Fan should NOT be activated during cooling mode

[paulnederland]

I have a Nest Thermostat that connects to an automation system that takes in one of three signals: Vent, Cool or Heat. The Vent request turns on a separate fresh air supply fan. The Cool request turns on the Air Handling Unit that automatically turns on its own blower.  I configured the thermostat for "gas" and selected to NOT control the fan. That works fine. Also, if I ask for Fan at the thermostat it only turn on the fan. Both request are correctly issued by the thermostat (FAN only energizes the fan G-wire and HEAT only energizes the W-wire)

The problem is the Cool request; it turns on both Y-wire and G wire. Is there a setting that I overlooked where the fan is NOT energized when Cooling is requested?

[paulnederland]

forgot to mention; my thermostat is model:

thermostat-1.4

[Markjosephp]

Hello paulnederland,

I'm sorry for the delay. I know it has been a while, but I wanted to ensure that everything was covered here.

To clarify, could you please tell us the type of Nest Thermostat you have? Also, regardless of what the setup process recommends, it appears that you're experiencing power issues with your cooling system. Do you have a C-wire or a common wire connected to your thermostat? Please check out this article to learn more about how it works, and let us know if you need more help.

Best,

Mark

[Jenelyn_O]

Hi there,

Checking in — could you please share with us the answer to the questions asked by Mark? It'll help us to move forward. 

I appreciate your help, Mark.

Thanks,
Jenelyn

[paulnederland]

Hi Mark,

The Nest thermostats I have are Model 1.4 running 1.1-11 software.

I have a C-wire connected to a dedicated 24VAC power supply.  For some strange reason, one of the thermostats lost power twice but has remained up for a number of weeks now, 

The reason for my unusual requirement for separate control of the Air Handling Unit is that I wanted to be able to control the fan for a vent-only operation of the Air Handling Unit and increasing the flow by a factor 2 while a damper opens up to allow outside air supply. I had hoped that i could use the thermostat for heating without turning on the fan by claiming the heating is electric. That setup actually worked. A heating call only energizes one wire. However, asking for cooling always energizes the cooling-wire AND the fan-wire. That makes it  difficult to distinguish between a Vent-only, or Cooling call.  I had to put some extra logic (per zone) in to ignore the fan-wire in case there is a Cooling call.   Unfortunately, I have many zones. 

I understand the reasoning to turn ON the fan when there is a cooling call because in most cases, a fan is used to transport cold air for cooling. I just wish there was a Nest Pro setting that allows for some fine tuning of the fan (+ make up air supply) operation.

[Markjosephp]

Hi paulnederland,

Gotcha! Thanks for the input. I'd love to know more about this.

Could you please share with us your thermostat's wiring? Also, gather the power readings on your actual thermostat's Technical Info settings.

I'll look forward to your response.

Regards,
Mark

[paulnederland]

I use the most simple Wiring:

R: Red: connected to 24VAC power

C: Blue: connected to 24VAC  common

W: White: connected to 24VAC  relay to signal Call for Heating

Y: Yellow: connected to 24VAC  relay to signal Call for Cooling

G: Green: connected to 24VAC  relay to signal Call for Fan

Power readings are: 

VIN 36.957

LIN 385

PSC: 385

The behavior of the thermostat in 4 different scenarios is as follows:

1. turn thermostat OFF; RESUT: no wires energized
2. turn heating ON and raise set temp above actual temp; RESULT: Energizes W (and nothing else)
3. turn cooling ON and lower set temp above actual temp and turn on fan timer; RESULT: Energizes G
4. turn cooling ON and raise set temp above actual temp and turn on fan timer; RESULT: Energizes Y and G

This las case is the problem; the thermostat is smart enough to realize that cooling can only be achieved by supplying cold air to the building.  However, this prevents an installation that by itself interprets a Y or W energizing signal as a need for running the fan, and in return interprets a G energizing as a need for outside, fresh air supply to vent the building. 

Once again, my question is; is there a way to decouple the Y and G signal? Maybe in a different thermostat than I'm currently using?

[Markjosephp]

Hey there,

Thanks for the details. Could you please check if the Fan settings were set to auto in the Google Home app?

Keep me posted.

Regards,
Mark

[paulnederland]

Mark,

For this thermostat, there is no auto setting in Google Home app, nor is there an auto fan setting in the thermostat interface.  

There is however, a "Fan Auto" in the phone app display when you:

1. select the thermostat
2. select Heat (or Cool)
3. select the fan icon
4. it displays: "fan Auto  Your fan runs automatically whenever your HVAC system needs it. You can run the fan manually by setting a timer". 

This is the problem; the system automatically turns ON the fan when Heat or Cool is needed.  There is nothing in any user interface setting that allows me to turn this OFF. 

This thermostat (model G4CVZ) is therefor NOT useful to me. Is there another thermostat that has more options (maybe in PRO?) that allows disabling the automatic part while still allowing manually turning ON the fan?

I really want to not throw the current 5 thermostats out and replace them with something that doesn't work either.