What Does Auxiliary Heat Mean On a Honeywell Thermostat?

If you have noticed on your honeywell thermostat ‘auxiliary heat’ or ‘Aux Ht’ and are wondering what it means then this is  a perfect guide for you. In this guide we shall look at what auxiliary heating is on a honeywell thermostat and how a heat pump works with it.

Generally ‘auxiliary heat’ or ‘aux ht’ display on a Nest thermostat means that a heat pump is utilizing supplementary heating to keep warming your home or office adequately. Auxiliary heat will typically  turn on when  the outdoor temperature drops lower than the recommended working condition of a heat pump, in many cases below 32 degrees.

In order to properly understand  what auxiliary heat is  it’s important to learn about what conditions aux heat can come on.

Conditions under which auxiliary heat will turn on

Auxiliary heating doesn’t have to always turn on. There are specific conditions necessary for it to automatically turn on and these include the following

1) Compressor not working:  when the outdoor condenser unit is not working or it is not connected to power, auxiliary heat will turn on. But  if the outside unit is operating as it out, there is no need for aux to turn on.

2) Frozen outdoor condenser unit: Another condition that can cause the aux heat to turn on is a frozen outdoor condenser unit. When you turn on a heat pump with a frozen condenser unit it will start defrosting the condenser coils and for this job to be completed, it needs the help of the secondary heat. That is another reason you could see an aux heat display on your thermostat.

3) Extremely low temperature: Heat pumps are efficient when it comes to heating your space compared to other heating options. They extract  heat from outdoor air and transfer it into your space.

 However there will come a time when the outdoor temperature drops so low such that the heat pump will not efficiently  transfer heat indoors.

When this happens aux heat will kick in to supplement the heat pump’s heating effort. They may simultaneously run to keep up with the heating.

Read also: Honeywell thermostat blowing fuse

 Is Auxiliary heat good or bad?

Auxiliary heat is secondary heat that supplements your heat pump’s primary heat. Aux heat is only necessary when your heat pump can not keep up. Auxiliary heat can be an expensive heating source, in fact it can be 2-5 times more expensive than a heat pump’s heating.

So Auxiliary heating is generally good only in extreme weather conditions where a heat pump becomes inefficient to provide sufficient heating. 

With that in mind. You may also have the option to manually turn on a supplementary heating source. This is known as Emergency heat.

In Other words, when you manually turn on aux heat, it’s known as Emergency Heat. 

How to turn on & off Auxiliary heat on a honeywell thermostat.

It’s worth noting that no heat pumps will have auxiliary heat capability. So only those that were installed with 2 stage heating sources will have this option. If your system doesn’t have it, you won’t see the option on the screen.

If you have a conventional  heating system alone( furnace) you’ll not see this option too. 

Turning on auxiliary heat on your thermostat depends on your thermostat model.

For T6 pro thermostat model,

  • From the display, click on mode, at the bottom left of your thermostat.
  • Then scroll to select EM  heat ( this only available on heat pump systems with heat pumps)

For RTH6500WF.

  • Press system and select Em Heat (heat pumps with aux. heat):
  • Controls auxiliary/emergency heat

Read also: Honeywell Thermostat Blowing Cold Air on Heat Setting

Troubleshooting Auxiliary heat blowing cold air on heat

There are times when you might turn on auxiliary heat or emergency heat and notice that it is blowing cold air on a heat setting.  This is due to a few reasons which include, tripped circuit breaker, incorrect thermostat wiring,  defective elements.

Tripped circuit breaker: 

Auxiliary heating sources are usually heating elements that work with the power supply. That means that power to the heating stips/ elements is cut off you wont have heat from aux heat.

The fix to fix this issue

check the circuit breaker for aux heat. Reset it if  tripped. If this does not resolve the problem, look for any other switch that supplies power to the aux.

Incorrect thermostat wiring

Heat pumps with auxiliary heat have a special thermostat wiring which is different from systems without a secondary heating source. Typically, for a system with Emergency/ aux heat, there should be an additional wire connected to the W2 terminal in addition to the wires connected to the compressor relay connectors.

Defective elements

Another reason your aux heating will blow cold air  is that they may be defective. This is rare but can occur especially if you use aux often or  your system is old

Aux heating uses the electrical resistance to get hot and warm the air passing through the heat exchange. The same way elements of the oven use electrical resistance to get hot, they are also susceptible to wear and damage over time.


We hope that at this point of this guide, you fully understand what auxiliary heat on your honeywell thermostat is and how to turn it on and off. You may also like to check the user manual for your specific thermostat.

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