r/Nest • u/HodginsBackmaker5681 • 2d ago

Heat Coming on in Cool Mode

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I just got a Nest thermostat installed this week. My electrician put it in. In the app, I have it set to Cool mode, and that’s what it says on the unit as well. But I’m noticing that since it was set up, it’s also sending a signal to my boiler to turn on and send heat to the radiators. My electrician insists there’s nothing with the wiring that should cause this, but obviously this shouldn’t be happening. When I set my comfort and sleep temperature programs, there is a “Heat To” field, but I set it super low in the 50s so it wouldn’t activate the boiler, which shouldn’t be happening anyway in Cool mode. Want to know if anyone has had this issue before and might help resolve it without me needing to talk to the electrician again.

I have a boiler/radiator system, here is a picture of the wire configuration. Can someone let me know if maybe it’s wired in a way that the system thinks I have a heat pump or something?

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u/AStuf Nest Thermostat Generation 3 2d ago

It is wired correctly assuming the red Rc and the brown C go to the a/c air handler. The white W1 and red Rh would go to the boiler. You can verify that it is a thermostat issue by removing the white wire (press the tab to release).

There have been a few systems that needed a resistor installed at the boiler so that it was not as sensitive to thermostat issues.

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u/HodginsBackmaker5681 2d ago

Thanks! So what would that mean if it’s a thermostat issue?

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u/AStuf Nest Thermostat Generation 3 2d ago

Try disconnecting both the W1 and Rh wires from the thermostat. Don't let them touch one another. Then turn on the a/c. If the a/c works and the boiler doesn't come on then the wiring is good but your boiler is sensitive. It a/c doesn't work then it is a wiring issue.

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u/ebusch73 2d ago

You would need a wire in the OB terminal for the thermostat to run in Heat Pump mode, so that's likely not it. Do you happen to have a picture of how the original thermostat was wired? Best guess would be that your Rc and Rh are reversed.

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u/HodginsBackmaker5681 2d ago

I don’t have a picture of the original configuration, unfortunately. So if it is wired correctly, do you think this is something that could be resolved in the app?

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u/ebusch73 2d ago

I don't think it would be related to any thermostat or app setting. It's either a basic wiring issue, or as the other poster mentioned something specific to how your boiler detects activation.