Hi,

Maybe this is discussed in here elsewhere, but I didn't find it.

I want to replace a Trane TCONT802AS32DAA 5 wire thermostat with a Trane TCONT900AC43UAA 3 wire. Is this possible? The current wiring is

Red wire to RC with a jumper to R
White wire to W1
Yellow wire to Y
Green wire to G
Blue wire to B

It is connected to Trane XV90 furnace and XL14i A/C unit.

It looks like the one you are using now is setup for a heat pump. The 3 wire thermostat you want to use will not work in that case. If you have just a plain A/C then it still will not work since you will have no Green connection for the blower.

Yes it is possible. U have to install a relay panel to tell the tcon900 that the equiptment is not communicating equiptment. Tcon 802 and 900 are the same thermostat except the 900 is for new communicating systems. If you want to upgrade try the tcon950 with the relay panel. It has many more great features and it is very easy to navigate.