Hi @AlexSBrady,
Thank you for posting in the community. I understand you are having issues with the wiring for your Grant Combi oil boiler as you are upgrading from a Nest Learning Thermostat (2nd gen) to a Nest Thermostat E. I'm happy to assist you with the information requested.
Could you tell me the country of your residence?
- Nest Thermostats bought for North America will generally not work with a UK/European heating system, and vice versa, due to the completely different voltage and wiring requirements (low-voltage HVAC vs. high-voltage boiler + Heat Link).
Since you are dealing with your boiler’s wiring, and especially since you mentioned having a Grant Combi oil boiler, I recommend that you contact a local professional installer in your area to check the compatibility and perform the necessary wiring.
Nest Thermostats generally work with most boiler systems that use standard low-voltage control (two wires to the R and W terminals). However, in the UK and Europe, Nest Thermostat installations, especially with high-voltage systems like boilers, often involve the Nest Heat Link.
Here are a few general things to keep in mind, although a professional is best for actual wiring:
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Before beginning any work, switch off the power to your system to ensure your safety and prevent blowing a fuse.
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When removing your old thermostat, take a picture of the wires and the terminals they are connected to for reference.
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The Nest Thermostat E supports modulating boilers that are OpenTherm compatible. It also works with condensing boilers that have standard on/off control.
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The wiring in UK/EU installations often involves the Heat Link unit operating on 230V.
You can use the Compatibility Checker for general guidance, but for boilers, especially an oil combi boiler, a professional's assistance ensures the correct and safe installation.
Let me know if you have any further questions.
Regards,
Anders
