# Thermostat wiring help



## danyell

I don't know what I've done wrong but clearly I have. I have an electric ac and gas furance. Last winter we assumed there was a problem with the thermostat since when we had people look at it they said the furance was fine. I bought a new thermostat a honeywell RTH2300b. The wires that came out of my wall are a blue wire, a red wire, green wire and a white wire. I don't have a yellow wire. The thermostat I got has a o/b, y, r, g, and w. I hooked it up by putting the white in w, the green in g, and the red in r. Now what do i do with the blue. if I put it in the ob the outside unit doesn't shut off if I put it in the y the heat doesn't work. What am I doing wrong?


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## Camojake95

I did HVAC for a few years and whenever I would do a replace an old system with a new one I would come across wiring that wasn't right. The first the I would do is look at the other end of the thermostat wire that hooks to your furnace. Usually the blue wire is a common wire but you never know how it's hooked up on the other end.


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## Stedman

Heres what the letters on your thermostat are:

o/b is for a reversing valve on a heat pump (you shouldn't use this one on your system)
Y: Condensing unit control power (air conditioning)
R: 24v from the transformer
G: Indoor fan/blower
W: Heating relays

Typically if you have only 4 wires the blue one is placed in the Y position.

If you hook it up that way and it doesn't bring the ac unit on make sure the thermostat is in "cool" mode and that the setting is at least 2 degrees lower than the temperature in the room. 

Another thing to make sure of is that there is a jumper wire between the RH and RC terminals on the thermostat wall plate. The reason there are 2 of them is for people who have different transformers for their heating and cooling systems.

Long story short, hook it to Y, make sure theres a jumper wire between RH and RC and turn it on. If it doesn't work then your new stat is faulty.


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## RussNield

So...sorry to revive an old thread but this one has similar tones to what I'm looking at.

My thermostat wiring had all the proper wires (I have hooked everything up and it seems to work) - except for it had a blue wire which the last tech that worked on the house cut and neatly tucked into the wall. 

Now I don't know why the tech that worked on the last thermostat would do this, but he gave me a bad vibe and I didn't want to call him up again just to change the therm. in my house to one I wanted to use (considering most of these are pretty easy to DIY).

I guess the answer would be - crawl up there and see where the blue wire goes, but was just wondering if i have all the necessary wires and a blue wire, is it important?


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## SABL

Hi Russ....welcome to TSF!!

The number of wires will depend on the HVAC equipment that has been installed. Whether a wire is needed (or not) will depend how the wires were connected at the furnace and which ones were used. You'll have to look at both ends of the wires......turn the breaker off that supplies electric to the furnace before opening the inspection cover.


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## Jthomas2011

We are having quite the same issue, except ours will come on, run,shut off and then doesn't come back on.....


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## pcride

As someone mentioned you will need to jump the wires at the furnace and this usually disables the fan on/auto function. It's common with the Honeywell


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## Tonyrc

Im having the same issue.(I have done my due diligence,youtube etc.)See pics.I didnt have a thermostat when I moved in,nothing was tagged so the wiring I have done is all wrong.Nothing is working.I need to know how I troubleshoot this or any specific wiring directions.
Electric Heat & Air Unit BDP Company-Carrier Model# 541DB048

Thermosatat Honeywell RTH2410B


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## pcride

Here's mine, I have AC unit and heater fuels by natural gas. This may not match yours depending on what you have, style and model, heat pump etc...


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## wmorris20

I wanted to let you know that your comment about the jumper between Rh and Rc helped me fix my issue. Thanks!


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