New Honeywell Thermostat Blowing Hot Air on Cool [Solved]

Is your new Honeywell thermostat blowing hot air on cool mode? It can be so frustrating to wake up in the middle of the night sweating only to discover that your newly installed Honeywell thermostat is not cooling.

new honeywell thermostat blowing hot air on cool
A woman adjusting thermostat temperature

There could be a few reasons why your Honeywell thermostat is blowing warm instead of cool air especially if it’s a new installation. These include:

  • Your outdoor ac unit has no power access (tripped circuit breakers)
  • Wrong thermostat wiring
  • Your system is in delay mode
  • Compatibility issues
  • Changeover valve issues

Read also: Honeywell Thermostat Not Cooling [8 Easy Solutions]

Why is My New Honeywell Thermostat Blowing Hot Air on Cool Mode?

Its not uncommon for a new thermostat to misbehave after an installation. This is mainly because of errors made during the installation, system compatibility issues and just a few more other reasons. Let’s now go straight into details.

1) System Is in Delay Mode

If you have set your Honeywell thermostat to cool mode and its flashing ‘cool on’ but blowing warm air instead of cold, your thermostat could be in delay mode. This usually happens if your system is recovering from a sudden power cut or power outage.

The thermostat has a delay mode functionality that protects your compressor and your entire equipment from short cycling. However, delay mode should not take more than 5 minutes.  If the delay mode is not an issue here, lets jump right into our second step of our troubleshooting.

2) You Are Using a Conventional System

When you are dealing with a Honeywell thermostat that is blowing hot air on cool mode, the first thing you want to do is verify that you actually have a heat pump and not a conventional system A heat pump should blow cool air on cool mode, but a furnace will not.

To know if you have a heat pump or not, try setting your thermostat to heat mode, and raise the temperature to a point that’s above the current room temperature reading. If the outdoor unit (condenser kicks on) then you’ll know for sure it’s a heat pump.

But this method can not work if there is power cut to your outdoor unit. In this case, the best you can do to tell if you have a heat pump, head over to your outdoor unit and check the label that is there on it.

If there is anything labelled ‘heat pump’ then you’ll know for sure your system is a heat pump. If there is no ‘heat pump’ label on it, you can copy the model number from the label and look it up on the internet. The result should display information about the kind of system you have (heat pump or not.)

3) Your Outdoor Unit Has No Power Access

When you are troubleshooting a thermostat that’s not cooling, its important to verify that your outdoor condenser unit has access to power.  You might have forgotten to turn on the circuit breaker for your ac condenser unit after installing your new thermostat.

If your condenser unit can’t turn on because of power issues, it won’t be able to dispose the heat from your home. And that could be the reason your thermostat is blowing warm air instead of cool air.

Check once more that the breakers are up and conducting power to your condenser unit. sometimes, you might have blown out fuses in your disconnect (outdoor device installed on your house wall). And other times, blown fuses on the control board could be the reason your system is not blowing cold.

If your condenser unit is running properly but your thermostat is blowing hot air on cool mode, then check your thermostat orientation.

Also read: Honeywell Thermostat Heat Not Working [7 Fixes]

4)Bad Thermostat Wiring

If your thermostat is blowing hot on cool mode, there could be a thermostat wiring issues. You must make sure that wiring from the old thermostat is correctly matched to the new thermostat. On top of that, both terminals R (for power) and Ric  (power for cooling) have to be  connected. If these two terminals are not connected by separate wires, then use a jumper (a short wire) to connect the two.

To make a jumper you can just cut and strip off a small piece of wire from one of those wires.

How should I wire my Honeywell thermostat?

Each terminal has a specific function. For example, wires that control cooling are different from ones that control the heating of your system. These wires need to be connected properly if your thermostat is to work without issues.

The O/B is responsible for controlling the changeover valve. Ensure its connected and there are no loose wires in your thermostat installation. Before you start touching your thermostat wires, verify that you have turned off power to your system in the circuit breaker. This is for your safety and also that of your equipment. Take a look at the wiring diagram for your Honeywell thermostat below:

S – Indoor and Outdoor Wired Sensors

Y – Compressor Stage 1 (Cooling)

Y2 – Compressor Stage 2 (Cooling)

G – Fan

C – Common

U – Humidifier, Dehumidifier, or Ventilator control

L/A – A – Input for heat pump fault

O/B – Reversing valve for Heat Pump systems

E – Emergency Heat

Aux / W2 – Heat Stage 2 (Heating)

W – Heat Stage 1 (Heating)

R – 24vac (Heating transformer)

Arc – 24vac (Cooling transformer)

5) Changeover Valve Issues

A defective changeover valve can be the reason your Honeywell thermostat is not cooling your home. A change over valve is a device on your heat pump that reverses the flow of the refrigerant. Because of that, your heat pump can work as a heater and as an air conditioner in the cooling.

However, if the changeover valve gets stuck in the heating position, your thermostat won’t do enough to help resolve this problem. Fortunately, changeover valves are replaceable. A defective changeover valve can be replaced and cooling for your home can be restored in no time. You’ll need to call professionals to come and replace it for you.

Also read: Honeywell Thermostat Won’t Turn on AC [Solved]

6) Thermostat Compatibility Issues

Your thermostat is not cooling your home probably because its not compatible with your existing system. It’s very tempting to buy a very nice smart thermostat without checking the compatibility with your system. Some newer thermostats are not compatible with older HVAC systems. If you are unsure whether or not your thermostat is compatible with your system, call HVAC professionals for advice.

7) Malfunctioning Control Board

control board compartment
Control board compartment

If your furnace control board is malfunctioning, it could prevent your system from responding to temperature adjustments from your thermostat.

The control board receives instructions about cooling and heating from your thermostat. Then relays this information to every other component of your system that is concerned with that specific instruction.

Sometimes the control board can become irresponsive to the thermostat and all that it needs to start working again is a reset. You can reset your furnace’s control board by turning off your furnace by either the power switch of the circuit breaker for about 5-to 10 minutes and restarting.

A completely dead furnace control board is replaceable. But for this fix you’ll definitely need the help of professionals.


Why Is There a Jumper Between R And RC?

The R terminal is for power (for the entire HVAC system) and the R c is power for cooling. These two terminals need to be connected separately to the furnace control board. If only one of them is connected, then a jumper will be needed to have both terminals connected.

If Rc terminal is not connected, your thermostat won’t be able to cool.

How Many Wires Are on A Heat Pump?

Depending on the type of heat pump you have, the number of connecting wires can vary from 5 to 8 with 2nd stage heat pumps having more wires than single stage heat pumps

What Color Is the C Wire?

The C-wires are typically blue in colour. C-wires are necessary in most modern thermostats, they supply them with continuous 24volts power supply

Final Thought

We hope that this troubleshooting guide about your new Honeywell thermostat blowing hot air on cool mode, has been useful. Otherwise, if the problem continues after trying out tips given in this article, then its best you call local professional HVAC guys.

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