why is my nest blowing cold air when the heat is on? it says it's heating but it's not

Diagnosis

This issue is almost always caused by a "Reversing Valve" configuration error. In heat pump systems, the reversing valve determines whether the system is in cooling mode or heating mode. The Nest Thermostat Learning 4th Gen uses a specific terminal (the O/B wire) to send a signal to this valve. If the thermostat is configured to send the signal on the "O" setting when your system requires "B" (or vice versa), the system will operate in the exact opposite mode of what you requested. When you set the thermostat to "Heat," the Nest sends power to the O/B wire. If the polarity is wrong, the compressor will engage and the fan will blow, but the reversing valve will remain in the cooling position, pumping refrigerant in the wrong direction and blowing cold air into your home. You can confirm this by checking if the outdoor unit is running while the indoor air is cold; if the outdoor fan is spinning but the air inside is freezing, the reversing valve is simply stuck in the wrong state due to the software setting.

How to Fix It

To correct the reversing valve polarity, follow these exact steps on your Nest device:

1. Press the thermostat ring to open the main menu.
2. Rotate the ring to Settings and press to select.
3. Scroll down to Equipment and select it.
4. Select Heat Pump from the equipment list.
5. Locate the O/B Setting. If it is currently set to "O," change it to "B." If it is currently set to "B," change it to "O."
6. Once the change is saved, exit the menu and return to the home screen.
7. Set the temperature significantly higher than the current room temperature to trigger a heat call.
8. Wait 3-5 minutes for the system to cycle. If the air coming from the vents is now warm, the configuration is correct.

If That Didn't Work

• Check for a "C-Wire" Power Issue: If you do not have a Common (C) wire installed, the Nest may be "power stealing" from the Rh or Rc wires. This can cause erratic signals to the reversing valve, leading to intermittent cold air. Install a Nest Power Connector or a dedicated 24V C-wire to ensure the O/B terminal receives a consistent, clean signal.
• Verify Heat Pump vs. Conventional Setup: If you actually have a furnace (gas/electric) and an AC unit rather than a heat pump, your system is "Conventional." If the Nest is accidentally set to "Heat Pump" in the Equipment menu, it will try to use the O/B wire for heating, which will trigger the AC compressor. Change the Equipment type from "Heat Pump" to "Conventional" in the settings menu.
• Inspect the O/B Wire Connection: Remove the thermostat display from the wall base. Ensure the wire in the O/B terminal is stripped exactly 3/8 of an inch and is pushed firmly into the connector. A loose wire can cause the reversing valve to fail to engage, leaving the system in its default (usually cooling) state.
• Test the Auxiliary Heat: If you have a heat strip (Auxiliary heat), try forcing the system into "Aux" mode. If the air is hot in Aux mode but cold in Heat Pump mode, the issue is definitely the reversing valve or the compressor, not the thermostat's ability to trigger heat.

Heads Up

• Avoid Rapid Switching: Do not toggle the O/B setting back and forth rapidly. Most heat pump compressors have a "short-cycle protection" delay of 5 minutes. If you change the setting and don't feel heat immediately, wait at least 5 minutes before adjusting again.
• Verify Wire Colors: While colors vary by installer, the O/B wire is typically orange or blue. If you see multiple wires in the O/B terminal, you have a wiring error that must be resolved before the software settings will work.
• Seasonal Testing: Always test your heat in the fall and your AC in the spring. Reversing valve issues are often missed until the first extreme temperature drop of the year.