[dboeth]

We have two electric central A/C units - one upstairs, one down. We discovered that the upstairs A/C unit was running 24/7 making the upstairs very cold (and a $680.00 electricity bill). The original Honeywell programmable thermostat was not shutting it off regardless of the room temp -vs- program.

Last night, I went to Home Depot and purchased a replacement 5-1-1 Honeywell thermo. After installing the new thermo, there was no power to it.

The old thermo was

Blue Wire to C terminal on thermo
Red Wire to R
Yellow Wire to Y
Green Wire to G
White Wire to W

The new thermo was wired

Blue wire (capped & taped)
Red Wire to R & Rc (factory supplied jumper)
Yellow Wire to Y
Green Wire to G
White Wire to W

I reinstalled the old thermo and there was no power to it either. I tried touching the red and green wires together and nothing happened, so I assume I have shorted something out.

My local utility sent a contractor out to seal our systems, so they have taped and applied a plaster/glue looking sealant on both units in the attic. I can't determine how to open the unit to access the control panel to see if I blew the fuse.

Before I tear everything up, is there another test/troubleshooting step I can take?

Thanks

[labman]

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. 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.

If you do not have 24 volts between the red and blue wire at the thermostat, you have no choice but to follow them back to wherever the transformer is and checking it. Shorting the red to the green should have started the blower. If the thermostat is A/C only, it is possible the green and white wires are not connected. Shorting red to yellow, should start the A/C. It must have 120 volts in and 24 volts out to the red and blue wires.

[dboeth]

The cause of the problem was a blown fuse on the circuit board located in the furnace unit in our attic. Apparently when the new thermostat was hooked up, the blue wire that was attached to the "C" terminal on the original Honeywell thermostat should have been capped and insulated rather than connected to the "B" terminal on the new thermostat.

The Honeywell quick start guide for the new thermo had the incorrect wiring instructions, but the actual user manual clearly said not to connect the fifth wire.

Lesson learned----read all of the materials before starting the job.