Nest Thermostat Blowing Cold Air on Heat [Fixed]

Last updated: May 20, 2023

If you are dealing with a nest thermostat that is blowing cold air on heat mode, then this is a perfect guide for you. It is not uncommon for heat pump’s heat and cool to be reversed. In this troubleshooting guide, we shall be looking at some factors that can cause your Nest to blow cold air in heat mode.

Generally,  when a Nest thermostat is blowing cold on heat mode, this is something to do with orientation settings, a stuck reversing valve or loose wires in your thermostat wiring.

wrong thermostat wiring can affect a nest thermostat
An open Nest thermostat

If your heat and cool are reversed on your Nest, it could be that the orientation is not set correctly (for heat pumps only). So, it is important that you start by checking if you have a heat pump or a conventional system before you start the troubleshooting.

To quickly tell the type of system you have follow these steps:

  • On your outdoor condenser unit, look for the system’s specification label and check if there is anywhere written ‘heat pump’.
  • Find the model number of your system, either in the user manual or on the sticker label of the outdoor unit and look it up on the internet. If the system is a heat pump this should show.
  • Another simple way to check whether you have a heat pump or not is to check your thermostat wiring. Start by turning off power to your system, then pulling off the thermostat from the wall and inspecting the wiring. If there is a wire in the O/B terminal, then you probably have a heat pump.
  • Try switching between heating and cooling. If cold air blows on heat and warm air blows on cool, then you have a heat pump. The O/B is just reversed and can be fixed by setting it correctly.

Possible Reasons Your Nest Thermostat Is Blowing Cold Air on Heat

When your nest thermostat is blowing cold air on heat or blowing hot air on cool setting the first thing you want to do is check the orientation OB settings of the thermostat.

To check whether the problem is with the Nest orientation settings or not do the following

1)Put the thermostat on heat and raise the temperature a bit higher than the current room temperature

2) Wait a few minutes and if cold air blows out of the vents. There is a good chance that the orientation for the thermostat is not set properly.

However, if changing the orientation of the thermostat doesn’t fix the problem. You need to check your thermostat wiring. Let look at the common reasons your nest could be cooling in heat mode

1) Check thermostat wiring

Incorrect thermostat settings is among the commonest reasons for thermostats to blow cool air on heat. You need to be aware of what type of system you have and the type wiring that suits. If you have a single stage heat pump, wire it accordingly. If it’s a 2nd stage heat pump, wire it accordingly too. Here are some wiring diagrams for heat pumps that could help you.

1st stage wiring
  • Y1  Compressor Relay (Stage 1)
  •  G  Fan Relay*
  •  O/B  Heat Pump Changeover Valve
  • C  24VAC Common Wire
1st stage heat pump with Aux heat
  •     Y1  Compressor Relay (Stage 1)
  •  G  Fan Relay*
  • O/B  Heat Pump Changeover Valve
  • C  24VAC Common Wire
  • W2/AUX  Heat Relay (Stage 2) / Auxiliary Heat Relay
2nd stage heat pump
  • Y1  Compressor Relay (Stage 1)
  • Compressor Relay (Stage 2),
  • G  Fan Relay*
  •  O/B  Heat Pump Changeover Valve
  • C  24VAC Common Wire
2nd stage heat pump with Aux heat
  • Y1  Compressor Relay (Stage 1)
  • Compressor Relay (Stage 2),
  • G  Fan Relay*
  • O/B  Heat Pump Changeover Valve
  • C  24VAC Common Wire

Read also: Honeywell Thermostat Blowing Cold Air on Heat Setting [Fixed]

2) Stuck reversing valve

A heat pump is equipped with a reversing valve. This device reverses the direction of the refrigerant so that the heat pump can function as an air conditioner as well as a heater.

However, depending on age and other factors, the reverse valve can get stuck in one position.

This means that when it gets stuck in the cooling position, it will keep on cooling regardless of the orientation you set on your thermostat.


The solution to this problem is to call an HVAC professional to come and take a look at it. Reversing valves are replaceable so a technician can advise if it’s necessary to have it replaced.

3) Thermostat settings

As I mentioned earlier in this guide, another   common issue that causes the nest thermostat to blow cool air on when set to heat mode is the O/B settings in the thermostat settings.

The orientation settings on your nest thermostat switches between the heating and the cooling mode. Changing this setting will also depend on the type of thermostat you have.

Read also: Nest Thermostat Keeps Changing Temperature


Properly set the OB settings.

To change the orientation of your thermostat, follow these steps:

For most nest thermostats, you will need to go to your nest home app then tap on your thermostat.

Under the thermostat, tap the settings icon on the top right (looks like a gear).

Scroll down until you find a heat pump. On heat up the current OB is usually highlighted. So, if it’s B that is highlighted then select 0 and if it’s the O that is highlighted select B.

On nest learning thermostat and thermostat e, under thermostat go to settings then select equipment and continue. If O is highlighted, select B and then try to call for heat. Otherwise, if its B is highlighted, select O and call for heat.

4) Loose wires

Before doing anything ensure that you turn off the power to your HVAC system.  

Now, if you have tried changing the orientation settings from your app and that doesn’t solve the issue, you might have a misplaced or corroded OB wire in your thermostat terminal.

Start by removing the face of your thermostat and check if the OB wire is connected into the right terminal. If you are using a heat pump this should go into the OB/ terminal

Read also: Nest Thermostat Not Blowing Cold Air [Easy Fixes]


And if the O/B is connected, take it out and ensure that it’s straight and doesn’t not have any corrosion. Also ensure that the wire is exposed enough (1/3-/1/2 inch).

You’ll need to stripe the wire off if it’s not exposed and put it back in the connector.

Read also: New Honeywell thermostat blowing hot air on cool


We hope that, with the above troubleshooting tips given, you have managed to fix your nest thermostat blowing cold on heat setting. Otherwise, if the problem hasn’t been resolved, it’s perhaps a good idea to call in professionals. Thanks for reading.

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