the three other wires to the R terminal on your thermostat momentarily to see which does what. If the air-conditioning compressor comes on, then you've got your Y wire. If the blower comes on, you've got your G wire. You won't hurt anything. Just don't cycle the air conditioner more than once.

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Originally Posted by labman

Having read your post, I thought it was obvious the heat was working. I hope you had the good sense to ignore the instructions to mess with the pair of wires connected to RH and W at the thermostat. It is just a matter of connecting the other 2 in place of the yellow and green wires.

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I based my post on the fact that I was not sure if he knew which of the two wires, he was using for the heat, was the hot wire. Therefore I told him to check them all to determine which was the hot wire.