Ask Me Help Desk - T8624D Wiring Quandry

Hi All,

Ive done a lot of research on here but am yet to find an answer specific to my problem so any ideas would be greatly appreciated.

I have recently installed a Honeywell T8624D to operate my Carrier Heating / Cooling system. THis thermostat is replacing a manual slider-type unit.

I have 4 wires to my tansformer as follows:
Red - Hot
Green - Fan
White - Heat
Black - Cool

I have wired the unit in the following manner with *some* success:
Red - Rc-Rh (jumpered)
Green - G
White - W1
Black - C

My research told me that Black was supposed to go to Y1 to operate the Cooling compressor but if I do that I lose all power to the thermostats LCD as I understand the C terminal must be wired to close the circuit.

So basically I am at a point where my heating and fan work fine, but I cannot get the Cooling Unit to start. I have tried a few suggestions of Jumpering W1 to Y1 and have not had any success either.

Again, any help would be most appreciated. Cheers.

Quote:

Originally Posted by DeeBetee

Hi All,

Ive done a lot of research on here but am yet to find an answer specific to my problem so any ideas would be greatly appreciated.

I have recently installed a Honeywell T8624D to operate my Carrier Heating / Cooling system. THis thermostat is replacing a manual slider-type unit.

I have 4 wires to my tansformer as follows:
Red - Hot
Green - Fan
White - Heat
Black - Cool

I have wired the unit in the following manner with *some* success:
Red - Rc-Rh (jumpered)
Green - G
White - W1
Black - C

My research told me that Black was supposed to go to Y1 to operate the Cooling compressor but if I do that I lose all power to the thermostats LCD as I understand the C terminal must be wired to close the circuit.

So basically I am at a point where my heating and fan work fine, but I cannot get the Cooling Unit to start. I have tried a few suggestions of Jumpering W1 to Y1 and have not had any success either.

Again, any help would be most appreciated. Cheers.

Does the new thermostat use a battery to operate. If so that is the only reason that I can think of that a thermostat has to have a common running to it.
Also I do not really understand if you have a heat pump or not, or if you have just an AC but maybe the thermostat is a heat pump. Also you say you have four wires to your transformer, if you are talking about a total of four wires on the transformer, two of those wires are for connecting to high voltage and the other two are the 24 volt out put.

There are three places that you need low voltage wiring. Starting at the furnace:

Red connects to R terminal
White connects to the W terminal
Green connects to the G terminal
Black connects to the Y terminal
Also you should have two wires that are going to the AC unit: color does not matter.
One of those wires connects to the C terminal
The other wire connects to the same terminal that the Black wire is on.

Now to the thermostat:

Red wire connects to the Rh that is jumpered to the Rc
White is connected to the W terminal
Green is connected to the G terminal
Black connects to the Y terminal.

If this helps please rate this answer Below.

I don't believe these thermostats have a battery, maybe a small capacitor as it remembers the time setting when I disconnect it, but only for a short while.

And yes, when I say 4 wires, I meat 4 wires coming out of the wall at the Thermostat end. I have not done any real investigation at the furnace end.

I am not 100% sure what a 'heat pump' is but I do have a Carrier Forced Air Furnace and Carrier AC unit which were previously all controlled by the old slider thermostat using the 4 wires I have mentioned.

Thanks a lot for your assistance. Im not sure if there is any more infor I can give you at this point but Ill be happy to find out more details if you need them.

Dan.

Use the wiring diagram I gave you matching colors except the black wire, the black wire will contol the AC and it goes to the Y terminal.

I have actually already tried the wiring guide you suggeted and the LCD on the thermostat does not power up - the ONLY way I can get it to power up is by having the C terminal hooked up.

It almost seems like I need another wire, or I need to jump one of the White, Green or Black to the C terminal?

Thanks

Quote:

Originally Posted by DeeBetee

I have actually already tried the wiring guide you suggeted and the LCD on the thermostat does not power up - the ONLY way I can get it to power up is by having the C terminal hooked up.

It almost seems like I need another wire, or I need to jump one of the White, Green or Black to the C terminal?

Thanks

In my first message to you I said this: "Does the new thermostat use a battery to operate. If so that is the only reason that I can think of that a thermostat has to have a common running to it."

Since you never said that it was not a thermostat that needed 24 volts to operate I assumed that it was a battery operated thermostat. Now I am of the opinion that you need a fifth wire from the furnace to the thermostat to make it operate properly. Or if you want you can buy an adaptor that will make it work with out the fifth wire. I have never used one but I am sure you can ask at a dealers shop and find out about it.

Ill check into that adapter option. Thanks a lot for all of your help and ideas.