Can Nest work with SEPARATE heating and A/C systems?

[RiverCityKen]

Hi there, all--

I've got a traditional, 5-wire thermostat controlling an Air Conditioning system, completely separate from a high-efficiency boiler heating system.  I'd like to replace the thermostat with a Nest, but I can't figure out how to utilize one of the red wires (RC or RH) to act as a C wire.   Since the boiler is also used for domestic hot water, it must run year-round, even during Air Conditioning season.  If I use the RH as the C wire, would one or both of the control system boards be blown up by in effect connecting them during the Air Conditioning season?

Thanks,

--Ken

[Markjosephp]

Hello Ken,

Thanks for reaching out! I understand your concern about replacing your traditional thermostat with a Nest Thermostat and the challenge of utilizing one of the red wires (RC or RH) as a C wire. It's crucial to handle this carefully, especially given the dual-use nature of your boiler system.

If you use the RH wire as the C wire during the air conditioning season, there's a potential risk of connecting both control system boards, which could lead to issues. It's not advisable to merge the wiring in a way that might damage the equipment.

To overcome this, consider these steps:

• Ensure that your HVAC system is compatible with the Nest Thermostat. The Nest website usually provides a compatibility checker to guide you through this process.
• If your system lacks a C wire, there are alternative methods to create one without risking damage. You might use an external 24V power source, such as a transformer, to provide power to the C wire on the Nest Thermostat.
• If you're unsure or uncomfortable with the wiring, it's always a good idea to consult with a professional HVAC technician. They can assess your system and provide a safe and effective solution tailored to your setup.
• Also, you can share with us a photo of your current thermostat's wiring setup so we can guide you on how to label it with the Nest Thermostat.

Let us know if you have more questions.

Best regards,
Mark

[RiverCityKen]

Thanks for answering, Mark.  I could easily add a 24V transformer, but if one of the 24VAC wires goes to the C terminal of the Nest, what do I do with he other wire??

--Ken

[Markjosephp]

Hi Ken,

You're welcome! If you're adding a 24V transformer to power your Nest Thermostat and one wire is going to the C terminal, the other wire should be connected to the Rc (cooling) terminal. This setup is commonly used when you have separate wires for heating (Rh) and cooling (Rc) systems. To be sure, can you post your thermostat's wiring configuration here?

To insert an image in a post:

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Keep me posted.

Regards,
Mark