Nest Thermostat Blowing Hot Air On Cool [Easy Fix]

Last updated: May 24, 2023

If you are dealing with a Nest thermostat blowing hot air on cool then this is the right guide for you.

Generally, if your nest is blowing hot air on a cool setting, this could be due to your thermostat setting, or you have a conventional heating system. Another issue that can cause this problem is an incorrect heat pump orientation settings (O/B) settings.

Let’s look at the following table which summarizes the possible causes for a nest not cooling.

5 Possible reasons a Nest thermostat is blowing hot air on cool

Possible cause	Main Fix
Thermostat settings	Endure you thermostat is in cooling mode (snowflake symbol)
You have conventional heating	Get an alternative cooling system
Stuck reversing valve	Replace solenoid or reverse valve
Wrong Nest orientation (O/B) settings	Correctly set the (O/B)
Wrong thermostat wiring	Ensure your thermostat is properly wired.

Lets now get into the details of the troubleshooting,

Thermostat settings

When you notice your Nest blowing hot air on cool, the first you might want to check is the thermostat settings. While this is no brainer, many times it turns out that the whole cause of the problem is just a thermostat that is actually not set to cooling.

Solution

Ensure that there is a snowflake symbol on the thermostat ( the snowflake symbol means the thermostat is set to cooling mode.

You have conventional Heating system

The truth is, if you don’t have heat but conventional heating (furnace) then you are going to have cooling.

And it can be really challenging to tell whether you have a heat pump or a furnace for many homeowners especially if you have just moved into a new home. But here is the trick you can use

Check the label or the sticker on the unit or if you can’t find it look up the model number of your system on the internet.

Another way to tell whether you have a heat pump or not is by checking the wiring of your thermostat. Typically for a heat pump system you are going to have a wire connected to the O/b terminal. If you don’t have a wire here you probably have conventional heating.

When you have heat pump systems and when you turn on heating, your outdoor unit should typically kick in.

If you believe you have a heat pump run a diagnostic test by following these steps:

Set your thermostat to cool mode ( snowflake symbol should display)
Set the temperature to something lower than the current room temperature
Wait for a feel minutes and feel the air that comes out of the vents, it is cool, if now you might want to check the O/B wire settings

Wrong Nest Heat Pump Orientation (O/B) settings

Now here is the thing, if your nest is blowing hot air on cool and blowing cool air on heat, you may have misconfigured O/B wire setting on your thermostat. O/B settings control the reversing valve.

Solution

To solve this problem you will need to reverse the O/B wire settings either on the thermostat or App.

If you are using Nest Learning Thermostat and Thermostat E, follow these steps:

On your thermostat, go to Settings Nest settings icon.
Select Equipment.
Select Continue and then Continue.
Select Heat Pump.
The Nest Thermostat defaults to an O setting for heat pumps.
If the O option is highlighted, select B.
If the B option is highlighted, select O.
To make sure the issue is resolved, test your heating and cooling again.

If you are using a Nest Thermostat follow these steps

Open the Home app Google Home app.
Tap your thermostat.
At the top right, tap Settings .
Select Thermostat.
Scroll until you find Heat pump. It should show which wire configuration is currently selected.
If the O option is highlighted, select B.
If the B option is highlighted, select O.
To make sure the issue is resolved, test your heating and cooling again.

Stuck reversing valve

Even when you have a heat pump, sometimes you may have it blowing hot air on the cool setting. This could be due to a stuck reversing valve.

A reversing valve helps to reverse the direction of the refrigerant so that the heat pump can work both as an air conditioner and a heating system.

The reversing valve can become defective over time in one direction (either in the cooling or the heating position) when this happens, that can cause the heat pump to either heat only or cool only.

Usually, when the reversing valve is defective, it is mostly its coli solenoid that is either cut or burnt.

Solution

To solve this problem you need to replace the solenoid coil, but this has to be tested with a multimeter to verify its the cause of the problem. Otherwise if it’s the entire reverser valve that has gone bad, you will need a replacement. To fix this problem.

Wrong thermostat wiring

Wrong thermostat wiring could also be the reason your nest won’t cool. Your nest thermostat wiring should match the type of system you have.

For example, if you have a single stage heat pump, your wiring will differ from that of a 2nd stage heat pump. Here are some wiring diagrams for different types of heat pumps systems.

1st stage heat pump

Label	Function
Y1	Compressor relay stage 1
G	Fan Relay
O/B	Heat pump changeover valve
Rc	24VAC power from cooling transformer
C	Common wire

2nd stage heat pump

Label	Function
Y1	Compressor relay stage 1
Y2	Compressor relay stage 2
G	Fan Relay
O/B	Heat pump changeover valve
Rc	24VAC power from cooling transformer
C	Common wire

Final thoughts

Well we hope that by now you have found a solution to a Nest thermostat that is blowing hot air in cool mode. Otherwise, if you are still facing the same problem we would advise that you perhaps call a local expert to help with the troubleshooting.