nest keeps blowing cold air when it's set to heat, the ac isn't even on

Diagnosis

The issue is almost certainly a "Reversing Valve" configuration error. In heat pump systems, the O/B wire controls the reversing valve, which determines whether the system moves heat into the home (heating) or pumps it out (cooling). If the thermostat is sending power to the O/B wire during a heat call, the system will blow cold air even though the thermostat display indicates "Heating." This typically happens because the 4th Gen Nest Learning Thermostat defaults to "O" (energize in cooling), but certain brands—most notably Rheem and Ruud—require "B" (energize in heating). When the setting is inverted, the thermostat tells the heat pump to reverse its cycle at the exact moment you want it to warm up. You can confirm this by checking if the outdoor unit is running while the indoor air is freezing; if the compressor is humming but the air is cold during a heat cycle, the reversing valve is being triggered incorrectly.

How to Fix It

To resolve this, you must manually override the reversing valve polarity in the equipment settings. Follow these exact steps:

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.
5. Locate the O/B setting. If it is currently set to "O," change it to "B." If it is set to "B," change it to "O."
6. Select Done or Save to apply the changes.
7. Wait approximately 5 minutes for the system to stabilize, then set the temperature 5 degrees above the current room temperature to trigger a heat call.
8. Verify that the air coming from the vents is now warm.

If That Didn't Work

• Check for a "C-Wire" Power Issue: If you do not have a Common (C) wire connected to the C terminal, the Nest may be "power stealing" from the Rh or Rc wires. This can cause erratic signals to the reversing valve, leading to intermittent cooling during heat cycles. Install a Nest Power Connector or a 24V C-wire transformer to ensure stable voltage.
• Inspect the O/B Terminal Connection: Remove the thermostat display from the base. Ensure the wire in the O/B terminal is stripped to exactly 3/8 of an inch and is inserted fully. If the wire is loose or frayed, it can cause a partial connection that triggers the wrong cycle. Use a small needle-nose plier to ensure the copper is making a solid connection.
• Verify Heat Pump vs. Conventional Setup: In the Equipment menu, ensure the system is actually identified as a "Heat Pump" and not "Conventional." If you have a dual-fuel system (electric heat pump and gas furnace), ensure the "Auxiliary Heat" is configured correctly, as a failure in the heat strip transition can sometimes trigger a system reset that defaults to cooling.
• Perform a Factory Reset: If settings are corrupted, go to Settings > Reset > All Settings. During the initial setup wizard, when prompted for the system type, be extremely specific about your brand. If you have a Rheem/Ruud, manually select the "B" setting when the O/B prompt appears during the setup flow.

Heads Up

• Avoid Frequent Toggling: Do not rapidly switch between heating and cooling modes to "test" the fix; heat pumps have built-in short-cycle timers that can lock out the compressor for 5-10 minutes to prevent hardware damage.
• Check Air Filters: A severely clogged filter can cause the evaporator coil to freeze, which may make the air feel cold even if the reversing valve is working correctly. Replace filters every 90 days.
• Voltage Warning: The Nest 4th Gen is designed for 24V AC systems. Never connect it to a high-voltage (120V or 240V) line-voltage system, as this will permanently fry the control board and void your warranty.