My new White Rogers programmable furnace stat says it needs 24 v dc to run it.
I installed it and am using the 2 AA batteries to power it, but I have 4 wires coming in now. I have A/C. The stat is running OK now but if I take the batteries out the display goes dead. I don't know how to run 24 v transformer power to this stat. I can fish another wire up to the stat, and I see a transformer behind the furnace at an AC outlet. Should 2 wires go up to the stat? One from trans. Ground, one from + trans. To the RED terminal?
It appears the (now) 4 wires go onto a terminal block just inside furnace chamber. Nothing goes to this transformer which was here when I moved in. I don't want to blow the furnace computer relay.
Can anyone advise?

I think you need to go back wherever you bought the thermostat and insist on more information, or see if you can find the manufacturer's web site.

Usually there is a 24 volt AC transformer in the furnace connected to a red wire running to the thermostat and a blue wire 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. It may be wired to have the 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.

There is no guarantee the existing furnace was wired as above. Start with the A/C. There should be a pair of wires running out to a relay in the condensing unit that controls it. Note their colors. Does your thermostat and furnace have terminal blocks marked W, Y, R, etc. The terminal the blue wire goes to may be marked com. If so, it may be as simple as connecting the proper colors of wire to them.

With power to the furnace, and nothing connected to the transformer behind it, do you have 24 V AC at the red and blue or common terminal? Or red and any of the other terminals? If so, it is unlikely the transformer behind it is needed for the furnace. If you don't have power at the furnace terminal block, perhaps you do need to run one wire to the red terminal and one to the common one from the other transformer. It is possible there was a transformer inside the furnace which went bad and somebody wired one in behind it.

It sounds like the thermostat uses the 2 AA batteries to power its logic and switches the power from the transformer. Is the gas valve and A/C relay marked as having a 24V AC coil? The gas valve will be in the gas supply line near the burner assembly. It will have 2 wires going to it, and may be part of other controls. I am still confused about the 24 volt DC.

Thanks for the reply.
The wires going up to my stat are: yel - yel screw on stat, red - RC (with short to RH), what - what, green to green.

The terminals on the stat available are: W, RH, B, RC, C, G, O, Y.
I think it is wired OK as it is. (furnace works)

From the A/C compressor unit, a white wire goes to a screw (grd) inside furnace, the blk wire goes to blk on terminal block inside furnace. This unit runs fine.

The terminal block inside (gas) furnace on right side of frame has... W, R, Y, G, C screws on it. Going down in order on a block with 3 screws for each block. This terminal block has a little stick-em that says "24 v Thermostat Connection" above it.

The (terminal block) W screw has the Wht wire with it, R screw has Red wire, Y screw has Yell wire to it, G screw has Grn wire to it. C is vacant.

Also on the W terminal is a BLK thick wire going to a fuse, the other side of this fuse is a yel wire going into a transformer built inside the furnace that says 25 v. ( there I found it!) the other yel wire from the transformer then goes out and goes to a "Honeywell" control box connector marked 25 v.

It appears by this wiring the red wire to stat should be "hot" with 25 v power. (I was wrong when I originally said 24 v DC, I guess it is AC).

But as I'm telling you this it seems the WHT wire to stat, is really hot, (as it goes to the transformer), and I am now totally confused.

To bring that furnace man in he always charges a lot, and I think this should be simple?
I appreciate the help.

It appears by your info and my last report of my furnace wiring... the thick black wire coming from my 24 v relay should not be connected to the WHT furnace block terminal (as it is now), it should be the thick BLK wire from the relay connects to the RED terminal block on furnace? Then it would be powering the RED wire up to the stat.
Do you agree??

Sounds good. One terminal of the transformer needs to be connected to the hot wire to the thermostat, usually red. The other needs to be connected to the other side of everything that has a wire from the thermostat.

It is so much easier to start new than straighten up something done wrong.

Most all thermostats use 24 volts. That is the voltage that they turn on and off to make the furnace to come on and off. That transformer that you see may possibly be the transformer that was used to power the zone valves in another furnace. The thermostat wires from the furnace to the thermostat should only be white and red. I do not see why 24 volts more would need to be connected to the thermostat. I think you are more confused than anything. Recheck the wiring diagram that came with the thermostat again.

Labman:
Your thermostat will work just fine with batteries. If you want to power the thermostat from the furnace & thus not need batteries, you need to run a wire from the C terminal on the furnace to the C terminal on the stat. It's just that simple.

Handyman2007:
Red=power to the stat, W=power out of the stat for heat, G=power out of the stat for the fan in cooling or "fan on", Y=power out of the stat for A/C condensing unit.