Nest Thermostat Blowing Hot Air on Cool [Solved]

If you are dealing with a Nest thermostat blowing hot air on cool mode, then this is a perfect troubleshooting guide for you. There a few reasons your Nest thermostat could be blowing hot air on cool mode, and these include:

  • You Nest thermostat is not compatible with your cooling system.
  • You have conventional system and not a heat pump
  • Wrong thermostat wiring
Nest thermostat blowing hot air on cool
A man adjusting temperature on a Nes Thermostat

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 heating on cool mode.

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 are using 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 d look pretty much like air conditioners. So, you can tell if your system is a heat pump by

  • By putting your thermostat on heat mode, and if your outdoor unit kicks on, then you’ll know that you have a heat pump.
  • checking the label on your outside unit. Look for any ‘heat pump’ label. 
  • Alternatively, get the model number and any other label information is necessary, 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, then you probably have a conventional system not a heat pump. And if you can see any white wire that is labelled w that’s a sign you don’t have a heat pump

If you have a heat pump, then your thermostat should blow cold air on cool mode. If not, try testing if either heating or cooling is working properly. 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 thermostat to cool mode.
  2. Drop the temperature to the lowest point and wait for 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.

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 both as a heater and an air conditioner, it might be working in the heater mode.

Changing orientations between heating and cooling depends on the heat pumps model that you have.

To change orientation on a nest thermostat, do this:

Go to the home app and tap settings on the top right. Then click on thermostat and scroll down to 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 properly. I your thermostat is still not blowing cold, try selecting another orientation and test your thermostat again.

If 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 start working as it should, you need to test the cooling and heating again.

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

O/B terminal on your Nest thermostat is responsible for charging your changeover valve either for cooling or for heating. If this terminal is loosely connected or not connected at all, it will cause problems with your thermostat controlling heating and cooling for your heat pump. To check if 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 O/B terminal is firmly connected. Loose wires can negatively affect how your thermostat operates. Ensure also that the end of the wire connecting on 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 of the heat pump wiring diagrams to help you out 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 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 be misbehaving simply because it’s not compatible 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 not compatible with Nest thermostats.

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

Sometimes systems may need the C-wire to be installed for them 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 soon or later start developing problems. 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 turn 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 independent 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 blowing hot air on cool mode, this guide provides the most common ones. In case 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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