Nest Thermostat Not Cooling (Blowing Hot Air on Cool) [Solved]

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This is a perfect troubleshooting guide for you if you are dealing with a Nest thermostat that is not cooling and blowing hot air on cool mode. There are a few reasons your Nest thermostat could be blowing hot air on cool mode, and these include:

  • Your Nest thermostat is not compatible with your cooling system.
  • You have a conventional system and not a heat pump
  • Wrong thermostat wiring

Read also: Nest Thermostat Not Turning on AC Compressor [Solved]

Why is My Nest Thermostat Blowing Hot air on cool mode?

So, let’s dive into details about why your nest thermostat is not cooling.

You Don’t Have a Heat Pump

A Nest thermostat will blow hot air on cool mode if you don’t have a heat pump. Your thermostat is probably blowing the hot air because you use conventional heating (furnace). Heat pumps work both as air conditioners and heaters. You need to find out for sure if you have a heat pump.

Typically furnaces (air handlers) are installed indoors, and heat pumps (air conditioners) are located outside.

How do I tell if I have a heat pump or not?

Heat pumps look pretty much like air conditioners. So, you can tell if your system is a heat pump by:

  • You’ll know that you have a heat pump by putting your thermostat on heat mode and if your outdoor unit kicks on.
  • Check the label on your outside unit. Look for any ‘heat pump’ label. 
  • Alternatively, get the model number and any other necessary label information on your system and look it up on the internet.
  • Another way you can know if your system is a heat pump or not is by pulling off your Nest thermostat from the wall. And if your O/B terminal isn’t connected, you probably have a conventional system, not a heat pump. And if you can see any white wire labeled w, that’s a sign you don’t have a heat pump.

If you have a heat pump, your thermostat should blow cold air on cool mode. If not, try testing if either heating or cooling is working correctly. You can do this by first putting your thermostat on ‘heat mode.’ Then raise the temperature on your thermostat above the current room temperature reading. If that works perfectly for heating, then try testing if cooling is working too by doing this:

  1. Set your Nest thermostat to cool mode.
  2. Drop the temperature to the lowest point and wait for a few minutes.
  3. Now try to feel the air that is coming out of your vents. Is that air cold? If not, let’s jump to the next step of our troubleshooting.
Adjusting a Nest Thermostat
Adjusting a Nest Thermostat

Thermostat Settings

 If your Nest thermostat is blowing warm air instead of cool, try changing the orientation of your thermostat. Because your heat pump works as a heater and an air conditioner, it might work in the heater mode.

Changing orientations between heating and cooling depend on your heat pump model.

To change the orientation of a Nest thermostat, do this:

Go to the home app and tap settings on the top right. Then click on the thermostat and scroll down to the heat pump. Click heat pump and see which configuration is currently there. If O is already selected or highlighted, select B, and test your thermostat if it has started working correctly. If your thermostat is still not blowing cold, try selecting another orientation and test your thermostat again.

Suppose you’re using the Nest E thermostat or the learning thermostat, tap settings on your thermostat and select equipment. Click Continue, then select heat pump. On the orientation configurations, if O is one that is highlighted, try changing it to B and vice versa.

However, to ensure that your thermostat has started working, you must test the cooling and heating again. If not, you may need to to a system reset.

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Changeover Valve Issues

A changeover valve that is defective or stuck in one position could be the reason your Nest thermostat is blowing warm air instead of cool. A changeover valve is a device in your heat pump that reverses the flow of the refrigerant so your heat pump can function as both a heater and an air conditioner.

On your Nest thermostat, the O/B terminal controls the changeover valve. If you have tried adjusting the orientation on your thermostat and your thermostat is still blowing hot air on cool, the problem could be with a changeover valve.

How can I fix a changeover valve that is defective?

A changeover valve can become defective with the age of your system. Luckily, it can easily be replaced. If you think you have a defective changeover valve, call an HVAC pro to come and take a look at your system. A technician will be able to get this problem sorted out in no time.

O/B Terminal not connected

The O/B terminal on your Nest thermostat is responsible for charging your changeover valve for cooling or heating. If this terminal is loosely connected or not connected, it will cause problems with your thermostat controlling heating and cooling for your heat pump. To check if the O/B terminal is properly connected, follow these steps:

  • Turn off your system power at the circuit breaker if you can’t locate your system circuit breaker, trip the main breaker.
  • Pull off your thermostat display from the wall.
  • Check if the O/B terminal is firmly connected. Loose wires can negatively affect how your thermostat operates. Ensure also that the end of the wire connecting to the O/B terminal is stripped enough to have proper contact with the terminal.
  • Put back your thermostat and test if your ac will start blowing cold air. Check how to install a Nest thermostat here.

Was Your Nest Thermostat Wiring for Heat Pump Done Correctly?

A mistake in your thermostat wiring could be causing many thermostat issues. Ensure that your thermostat wiring is done properly for your specific heat pump. Below are some heat pump wiring diagrams to help you with your nest thermostat wiring.

For a 1st stage heat pump, these are the terminals that are supposed to be connected.

  • Y1 Compressor Relay (Stage 1)
  • G Fan Relay*
  • O/B Heat Pump Changeover Valve
  • Rc 24VAC power from the cooling transformer
  • C 24VAC Common Wire
1 stage heat pump wiring diagram

For a 1st stage heat pump with AUX heat, the following are the terminals that must be connected for it to work properly:

  • Y1 Compressor Relay (Stage 1)
  • G Fan Relay*
  • O/B Heat Pump Changeover Valve
  • Rc 24VAC power from cooling transformer
  • C 24VAC Common Wire
  • W2/AUX Heat Relay (Stage 2) / Auxiliary Heat Relay
1 st stage heat pump with AUX heat

You can read more about Nest thermostat wiring for your specific heat pump here

Your System Is Not Compatible with Your Nest Thermostat

Your Nest thermostat could misbehave simply because it’s incompatible with your current HVAC system.

While Nest manufacturer claims the thermostat is compatible with 95% of 24v volts systems. There are still 24 volts systems from different manufacturers that are incompatible with Nest thermostats.

If your system supports high voltage (120-240volts) for thermostats, it won’t work with your Nest thermostat.

Sometimes systems may need the C-wire to be installed to be compatible.


Is C Wire Required for Nest?

While the manufacturer says the C wire is not needed on many systems. A Nest thermostat with no C-wire connection will develop problems sooner or later. The most common issues you might experience from a Nest thermostat without a C wire include your thermostat getting too hot or too cold. If your c-wire is missing, you can try out these 2 tips to sort out the problem:

  1. Use an adapter that turns 4 wires into 5 wires: The adapter is typically installed at the furnace. On one end of the adapter are four wires that go onto the control board of your furnace and on the other side is an output of 5 wires that go into your thermostat.
  2. Use the G wire for the fan. You’ll pull the wire from the G terminal and put it on the C terminal of your thermostat. Then you’ll need to go down to your furnace’s control board. Pull off the G-wire and insert it onto the C terminal on the control board. Then make a jumper between Y and G. The only downside with this method is that the fan won’t work independently of heating or cooling.

Final Thought

We hope this guide has helped you in one way or another. While there could be different reasons for your Nest thermostat not cooling and blowing hot air on cool mode, this guide provides the most common ones. If the problem doesn’t go away after trying out these tips, perhaps call in local HVAC experts for help. Thanks for reading.

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