Odd two-stage fan issue

[Tangobravodelta]

I am trying to understand an issue with my new Nest 4th Gen thermostat. Before switching to the Nest, I was familiar with how the cooling cycle would start with the first stage fan and after a moment the second stage (much louder and more forceful) would kick in. With the Nest the second stage does not kick in -- except in a specific circumstance.

Yesterday the unit had been off all day (because of this issue) and at bed time I turned it to cooling mode. It started the scheduled cooling to go from the room temp of 75 to the set 65. It immediately started the low fan and then soon the high fan. It was pumping cold air with a strong air flow. All good.

I was awake for about 20 minutes and it continued normally. Then I woke at 2:40 am and it was 70 degrees. The high fan was not running. I went to the hallway and heard the attic furnace running the low fan.

I woke again at 6 am to the same conditions. Still 70 degrees. So I switched it to the off mode.

I have seen this pattern before where the second stage fan kicks on properly for the first cycle of a scheduled cooling period, but then the temp never reaches the set temp overnight and only the first stage fan runs with very little cooling output.

This pattern confirms it can activate the second stage fan, right? So it shouldn't be a wiring issue?

The wiring is yellow to Y1, green to G, red to RC, white to W1. That is the same as with the previous thermostat, except that the previous wiring had a jumper from RC to R. If that could be the problem, how is that fixed?

[WorldOfJohnboy]

I can't say with certainty how your wiring should be, but did want to say that I was having a "relatable" issue with my Nest Learning Thermostat (I think Gen 2) early on, after coming from a "basic" thermostat.  Essentially, my furnace would kick on to start the cooling process, but when my AC unit kicked on it would sometimes trip my circuit breaker.  I only had a 4 wire setup. 

Once I swapped it for a 5-wire (adding the Blue Common), my issues went away.  Not sure if your HVAC can take a 5-wire without expert help, but thankfully mine did and it resolved my issue.

[ByronP]

Hi Tangobravodelta,

Thank you for posting in the community. I understand that you're experiencing trouble with your 2nd stage fan that isn't active after setting your temperature for the second time on your Nest Learning Thermostat 4th Gen. I appreciate the help, @WorldOfJohnboy. Let's work together to find a solution.

For further assistance, please answer the following questions:

• When did cooling last work?

• What type of system do you have? (Heat Pump or Conventional Type)

• When was the last maintenance service and cleaning?

• Do you have a photo of your old thermostat wiring?

• Do you see a power error code on your Nest Learning Thermostat 4th Gen?

• Is this the first time you’re calling for cooling this year?

Please keep me updated.

Regards,

Byron

[Tangobravodelta]

Cooling last worked a few months ago with no problems. Heating was working fine before I installed the Nest. (At first I thought the Nest was working properly but then I didn't need either heat or cooling for several weeks.) It is a conventional system with natural gas for heat. The filter was changed about two months ago. There is no power error code on the Nest. This is the first time I was calling for cooling this year. I have photos of the previous thermostat wiring and the Nest wiring but this page says I don't have permission to upload images.

[ByronP]

Hi Tangobravodelta,

Thank you for your response and the detailed information you provided. I apologize that the image of your previous thermostat wasn't included due to submission troubles. Let's work together to find a solution.

To help me understand the situation better, please answer the following questions:

• Have you recently experienced a brownout or blackout?

• Has your utility company installed a box on your HVAC system that temporarily turns the system off during high-demand (peak) times to save energy?

• Have you experienced a period of extremely hot weather lately?

Additionally, if your Nest Learning Thermostat 4th Gen is set to learn to turn on or off at specific times, that could be why it's not working when expected. You can find helpful instructions on how to turn off temperature learning schedules.

Please keep me updated on your findings.

Regards,

Byron

[Tangobravodelta]

No brownouts or blackouts. No new box on the HVAC system. No hot weather lately. I originally had heating and cooling schedules set, but when the problem arose I deleted those schedules and it is now just set manually.

[ByronP]

Hi Tangobravodelta,

Thank you for your prompt response. I appreciate your answers. Let's explore additional options.

Please check power readings from your Nest Learning Thermostat 4th Gen. Please do the following:

1. Go to your Google Home app.

2. Tap on settings and then select your device.

3. Tap on Technical Info.

4. If you have a voltmeter, you can use it in the wiring to check the power readings.

Please review if you have a low VOC. Here is the table to review the expected power reading:

Please let me know how it goes.

Regards,

Byron

[Tangobravodelta]

When I select the device, there is no "technical info." There is "device information" and under "technical information" there I see nothing on power readings, just battery state (OK) and battery voltage (3.89 volts). Nothing says low VOC.

[ByronP]

Hi Tangobravodelta,

Thank you for your prompt reply, and my apologies for the misunderstanding. Let's explore further options.

Based on the battery voltage, your Nest Learning Thermostat 4th Gen appears to have sufficient power to operate the fan. Optimal performance of your HVAC system relies on adequate airflow. A clogged air filter can restrict air over the cooling coils. Please follow these steps to inspect the air filters:

1. The air filter is usually behind a grate in the hallway. Please check it using your furnace's manual.

2. If you can't find it, check for it inside the furnace near the fan.

3. If the air filter seems dirty or there seems to be a clog, you might need to replace the filter.

Please let me know how it goes.

Regards,

Byron