So, the Nest thermostat is the dining room on the sunny side of the house in the PM.
During the summer hot months I usually leave the house at 76 degrees during the day and turn it down to 70 degrees at around 6PM with the intent of getting the house temperature down to about 72 degress by the time I hit the bed at 9PM.
Here's what usually happens. As soon as I set the temperature to 70 degrees at 6PM the AC kicks in and runs for about an hour then I hear it stop since I'm usually in the living room on top of the AC unit. I go to the Nest and it says "Delayed for XX minutes" usually XX = 2 - 4 minutes depending on how fast I get to the Nest from the living room couch after realizing the AC has turned off. I don't usually look at the temperature but I don't think it has decreased from the originally setting before the AC turned on and in fact (I have a new theory) that it may in fact have remained the same or increased because the sun is beating down on the dining room where the Nest is located.
Anyway after the delay the AC starts up again and seems to continue running, even for several more hours, until the desired temperature is reached.
I've run the AC at earlier times during the day like in the AM or early afternoon and the AC seems to run continuously without the "delay" interruption.
I've noticed the above behavior more than 20 times, researched the internet for something similar and contacted my HVAC installer. The HVAC installer thought it might be the Nest Thermostat so I bought a new Nest Thermostat and it behaves the same way. The HVAC installer doesn't think it is the HVAC either.
After lots of theories involving the potential interaction between the Nest and HVAC status which my installer says doesn't exist my installer has suggested that we replace the Nest with a mechanical theromostat to see if the problem is solved. This won't happen until next summer. I believe the mechanical thermostat will solve the problem but I want a WiFi thermostat (being an engineer with a passion for control). So I'm willing to live with the Nest in that it has only a slight effect on the HVAC longevity since it requires an extra restart of the AC once a day during the hot summer months.
But, being an engineer, I have theory on what I think is going on and would like to have the group and Nest engineers comment on.
I believe that at 6PM on a hot summer's day, even with the AC just turned on, the temperature (76 Deg) in the dining room (where the Nest is located, which is on the sunny, unshaded part of the house) does not decrease or possibly even increases over a 40-60 minute period. I believe the Nest has logic in it that measures the change in temperature from when it turns on the AC to see if it is changing and changing in the correct direction. If there is no perceivable change detected or the change is in the wrong direction (i.e. hotter) the Nest Thermostat thinks that the AC has not in fact turned on, so it sends a TURN OFF command to the AC and since the Thermostat has not reached it's goal temperature the Nest tries to START the AC but goes into DELAY mode for a few minutes to prevent harming the AC unit. Once the AC starts up again at about 7PM, the sun is setting and by 8PM the temperature in the dining room has started dropping and will continue to drop over the succeeding hours until the set temperature is reached.
So why hasn't anyone mentioned this issue on the internet? In order for Nest users to see this effect they'd have to have a Nest Thermostat located on the sunny side of their home and away from cooling vents, and lower the Nest Temperature by >5 Degrees during the hotest time of day.
Thanks for taking to the time to read and comment.
Also, the C-Wire is connected and all Nest features like Eco Temperatures, Airwave, Auto-Schedule, Early-On, Cool to Dry, Sunblock are turned OFF.
