Two thermostats and HVAC units linked to one sensor but turn on at different times--how to fix?

[romperoom]

I purchased a home that has two HVAC units/air handlers that operate through the same ductwork. Such a system is supposed to be "twinned" such that the two units operate as if they are one unit. However, the previous owner purchased a (very nice and expensive) system that cannot be twinned (per manufacturer). Instead it was operated by two separate thermostats located three inches apart on the wall and which had to be set to exactly the same temperature.

The key problem with such a setup is that if the thermostats don't call for heat/cooling at exactly the same time, the system is ineffective and inefficient because one unit blows the hot/cold air backwards through the other connected unit and the shared return duct. (Twinning would prevent this, but my unit can't be twinned without replacing the relatively new units.) 

My solution was to purchase two nest thermostats and one temperature sensor and link both thermostats to the same sensor. Nest thermostats (unlike competitors) allow this. I thought I was being incredibly clever because the sensor transmits an identical temperature to both thermostats, each thermostat was programmed identically (in the app in time and temperature), and thus both units should turn off and on at exactly the same time.

But, this isn't what happens! One thermostat will regularly call for cooling while the other does not, and that thermostat will run for several hours longer than the other (and it seems random which one runs). How is this even possible?!

This is an extremely long shot, but I'm wondering whether there are ideas for possible solutions. One guess is that the thermostats differ (somehow) in the precision of the temperature (so that while I think they are set to 70 one is in fact set to 69.75 and one is set to 70.25 or something and thus the 69.75 turns on first and runs longer). Can they be calibrated somehow? Reset? Can I do something in the app? Any suggestions, however, off the wall, are much appreciated! Thanks!

[Jeff]

Hi romperoom,

Sorry for any confusion you're experiencing with your Nest Thermostat setup. It looks like you've done a lot of work trying to sort this out, and I'll be happy to help you keep looking into things. My first thought was that it's possible that the two Nest Thermostats are slightly off from one another in their learning progress. As a test, try putting both thermostats in a manual schedule and see if they stay synced up. If they still differ, we'll have to look at other options. Let me know how that test goes.

Thanks,

Jeff

[Ryan_G]

Hey everyone,

I'd like to jump in here and check if you've seen Jeff's response. Let us know if you need more help by replying to this thread.

Thank you for your help, Jeff.

Regards,
Ryan 
 

[romperoom]

Hi,

Thanks. Yes, I have put them on a manual schedule from the beginning. I really think its something about the precision of the temperature setting.

After sending this, I went back again and cut-and-pasted the exact daily temperature settings from one thermostat into the other (i.e. so that all 7 days in each thermostat was set to the same temperature and time by cut-and-pasting from the exact same parameters). That has definitely helped and the daily run time differences did not exceed 15 minutes on any day last week (down from several hours in the past). But I'm still surprised there is any variation. Why does one run 15 minutes longer? It seems like digital devises like this should be extremely precise.

Thanks!

[Ryan_G]

Hey romperoom,

Thanks for the reply and for the additional details of the issue.

The two Nest Thermostats may start cooling or heating at a different time as they have their individual sensors. This may be due to the built-in system delay to the device.

You could do a complete data reset on your devices and add it to your Google Home app once again.

Regards,

Ryan

[romperoom]

Thanks! That first article you linked is quite helpful RE the rounding of temperatures and the built-in delay. I'll try resetting the two devices at the same time and hopefully starting from scratch will resolve the discrepancy. 

Thanks!

[Ryan_G]

Hey romperoom,

Thanks for the reply.

We're happy to hear that the article helped you. We'll keep this conversation open for the next couple of days to give you more time to check how your Nest Thermostat behaves. 

Regards,

Ryan

[Jake]

Hey romperoom,

I wanted to check in with you. Please let me know if you have any questions, as I will be locking the thread in 24 hours. 

Best regards,

Jake