Hi. I just replaced my old mercury thermostat with a Honeywell RTH2310. The old wires were : yellow connected to a "Y" screw, Green connected to a "G" screw, White connected to a "W" screw, red wire connected to "Rh" screw and a Blue wire was connected to a blank screw, but was right next to an "Rc" screw. On the new Honeywell, I connected the obvious, yellow to Y, green to G, white to W, red to Rh and the blue to the Rc. Was this OK to do? What was the blue wire for and why was it not connected to anything specific on the old thermostat? Thanks.

You would have to go to air handler to see if connected. Blue and yellow I see sometimes as cool(outside compressor contactor control wire)

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Originally Posted by Stratmando

You would have to go to air handler to see if connected. blue and yellow I see sometimes as cool(outside compressor contactor control wire)

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Huh? Perhaps I didn't phrase my question correctly... When I connected the new Honeywell RTH2310, I connected the red wire to the "RH", the blue wire to the "RC", yellow to Y, green to G, white to W etc. Everything was working OK when I turned it all on and checked if the heat ran and then tested the A/C. My question was if anyone knew if the blue wire actually does anything? So, while waiting for a response, I opened the Honeywell control panel and loosened the blue wire screw, and tested the unit again, by turning on the Heat and then the A/C. Everything seems to work. I don't have the programming set yet, because today, the temperature is mild and don't need Heat or A/C today.

Here is a wiring color map for thermostats. Convention is for blue to be used in 2nd stage cool, them heat or common/neutral. If the blue is one of these it doesn't make sense to connect it to this thermostat, since this is a battery powered single stage thermostat. That explains why removing it does nothing. However, if you have a transformer in your air handler to power a thermostat without batteries, and the blue is your common/neutral, you probably blew the transformer.

You might want to find where that blue goes in case you need to use it in the future.

FYI Rh and Rc are for those rare dual heat and cool transformer systems. Which is why they were jumpered together.

Thanks for responding. As it turned out, yesterday it got quite warm and the automatic programming did not kick on to cool the house at the time I had programmed in there. So I tightened the screw to that blue wire in the slot. Then the programming mode suddenly kicked on and the A/C started. I'll probably call a service technician though, because now there's a puddle on the floor in front of the Heil, so something's probably clogged and needs to be cleaned out.

I understood your question, So you looked in the Air Handler and saw it was Connected, Not connected, Both?