[malbritton]

Hi. I am adding AC to an older furnace and I can't figure out how to wire the 24V Thermostat wires. I only see three places to hook wires in, labeled: TH, TH-TR, TR. Each terminal has two places to hook in wires. Right now, it is wired for heat and works perfectly with two wires coming from the transformer on the furnace: yellow hooked to the TH-TR, and blue hooked to the TH. There are two of 5 wires from the thermostat hooked up: red to TR and white to TH-TR.

I don't know where to hook up the yellow and green wires from the thermostat or the red and white wires from the AC condenser. (I was advised not to use the blue thermostat wire).

Do I need a fan control unit, or can I work with what I got?
Very many thanks,
Matt

[hvac1000]

You will need a fan center. You will also need 4 wires and a heat and cool thermostat. Now you could get buy with a relay BUT your old 24 volt transformer probably would not be strong( volt amp-rate) enough for all the circuits.

[Doc Harrison]

Yes you need a fan control unit and a new thermostat. Buy a heating/cooling thermostat (preferably a Honeywell T-87 series). I cannot tell for certain if you have 24 volts going to your current (no pun intended) thermostat. On many older furnaces, the unit operated off the millivolts created by the thermocouple. If you have a standing pilot, this may be the case. Either way you will need the relay in the fan control unit to energize the fan when calling for cooling. It will require some high voltage work to make this connection so please use caution. There will be a wiring diagram in the control unit and thermostat installation instructions, but any additional questions, please write in.