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Pure L · Jun 28, 2007, 06:47 PM

Hi all.

I just installed a new thermostat thinking this would solve my problem of not being able to engage the fan using the old thermostat (using "on" or "auto".) I was wrong. I'm getting the same behavior with the new thermo.

According to another HVAC forum, it turns out the reason I cannot engage this fan using the thermostat is that I'm missing a connection. Currently, there are 2 wires that run from the thermostat to the control board of my 24v, gas furnace. These 2 wires are "rc" and "w". Now, along with those 2 wires are 2, totally unconnected green and red wires which, I'm assuming, could play a crucial role in the wiring-up of the fan to the thermostat. (They are not connected to anything at either end: the thermostat or the cirucuit board.)

Also, something else I discovered is the fact that I can take a raw wire and run it from "rc" to "w" (with the thermostat disconnected, of course) and the blower/fan will indeed run. I understand that this probably isn't safe hence my query to this board.

If anybody would be willing to lend a hand I'd be much obliged. Thanks!

Stratmando · Jun 28, 2007, 07:02 PM

If you only have 2 conductors, you can't separate to fan only. Would use 1 of the unused wires. Don't like to say can't.
Sometimes Red is hot from transformer.
Green is fan
White is heat
Yellow or blue common for compressor unit(Cool)

T-Top · Jun 28, 2007, 07:47 PM

Do you have a mobile home system? It sounds like it, if you have a A/C with a heat only thermostat. If the A/C runs but not the blower I would say you have a bad relay in the blower control part or a bad blower motor. If nothing runs it may be as easy as switching the blower control from heat to cool in side the furnace. Its hard to say need more info. The RC to W throws me off. RC is for a transformer from the A/C unit and RH is for a transformer from the heater. If you have only two wires on the stat that runs heat and cool they need to be on RH and W.

Pure L · Jun 28, 2007, 07:50 PM

Thanks for the replies.

T-top, I don't have AC. I should've specified that... sorry.

This is only a gas heater that has never had the option of running the fan/blower on the furnace. I'd like the thermostat to control this option.

I'm wondering if I could use either the red or green wire?

Like I mentioned, jumpering the "rc" to "w" with a raw wire turns the blower on. There's got to be a more elegant way though.

Pure L · Jun 28, 2007, 08:20 PM

T-top,

OK. On the circuit board of the furnace, there are 5 connections: W, Y, R, C, G.

The black wire is connected from the "R" on the circuit board to the "Rc" on the thermostat. (That's the way the old one was too... to tell you the truth, there have been other strange issues with this furnace, come to think of it... )

Anyway, the yellow wire runs from "w" on the circuit board to the "w" on the thermostat.

The new thermostat has 6 connections: G, Y, W, RC, and in the middle, one marked "not used". Ugh. Like I said, currently, "Rc" on the thermostat is connected to "r" on the circuit board. "W" to "w".

Something else that's strange is the fact that there is a loose, orange wire on the old thermostat. That wire is extremely short, orange and connected to the "rh" on the old thermostat wall-mount. I don't know if this orange wire was connected to anything or not. Given it's minuscule length (about 2.5 inches) I'd say it wasn't. However, given the current connections could you see a purpose for it?

Thanks!

labman · Jun 28, 2007, 08:52 PM

Connecting the green wire to G at both ends should make the fan option work. Some of the rest doesn't sound right. The little orange wire could have been a jumper. Normally a red wire runs from R at the furnace and is jumpered to connect to both RH and RC at the thermostat.

You might explain what the other problems have been.

Stratmando · Jun 29, 2007, 05:50 AM

Once you hook up green(Fan) wire to thermostat, will need to separate 2 wires in furnace. Green goes to fan, and other wire will remain connected to where it is now(Heat)

Pure L · Jul 1, 2007, 12:25 PM

Originally Posted by Stratmando

Once you hook up green(Fan) wire to thermostat, will need to seperate 2 wires in furnace. Green goes to fan, and other wire will remain connected to where it is now(Heat)

Is there any harm in just running the fan/blower?

I mean, if this wasn't hooked up before do you suppose there could be some specific reason?

Thanks!

labman · Jul 1, 2007, 12:39 PM

Normally, no. Most thermostats and furnaces are set up to do so. If you look at the sticky at the top of the forum, there is a discussion of it. There is also a good explanation of thermostat wiring.

Pure L · Jul 1, 2007, 05:36 PM

Originally Posted by Stratmando

Once you hook up green(Fan) wire to thermostat, will need to seperate 2 wires in furnace. Green goes to fan, and other wire will remain connected to where it is now(Heat)

Stratmando,

If I add another connection to what I've got ("w" to "W" and "Rc" to "R") I'd have 3 wires, no?

Say, if I added that green wire and ran it from "g" to "G"?

Not too sure if you're suggesting I lose one of the connections or not?

Thanks!

Stratmando · Jul 1, 2007, 05:47 PM

I was saying at thermostat,Red to R, white to heat(already is), now you are going to hook up green to g on thermostat, and go to air handler and separate the fan relay from heat.
Sounds like both together there.
No reason you can't run blower by itself. See if schematic attatched and follow back from thermostat connections,