Thermostat Wiring

So why does a gas furnace control board have a contact for the yellow wire? It makes no sense to me.
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On 07/07/2014 08:55 PM, Davej wrote:

What makes no sense is your vague question
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On Monday, July 7, 2014 9:07:04 PM UTC-5, philo  wrote:

I think it is because while the green wire or the yellow wire will turn the blower fan on -- the yellow wire will also cause the blower fan to remain on for a minute or so after the compressor turns off.
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Davej wrote:

Y wire is for a/c unit compressor(ODU). Don't use it if you don't have a/c in your system. Color does not mean much. If you try to match colors what are going to do when colored wire runs out?
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On 07/07/2014 10:14 PM, Davej wrote:

Wire color means nothing , if you want help post a link to the schematic.
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On Monday, July 7, 2014 10:14:57 PM UTC-5, Davej wrote:

So perhaps that was at least part of my trouble. When my furnace was instal led they did not connect the yellow wire to the furnace control board. Perh aps they were afraid there might be another 24V transformer somewhere and d id not want to mess with it. With the yellow wire connected the thermostat now turns off the outdoor A/C compressor and then the blower fan turns off a few minutes later.
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On Tuesday, July 8, 2014 8:08:01 PM UTC-4, Davej wrote:

Assuming the wiring followed typical convention, yellow is for AC. If you have AC, it has to be connected or the AC won't run. If you don't have AC, then you don't need the yellow wire.

stat now turns off the outdoor A/C compressor and then the blower fan turns off a few >minutes later.
It's not at all clear what's going on here, what was connected, what was working or not working. But if this was a typical install following wire colors, the AC would not run without the yellow wire attached to the furnac e. The yellow wire is the call for cooling.
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trader_4 wrote:

Also there could be 24V AC x-former for cooling and heating separate. Rc and Rh terminals are usually tied together having one x-former. Simply consult the user manual for your 'stat for typical wiring example drawings. No buts, no ifs, just do it right. No exceptions. Blower does not turn off right away, turn off delay is adjustable.(by jumper or dip switch) The reason is obvious.
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On 7/7/2014 9:55 PM, Davej wrote:

Some applications use it.
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On Tuesday, July 8, 2014 7:46:45 AM UTC-4, Stormin Mormon wrote:

It's because the typical gas furnace can have AC added as an option. Yellow is the control wire for AC and the furnace control board runs both the heat and the AC, if the AC is installed. It's a minimal addition to the control board circuitry and makes more sense than having two different versions, one for furnaces with AC and one without. If there is no AC, then there is no need for connecting a yellow wire. It might still be done though, to allow for AC in the future.
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Stormin Mormon wrote:

Question to OP, would you like two 'stats and two control borads for your system? One combined dual duty controller and one 'stat, shared blower. Which do you think is better?
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