So why does a gas furnace control board have a contact for the yellow wire? It makes no sense to me.

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

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.

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.

Hi,
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?

On 07/07/2014 10:14 PM, 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.




Wire color means nothing , if you want help post a link to the schematic.

On 7/7/2014 9:55 PM, Davej wrote:
So why does a gas furnace control board have a contact for the yellow wire? It makes no sense to me.


Some applications use it.

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On Tuesday, July 8, 2014 7:46:45 AM UTC-4, Stormin Mormon wrote:
On 7/7/2014 9:55 PM, Davej wrote:

So why does a gas furnace control board have a contact for the yellow wire? It makes no sense to me.





Some applications use it.





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.

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. 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.

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.

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.

Stormin Mormon wrote:
On 7/7/2014 9:55 PM, Davej wrote:
So why does a gas furnace control board have a contact for the yellow
wire? It makes no sense to me.


Some applications use it.

HHm,
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?