trader_4 wrote:
On Tuesday, July 8, 2014 8:08:01 PM UTC-4, Davej wrote:
On Monday, July 7, 2014 10:14:57 PM UTC-5, Davej wrote:

On Monday, July 7, 2014 9:07:04 PM UTC-5, philo wrote:

On 07/07/2014 08:55 PM, Davej wrote:



So why does a gas furnace control board have a

contact for the yellow wire? It makes no sense to me.



What makes no sense is your vague question



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.





So perhaps that was at least part of my trouble. When my furnace was installed they did not connect the yellow wire to the furnace control board.


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.




Perhaps they were afraid there might be another 24V transformer somewhere and did 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.

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 furnace.
The yellow wire is the call for cooling.

Hi,
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.