New Honeywell Thermostat Blowing Hot Air On Cool [Solved]

Last updated: April 09, 2023

Is your new honeywell thermostat blowing hot air on a cool setting? If so then this troubleshooting guide is specifically for you.

When your honeywell thermostat is blowing hot air on cool mode, the following are the possible  main causes for this problem

  • A defective or incompatible Thermostat
  • A Stuck Changeover valve
  • AC Condenser unit not working
  • Bad thermostat wiring
  • Defective circuit control board
  • Incorrect thermostat settings
  • O/B terminal not connected
  • You have conventional heating (not a heat pump)
wiring a new thermostat

 However, before we jump into details of our troubleshooting guide , you need to know that this guide applies to nearly all new thermostats including the following listed brands below:

  • Honeywell thermostats
  • Sensi thermostats
  • Ecobee
  • Google nest thermostats
  • Any other smart thermostat

Reasons A Honeywell Thermostat is Blowing Hot Air on Cool

Possible causeMain Fixes
Incorrect thermostat settingsSet your thermostat to cool
Bad thermostat wiringWire your thermostat correctly
You have conventional heating (not a heat pump)Get an air conditioner or heat pump
AC Condenser unit not workingTurn On condenser unit
A Stuck Changeover valveReplace changeover valve
Defective circuit control boardReplace circuit control board
A defective or incompatible ThermostatReplace thermostat
O/B terminal not connectedConnect the wire to the terminal

1) Incorrect thermostat settings

This is the first thing you want to check and correct when you notice that your Honeywell thermostat is blowing hot air on cool. Wrong thermostat settings are the number one culprits for the cooling and heating problem. Although this is no brainer, many homeowners forget to switch between heating and cooling on their thermostat.

If you are using a Nest thermostat, change the orientation of the thermostat by doing the following .

You need to be sure that you are using a heat pump for controlling your temperature.

If you are using nest learning, or Nest e do the following:

 Go to your thermostats settings and on ‘equipment’ select, heat pump. You will see a switch button for switching between O and B. The default setting is O.

In case option O is highlighted, select B. And if B is one that is highlighted, select O.

Test the system by  calling for cooling.

If your system starts cooling, congrats because you have just fixed the problem. Otherwise let’s continue our troubleshooting steps.

2) OB terminal not connected

After attempting the first  step of  troubleshooting this problem but the your honeywell continues to heat on cool setting even after correcting the orientation settings, there would be a problem with the wiring of your O/B terminal. However, take note that if you are using conventional heating, you should not have a wire in the O/B terminal.

How to check if a wire is properly connected to the O/B terminal.

Step 1. Make sure you turn off power to your HVAC system, either by the switch or at the main circuit breaker or both.

Step 2. Take the thermostat display from the wall and inspect the wire connecting to the O/B terminal. Check if there are any corrosions on or if it’s loose. If necessary, take it out and try stripping it long enough to have proper contact with the terminal.

Step 4 Put back the wire into the connector and put back the thermostat display

Step 5. Turn on your system and test if cooling works

3) Bad thermostat wiring

Bad thermostat wiring could also be a good reason why your honeywell thermostat is blowing hot on cool setting Apart from checking the wire connecting to the O/B terminal, ensure that all other wires are properly mapped to the correct terminal. That is especially if you just upgraded from a Honeywell thermostat to a nest thermostat or between any other thermostat brands.

Ensure that there are no loose wires.

If you are using a Honeywell thermostat, check whether  there are separate wires connecting to the Rc( power for cooling) and RH( power for heating) or not. If there are not, put a jumper between the two terminals and test for cooling.

PS: before opening the thermostat, remember to turn off the power for safety.

Below is a quick wiring color code for a Honeywell thermostat. If you are using a nest thermostat, check our nest thermostat wiring guide. For any other thermostats check the respective manuals.

  • Aux / W2 – Heat Stage 2 (Heating)
  • C – Common
  • E – Emergency Heat
  • G – Fan
  • L/A – A – Input for heat pump fault
  • O/B – Reversing valve for Heat Pump systems
  • R – 24vac (Heating transformer)
  • S – Indoor and Outdoor Wired Sensors
  • U – Humidifier, Dehumidifier, or Ventilator control
  • W – Heat Stage 1 (Heating)
  • Y – Compressor Stage 1 (Cooling)Y2 – Compressor Stage 2 (Cooling)

4) You don’t have a heat pump

Other problems that cause your Honeywell thermostat to blow hot air on a cool setting is simply because you don’t have a heat pump. A heat pump works by heating when you need heat and cooling when you need cooling.

However, if you have a furnace or conventional heating only, it wont cool your home even when you set your thermostat to cool. But  if you are not sure whether you have a heat pump or not, try to turn on your system to heat. If the outdoor unit does not kick in, then you might not have a heat pump.

But a more accurate way to know whether  you have a heat pump or not is to manually check on the label of your outdoor condenser unit. In most cases you’ll find it there.

However, if you believe you have a heat pump but your thermostat keeps blowing hot air on cool mode, check where the condenser unit is working or not.

Read also: Honeywell Thermostat Not Showing Cool Option [Solved]

5) Ac condenser unit not working

The condenser unit plays a big role during the process for cooling. That is because  condensation of freon takes place there. That’s where heat from your home is disposed of outside with the help of a condenser fan.

If the condenser is not working or there is no power supplied to it,  cooling in your home simply  won’t take place.

If your outdoor condenser unit won’t turn on when you turn on your thermostat, check whether there is power to it. You can go to the  circuit breaker and check on the breaker switch  labeled air conditioner. If it’s tripped reset, it into the original position and check if your condenser has started working.

Other times it’s the compressor capacitor that becomes defective. In that case you’ll need to replace it.

Troubleshooting an ac compressor that won’t turn on

Possible causeMain fixes
No power/ tripped breaker switchTurn on tripped circuit breaker
Defective capacitorReplace capacitor
Stuck contactor in the off positionClean or replace contactor
Defective fan motorReplace fan motor

6) A stuck changeover valves

A stuck reversing valve on a heat pump can cause your honeywell thermostat to blow hot air on cool mode. To better understand this problem, we need to understand how a reverse valve works.

A reverse valve on a heat pump simply helps to switch the direction of the refrigerant so that your heat pump can cool or heat. In short a reverse valve reverses the flow of freon so that it can function as an air conditioner and as a heater.

Read also: Thermostat Set To 74 But Reads 78 [Causes & Fixes]

However, with time and due to other factors a reverse valve can get stuck in the heat position thus causing your thermostat to blow hot air on the cool setting.

Another common issue that can cause a reverse valve to get stuck is a broken solenoid. A solenoid is responsible for switching from one mode to another. A broken solenoid can cause a reverse valve to get stuck.

However, a solenoid is an inexpensive component and can be replaced cheaply. We recommend that you take up the task of replacing your solenoid only if you know what you are doing. Otherwise it’s best to call in professionals for help.

Troubleshooting a stuck reverse valve

                  Possible causeMain fix
Aged reversing valveReplace reverser valve
Broken solenoidReplace solenoid

7) A defective circuit control board

 A circuit control board compartment

A  circuit control board is the heart of your HVAC system. This simply because  it performs many different functions from relaying thermostat instructions to sensing errors and displaying error codes. The control board also monitors input sensors for the safety of your equipment.

If a CCB is defective, it can ignore cooling or heating instructions from the thermostat. And this could possibly be the reason why your thermostat is not blowing cold air on the cool mode.

Here is a quick overview of troubleshooting a bad circuit control board.

Possible causeMain Fix
Blown out fuse on the control boardReplace the 3 or 5 Amp fuse
Defective furnace transformerReplace furnace transformer
Defective transistor or relayReplace control board
Defective control boardReplace control board

8) A defective or incompatible thermostat

Now this is an obvious reason. If you are using any of the smart thermostat brands including Honeywell, google nest, Ecobee, sensi or any other new thermostat and keeps blowing hot on cool, the problem could actually be a defective or an incompatible thermostat.

These days, there are a lot of powerful programmable thermostats that have vast features and it’s tempting to get a new thermostat without actually checking the compatibility. Soon we shall have a thermostat compatibility checker on our website to help you make a wise choice before ordering a thermostat.

But what I am trying to say is, if you believe your thermostat is incompatible with an existing HVAC system, either use a compatibility checker or call a local HVAC professional for advice.

Most likely if your thermostat is incompatible, you’ll need to have it replaced. Other times, it’s just a thermostat that is bad and may just need a cleanup or replacement.

But what are the signs of a bad thermostat? Here is a quick table for your quick reference.

Possible cause                    Main Fix
Inconsistent temperaturesClean thermostat, adjust. In some cases, replace thermostat
AC or Furnace runs constantlyCheck the wiring
Air conditioner or furnace won’t startRecalibrate or properly wire your thermostat
Thermostat has no powerTry changing batteries

9) Leaking freon

Freon is an important element in the cooling and heating process of your air conditioning system. It’s the freon that absorbs the heat from your indoor air and disposes it via the outdoor condenser unit.

However, when there is little or no freon in your Heat pump or AC, your system won’t be able to blow cool air even if you put your thermostat on cool mode.

Freon may leak out over a long period of time due to many factors that include an aged system or other hostile weather conditions.

Troubleshooting a leaking system is not an easy task. So if your suspect freon is leaking out of your system consult an HVAC professional. Here are just a few signs to tell you that you have a system that is leaking out freon.

  • Air conditioner blowing warm air (not cool)
  • High electricity bills
  • Ice building up on the refrigerant lines
  • Strange sounds coming from the condenser unit
  • The set temperature is never reached

Usually when a licensed professional verifies that your system is low on freon, he/she will typically pinpoint the source for the leaks and seal them off before recharging your system with any more freon.

10) Delay mode

Delay mode on any thermostat is simply a minimum time off or a period of short time usually up 5 minutes when a thermostat doesn’t turn on the heat pump compressor to prevent it from coming on too quickly in what is commonly known as short cycling, this usually happens a lot if you had power outage or you suddenly turned off your system and tried turning it on.

During the time delay , your compressor will not turn on but the fan can blow. That could be the reason you are experiencing something like your thermostat blowing warm air on the cool setting.

However, you have to note that delay mode doesn’t take more than 5 minutes. And depending on your thermostat model, there is usually a delay message only on your thermostat display to notify you about this.

If 5 minutes elapses and the problem is not resolved, there could be another issue preventing  the cooling on your system

Final thoughts

Now that you have learnt what causes a honeywell thermostat to blow hot air on cool, we hope by now you have fixed the problem.

HVAC systems are designed to last for at least 10 years. But these systems can last even longer if you do not skip annual tune ups. 

That said, if you have not fixed the cooling problem by the end of this guide, perhaps consider calling experts for help.

Otherwise thanks for reading our troubleshooting guide.

More resources:

How do I wire a honeywell thermostat

Sensi thermostat not cooling

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