Furnace / Nest Mystery

Old 11-26-21, 05:19 AM
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Furnace / Nest Mystery

Iím having an issue with my furnace / nest thermostat that only occurs at night. During the day everything works fine. Starting at around 9PM, at the next call for heat, the furnace will fire (natural gas), then shut down with an error code 13. This locks it out, so we have no heat. It will eventually restart in the morning after about six plus hours. Sometimes when it restarts it will shutdown with error code 33, then run with fan only (no heat), then restart and work for the rest of the day.
This is a six year old horizontal Carrier unit installed in the antic serving upper floor, model 59SP5A040E14. We have a similar unit and thermostat in the basement serving the first floor with no issues.
We have had a tech look at the furnace and everything looks ďperfectĒ. Clean condensate drain, fan speed high, gas pressure correct, combustion / flue gas excellent, flutes clean, flame sensor resistance correct, new air filter, all vents open. Again, it operates fine during the day.
I did notice something funny in the nest history. During the day, The heat will go on for 15-20 minutes to reach setpoint. But at night, the nest is calling for heat for only a minute or two and then stopping. It then starts a call for heat a few minutes later and the furnace is not working, so the call for heat continues for the rest of the night. There are also a few periods during this time when the nest stops calling for heat. Iíll try to attached an image.
Canít figure out if it is an issue with the furnace or the nest. The furnace is getting errors that imply it is overheating, but we canít find a cause. The only difference between night and day could be outside air temperature (colder at night), but that doesnít seem to be a consistent factor. Why would the nest history show these strange heating patterns at night? There are no setpoint changes in the schedule.
Any ideas would be appreciated!


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