Thermostat says CH active but boiler not activated

[CEA5T]

I am a new owner of a Nest learning thermostat, having totally overhauled my system from gravity fed to fully pressurised, including replacement pipe work throughout. The engineer chose to fit Nest to manage the system, whereas I had been used to Hive.

I have programmed very simple schedules for CH, namely target 20c from 07:00 to 21:30, otherwise 12c. For several weeks, this has resulted in the CH firing-up in pre-heating mode at 06:00, so all good.

Until this morning.

At 06:15 I arose and the thermostat showed orange, reporting pre-heating with a target of 20 and sensing actual room temp of 17 or 18. But the boiler was inactive until 07:30 when it sprang into action! None of the Nest settings had been recently adjusted, and no disturbance in electricity supply had occurred.

HW is programmed to be active between 06:30 and 07:30, but this also seemed to have failed to occur. I wonder whether the CH activating at the very time the HW was due to end is a coincidence, or whether there may be some interaction taking place? (I had previously suspected radiators were cooling during periods the HW cylinder thermostat was calling for heat).

I am wondering whether my Nest thermostat is suffering from ‘learning difficulties’ or maybe I am just too old to understand the technology? Any pointers would be greatly appreciated.

[CEA5T]

Just adding to the mystery, the smart meter recorded 2.4kWh of gas being used by the boiler between 03:00 and 03:30 that morning. Nest was not scheduled for this, and frost protection could not have been triggered as overnight temperatures quite mild. I guess it will remain a mystery.

[luciaaa]

Hi, @CEA5T!

Thank you for reaching out to the community. I'm sorry to hear that your thermostat isn't following the schedule and your boiler isn't turning on, even though the thermostat is set to heating. I understand the importance of a properly functioning thermostat, especially during winter. I'm happy to help!

It seems this inconvenience is related to the preheating feature, specifically True Radiant. This feature might be preventing the system from heating up in time, causing delays of up to half an hour.

True Radiant learns how long your radiant system takes to heat up. It turns the heat on early to reach the target temperature and then turns it off early to avoid overshooting.

To calculate the necessary preheating time, the thermostat uses historical data (previous heating times) and current internal and external temperatures. For example, on colder days, the thermostat will activate heating earlier to reach the set point temperature.

For more information, please refer to this article: True Radiant.

To effectively troubleshoot, please answer the following questions:

• When was the Nest thermostat installed?
• Is the display securely seated on the base?
• What is the make and model of your HVAC equipment?
• Are the power breakers on, and is your home receiving power?
• Was the Nest thermostat recently reset to factory defaults?
• Is there a pre-heating message on the display?
• Does pre-heating activate too early on some days but not others?

Additionally, please provide the following information:

• A screenshot of the expanded energy history. Refer to this article for instructions: Thermostat Energy History.
• A screenshot or photo of your schedule showing all set points

Thank you for your cooperation. I look forward to your response.

Best regards,

Ana