Installed a new Hunter Thermostat today and I am getting "No AC Power". So I looked in the fuse box and the 5V that connects the Black and Red wire was blown. I since replaced the fuse with a new one and it also blew. Please help. It gets pretty cold at night.

Where is this black wire? Coming off the transformer and connecting to the red wire through the fuse?

I would pull the thermostat off and make sure the red wire isn't shorting to anything.

Did the new thermostat have the same terminals as the old? Is it wired up much like below?

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Thermostats often have RH and RC. Unless you have a second transformer, jumper the red wire to both. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.


To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

Usually hanter thermostats have 4-wire system. That mean you have to connect two transformers (heat and AC) together. Be careful voltage must be 24V not 48V(they have to be connected in parallel). So as Labman said you need an elec. Meter. Good luck.