# How can I hot wire my AC unit from my furnace without a hot red wire?

My power control board inside my furnace seems to be broken and a service technician cannot come out until after the memorial day weekend. My thermostat is not receiving power. Using a multimeter I see only 25 VAC between the common and the W2 wire. Everything else is zero or near zero. Using the fact that 25 V is present on the wire connectors, is there a way to manually start and stop my heat pump until the service technician is able to visit?

My research shows that the white wire is for heating and the red orange and yellow wires are what is necessary to operate the AC. The blower within the furnace is also continually running so there should not be an issue with it freezing over.

My thinking was that I could disconnect the white wire from the cable that runs from the furnace to the AC unit and connect the R, O, and Y together, and connect them to the hot spot (W) on the control board. This would put 25 V on those three wires and return through the common which is still connected on the other end. Will this work? Or is there a better way to accomplish this?

• Those numbers don't make sense. I suspect a flaw in your methods of data collection Commented May 25, 2019 at 22:10
• @Harper What numbers don’t make sense? What would you expect? Commented May 25, 2019 at 22:13
• To start with I would expect to see it at R. What are you using for "common"? Commented May 25, 2019 at 22:18
• When I Google “thermostat wire voltages” I find a chart that shows that both red and common are 24 V from the transformer. My assumption was the common wire was a common neutral/ground. What is the best way to check each wires voltage to see what his hot and what is not? I’m using a good multi meter and I am checking for VAC, And each wire position is labeled with a letter which match the color of the wire, except for a brown wire on W-2. Commented May 25, 2019 at 22:32
• OK common mistake, assuming chassis is common like it is on a car. Nope, both 120/240V and thermostat wiring is wired as an isolated system. Now, 120V has a neutral-ground equipotential bond that is supposed to hold neutral near ground. Some people think that means it's not an isolated system; regardless it should be wired like one. Neutral is current return, ground is safety shield. Anyway thermostat wiring really is isolated, and you must measure between points on that system or your results will be garbage. Commented May 25, 2019 at 22:37