What is AUX Heat on Honeywell Thermostat? [Simple Guide 2025]
When temperatures drop below 40 degrees Fahrenheit, you might notice your Honeywell thermostat displaying “AUX heat.” This auxiliary heating mode activates automatically when your heat pump struggles to maintain your desired indoor temperature.
Many homeowners, including myself, often wonder what auxiliary heat means on their Honeywell thermostat. While it’s commonly confused with emergency heat, auxiliary heat actually serves as an automatic support system for your heat pump. It typically turns on when indoor temperatures fall 1.5 to 2 degrees below your set temperature.
In this comprehensive guide, we’ll explore everything you need to know about AUX heat on Honeywell thermostats. We’ll cover how it works, when it should activate, common problems you might encounter, and practical solutions to help you maintain optimal heating efficiency in your home.
Understanding AUX Heat on Honeywell Thermostats
On Honeywell thermostats, AUX heat stands for auxiliary heat – a backup heating system that works alongside your heat pump. Specifically, this secondary heat source kicks in automatically through your thermostat’s built-in controls.
The auxiliary heating system operates through a sophisticated mechanism. Your Honeywell thermostat monitors both indoor and outdoor temperatures to determine the most energy-efficient heating method. Furthermore, when the heat pump struggles to maintain comfort levels, the thermostat signals the auxiliary heat strips to provide additional warmth.
Your Honeywell thermostat activates AUX heat under several specific conditions:
- When outdoor temperatures fall below 40°F
- During heat pump defrost cycles
- If indoor temperature drops 1.5-2 degrees below the set point
- When raising the temperature setting by 2-4 degrees
The auxiliary heat serves as a “boost” to your heat pump, particularly during challenging conditions. Consequently, both systems may run simultaneously to achieve your desired temperature. The thermostat display shows “Aux Heat On” to indicate when this backup system is active.
Heat pumps maintain high efficiency until temperatures reach about 32°F. Beyond this point, the auxiliary heat becomes essential for maintaining indoor comfort. Although this backup system uses more electricity than the heat pump alone, it effectively brings your home to the desired temperature in less time.
Common AUX Heat Problems
Many homeowners face issues with their Honeywell thermostats’ auxiliary heating system. Initially, let’s examine these common problems and their impact on your home comfort and wallet.
AUX heat running too often
Your auxiliary heat might activate unnecessarily even when outdoor temperatures are above 40°F. One primary cause is incorrect thermostat settings – particularly the “comfort” versus “economy” mode selection. In comfort mode, the system prioritizes quick heating over efficiency, often triggering aux heat with just a small temperature adjustment.
Several indicators suggest your aux heat is running more than necessary:
- Activation during mild outdoor temperatures
- Turning on with minimal temperature adjustments
- Running when your home is already warm
- Engaging immediately after programming changes
A notable issue occurs when the thermostat’s “aux heat droop” setting isn’t properly configured. This setting determines how many degrees the indoor temperature can drop before auxiliary heat kicks in. Moreover, some basic Honeywell models eagerly switch to aux heat when set above 73°F, regardless of outdoor conditions.
High energy bills
The financial impact of excessive aux heat usage is significant. Your energy costs may increase by up to 50% when the system frequently relies on auxiliary heating. This substantial rise occurs because the system runs both your heat pump and backup heat source simultaneously.
The problem often stems from competing heating and cooling functions. For instance, if your heat pump’s reversing valve malfunctions, the system might operate in cooling mode while aux heat tries to warm your home. Therefore, these opposing actions create a dramatic spike in utility costs.
Additionally, issues like refrigerant leaks or ice accumulation on the outdoor unit force your system to depend more heavily on auxiliary heat. This dependence leads to extended running times and, subsequently, higher energy consumption.
Troubleshooting AUX Heat Issues
Resolving aux heat issues requires systematic troubleshooting. Let’s examine the key areas to check and fix common problems.
Check thermostat settings
First, verify your Honeywell thermostat’s configuration settings. Access the installer setup menu by pressing and holding the Fan and Up buttons. Notably, the Auxiliary Heat Control setting (Function 26) offers two modes:
- Comfort mode (0): Prioritizes quick heating, triggering aux heat more readily
- Economy mode (1): Delays aux heat activation to save energy
Second, ensure your system type matches your heating setup. Incorrect configuration might cause the heat pump to operate in cooling mode instead of heating.
Inspect outdoor unit
Begin with basic power verification. Check that all HVAC system breakers are in the ON position. Accordingly, inspect the unit’s power switch and ensure all access panels are properly closed – many systems include safety switches that prevent operation when panels are open.
Third, examine the outdoor unit for ice accumulation. Your heating system relies on aux heat during defrost mode to maintain indoor comfort. If the defrost control board malfunctions, ice buildup can restrict critical heat transfer.
Test heat pump operation
Start by measuring the voltage between R (or Rc) and other thermostat wires. The system should operate between 20-30 VAC – readings outside this range might indicate transformer or wiring issues.
Indeed, if your heat pump runs in cooling mode during heating calls, verify the changeover valve configuration. Look for error codes through the small sight-glass LED on your furnace – these codes often indicate specific system problems.
Primarily, if these steps don’t resolve the issue and you have a dual-fuel system (heat pump with gas/oil backup), contact an HVAC professional. Some problems require specialized diagnostic tools and expertise.
Smart Ways to Reduce AUX Heat Usage
Reducing auxiliary heat usage starts with understanding smart temperature management. Primarily, gradual temperature adjustments prove more efficient than dramatic changes in your Honeywell thermostat settings.
Optimal temperature settings
Setting your thermostat between 60-68 degrees helps minimize aux heat activation. In fact, raising temperatures in two-degree increments prevents unnecessary aux heat engagement. As opposed to making large temperature jumps, this gradual approach allows your heat pump to work efficiently without backup heat.
For optimal performance, consider these temperature guidelines:
- Keep steady settings throughout the day
- Avoid raising temperature more than 3 degrees at once
- Set aux heat lockout when outdoor temperatures exceed 40°F
- Maintain minimal temperature setbacks of 1-2 degrees during cold weather
Home insulation tips
Given these points about temperature control, proper home insulation becomes equally crucial for reducing aux heat dependency. A well-insulated house retains heat more effectively, reducing the strain on your heating system.
Focusing on air sealing proves particularly beneficial in preventing heat loss. By addressing common sources of drafts and heat escape, your heat pump maintains desired temperatures without relying on auxiliary heating. This approach not only minimizes aux heat usage but also leads to noticeable reductions in energy costs.
The combination of strategic temperature management and improved insulation creates an efficient heating environment. By implementing these measures, your Honeywell thermostat’s aux heat feature activates only when truly necessary, primarily during extreme temperature conditions or brief defrost cycles.
Conclusion
Understanding your Honeywell thermostat’s AUX heat feature helps maintain comfortable temperatures while keeping energy costs under control. Through my research and experience, I’ve found that this backup heating system works best when properly configured and maintained.
Smart temperature management makes a significant difference in AUX heat usage. Rather than making large temperature adjustments, small incremental changes allow your heat pump to work efficiently without engaging the backup system. Additionally, proper home insulation and regular maintenance checks prevent unnecessary AUX heat activation.
Remember that AUX heat serves as a support system, not your primary heat source. While it’s essential during extremely cold weather or defrost cycles, minimizing its usage through correct thermostat settings and gradual temperature adjustments will help reduce your energy bills.
Most homeowners can handle basic troubleshooting steps themselves. However, if you notice persistent issues with your AUX heat system or unusually high energy bills, consulting an HVAC professional might save you money in the long run.
