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Discussion Starter · #1 ·
I’m 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 “perfect”. 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. I’ll try to attached an image.

Can’t 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 can’t find a cause. The only difference between night and day could be outside air temperature (colder at night), but that doesn’t 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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It's possible it could be the nest thermostat. Is this a newly installed nest?

If the nest thermostats are the same, just swap them out and see if that's the issue. Just a suggestion.
 

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When a temp limit trips, in some units the power is cut to the entire heating control/t-stat circuit which could cause the nest to sense that and terminate the call for heat.

Did your tech check temperature rise - supply temp - return temperature, measured near the unit?

If you have another non-nest thermostat, wise to try it as nest may cause weird issues with control boards due to current leakage, noise, etc. It has a very poor reputation.
 

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Hi, you could try jumping R and W and the thermostat location and see if the furnace continues to runs, is this something that just happened?
 

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Discussion Starter · #5 ·
When a temp limit trips, in some units the power is cut to the entire heating control/t-stat circuit which could cause the nest to sense that and terminate the call for heat.

Did your tech check temperature rise - supply temp - return temperature, measured near the unit?

If you have another non-nest thermostat, wise to try it as nest may cause weird issues with control boards due to current leakage, noise, etc. It has a very poor reputation.
Temp rise was 68F inlet, 130 discharge. Unit lists recommend range of 40-70 degree delta T, 165 max discharge.

I don't have a non-nest t-stat, but I have switched the upstairs and downstairs nests and will see if that makes a difference.
 

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Discussion Starter · #6 ·
Hi, you could try jumping R and W and the thermostat location and see if the furnace continues to runs, is this something that just happened?
I'm going to try switching upstairs and downstairs t-stats tonight.

This starting happening once nighttime temps drops below about 40F.
 

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Temp rise was 68F inlet, 130 discharge. Unit lists recommend range of 40-70 degree delta T, 165 max discharge.

I don't have a non-nest t-stat, but I have switched the upstairs and downstairs nests and will see if that makes a difference.
Was the thermometer inserted into the supply air duct out of sight of the top of the furnace?
It can pick up radiant heat and give a false high reading if inserted too close to the outlet of the furnace.

130 - 68f = 62F.

While this is within normal range, it is high for a modern furnace.
When the range is 40 to 70F, it should really be closer to 50F than 70f.

If the readings are accurate, I recommend raising the fan speed if possible to get the supply temp down to around 120F. If the main limit is indeed tripping on occasion, increasing the fan speed should solve the problem. (on paper, you should not be having limit trips with a discharge temperature of 130f, what happens in the real world can be a different story!)

You did type "fan speed high" - does you mean, it is already on the maximum speed?
It should have a lower rise than 62F when the fan is on high speed, it's only a 40k btu.

Does your nest thermostat has a wire connected to the C terminal?
 

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What does the schedule look like? Your history looks the opposite from mine. I have my temp schedule set on 64, from 12 midnight, to 8 AM. Than the temp is set on 71, from 8 AM to 12 midnight. So my furnace hardly runs from midnight to 8 AM. Looks like your furnace is running all the time during the early morning hours. Could it be your schedule setting?
 

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Discussion Starter · #10 ·
Was the thermometer inserted into the supply air duct out of sight of the top of the furnace?
It can pick up radiant heat and give a false high reading if inserted too close to the outlet of the furnace.

130 - 68f = 62F.

While this is within normal range, it is high for a modern furnace.
When the range is 40 to 70F, it should really be closer to 50F than 70f.

If the readings are accurate, I recommend raising the fan speed if possible to get the supply temp down to around 120F. If the main limit is indeed tripping on occasion, increasing the fan speed should solve the problem. (on paper, you should not be having limit trips with a discharge temperature of 130f, what happens in the real world can be a different story!)

You did type "fan speed high" - does you mean, it is already on the maximum speed?
It should have a lower rise than 62F when the fan is on high speed, it's only a 40k btu.

Does your nest thermostat has a wire connected to the C terminal?
There is no C wire.

Fan speed was increased by one of the techs who came out. Not sure if it's at max.
 

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Discussion Starter · #12 ·
What does the schedule look like? Your history looks the opposite from mine. I have my temp schedule set on 64, from 12 midnight, to 8 AM. Than the temp is set on 71, from 8 AM to 12 midnight. So my furnace hardly runs from midnight to 8 AM. Looks like your furnace is running all the time during the early morning hours. Could it be your schedule setting?
The history shows heating during the night because the thermostat is calling for heat and the furnace is not working. We have it set for a constant 70F during day and night. If the furnace would turn on, the history during the night would like the same as the day.
 

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Just a simple question... how does anyone with a Nest really objectively know if what it is doing in its algorithms is correct... or even if they are doing what they are designed to do?
 

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The history shows heating during the night because the thermostat is calling for heat and the furnace is not working. We have it set for a constant 70F during day and night. If the furnace would turn on, the history during the night would like the same as the day.
Night will run a lot longer depending on the size. Have you verified that it's not running at night? Is it actually cold in your house? Nest logging is very limited, I wish they would also log temperatures like ecobee does.

If it's not working, is there an error flash code when you look at the control board?
 

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Just a simple question... how does anyone with a Nest really objectively know if what it is doing in its algorithms is correct... or even if they are doing what they are designed to do?
That's easy, because they don't do much at all. They are a basic scheduled wifi thermostat, nothing special at all. (granted unconventional scheduling) It's logging is nearly useless.
 

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Discussion Starter · #17 ·
Night will run a lot longer depending on the size. Have you verified that it's not running at night? Is it actually cold in your house? Nest logging is very limited, I wish they would also log temperatures like ecobee does.

If it's not working, is there an error flash code when you look at the control board?
Yes, it is very cold in the house at night. The furnace is not running. I also wish it logged the temperature. We inherited these when we bought the house.

The furnace flashes code 13.
 

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Both Ecobee (which I now have) and Nest could take a cue from the old Filtrete/Radiothermostat. That app showed real time bar graphs of set and actual temperature and unit run time per hour. One glance at that and you could tell immediately how well your system is working... and especially if something is wrong. Unfortunately my Ecobee app has nothing like that.

Nest logging is very limited, I wish they would also log temperatures like ecobee does.
 

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There is no C wire.

Fan speed was increased by one of the techs who came out. Not sure if it's at max.
Nest power-steals without the common and may cause problems with control boards.

Was the temperature rise (supply and return temps) you posted taken before or after fan speed adjustment?
 

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Both Ecobee (which I now have) and Nest could take a cue from the old Filtrete/Radiothermostat. That app showed real time bar graphs of set and actual temperature and unit run time per hour. One glance at that and you could tell immediately how well your system is working... and especially if something is wrong. Unfortunately my Ecobee app has nothing like that.
Look at the web portal. You'll like it.
 
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