Two furnaces, two nest thermostats, only one furnace has AC

[Robt2]

Hi, my home has two furnaces each controlled by a nest thermostat but they were purchased at different times, the thermostats that is.  The large furnace has AC and services the basement and main floor.  The smaller furnace does not have AC and services the second floor.  There are ducts that have manual dampers that allow cool air from the large furnace to go to the cold air return of the small furnace so that the cool air can be distributed.  i also have a nest sensor.  my question is how can i get both furnaces to start and stop at the same time using the sensor or the temperature on the second floor.  

[mejiacruz]

Hi @Robt2,

Your home has a complex HVAC setup with two furnaces, two Nest thermostats, and a manual damper system. To effectively synchronize the operation of both furnaces based on the second-floor temperature, I'll need some more information about your system:

• What are the approximate capacities of your two furnaces (in BTU/h)?
• What are the exact models of your two Nest thermostats?
• Are the manual dampers located in the ductwork easily accessible?
• Where is your Nest sensor currently located, and to which thermostat is it connected?
• Are you experiencing any issues with the current system, such as uneven heating or cooling?
• Have you attempted to synchronize the furnaces previously? If so, what were the results?

Providing these details will help me offer tailored recommendations to optimize your HVAC system. Please feel free to share any additional information that may be relevant.

I look forward to assisting you.

Best regards,

Josh

[Robt2]

Sorry for my late response and thank you for reaching out.

Furnace 1 services the basement and main floor. it is 90,000 BTU/hr heating and a 3.5 ton cooling

Furnace 2 services the second floor.  It has 54,000 BTU/hr heating

Furnace 1 is controlled with a nest display-3.4 with backplate-5.3, software 6.2-27, thermostat 1

Furnace 2 is controlled with a nest display-3.4 and backplate-5.4, software 6.2-27, thermostat 2

The two transfer ducts are 8" in diameter and they are easily accessible, both are wide open

the nest sensor is connected to the furnace 1 nest thermostat and is on the second floor, 18" from the the furnace 2 thermostat at about the same elevation, 2" higher.

Control of heat is easy.  Set both thermostats to heat mode, set the schedule, close the dampers and each furnace does its job.  For cooling, set both thermostats to cooling, utilize the same schedule, open the dampers fully and you would think since the sensor is close to thermostat 2 they would follow each other, but they dont. AC will come on with furnace 1 maybe furnace 2 fan will come on(it has no AC so all it can do is turn on the fan).  it is like they are reading 0.5c different in temperature because they just dont follow each other.  i can get them close and can work at times but it is not predictable.  just now the furnace 2 shut off but furnace 1 is running.  second floor temp (sensor) is 22c, thermostat 1 reads 21.5, thermostat 2 reads 21.5.

I really appreciate any guidance and please ask any questions for clarity.

[mejiacruz]

Hi @Robt2,

Based on the information you've shared, what I can suggest is:

Create a dedicated cooling schedule:

• In the Nest app, create a new schedule specifically for the thermostat controlling the AC unit.
• Set the schedule to activate during your desired cooling periods.

Prioritize sensor temperature:

• Within the schedule, ensure the thermostat primarily relies on the sensor temperature for cooling decisions.
• This might involve adjusting sensitivity settings or disabling local temperature readings during specific times.

Set a temperature differential:

• Set the thermostat on the second floor (without AC) to a temperature 1-2 degrees lower than the desired second-floor temperature.
• This will create a slight pressure differential, encouraging airflow from the first to second floor.

Utilize fan settings:

• Ensure the fan on the second floor furnace is set to "auto" or "on" during cooling periods to maximize air circulation.

This can help you to synchronize your furnaces and help with the fan move the air coming from the AC. Please try this and let me know if this helps you to get the desired behavior.

Best regards,

Josh

[Robt2]

i have been utilizing a dedicated cooling schedule on thermostat 1 and it uses the sensor exclusively during the cooling season.  with the ducts that transfer cool air to furnace 2 there is no trouble cooling as long as the fan on furnace 2 runs. 

The issue is the coordination of furnace 1 operation with cooling with furnace 2.  you reference adjusting sensitivity settings, how would i do that? 

[mejiacruz]

Hey there!

I’m thinking that the best way to get this synchronized is working with one of our Nest support specialists, they deal with these kinds of things all the time. They have some advanced tools and a deep well of knowledge about your specific issue. They'll be able to take a closer look at what's going on and get your devices synchronized and running in no time.

Here's the contact us form

I know this isn't the answer you were hoping for, but I truly believe our Nest support team is your best bet to get your devices synchronized.

Please make sure you leave your Community Thread and Community Username.

Regards,

Josh.

[mejiacruz]

Hello again,

We haven’t seen your form submission come through - were you able to access it alright? Let us know if you’re running into trouble or still need our help!

Regards,

Josh

[Robt2]

I did fill it out and sent it in prior to your recent message Josh.  since you did not get it i am assuming i did something wrong and i submitted it again.  please let me know if you received it.  Thank you for following up.  i appreciate the enquiry and support.  Robert

[pereztzum]

We've received your form and appreciate the details you provided. Please keep an eye on your email, as someone from my team will contact you shortly to help.