What Is Auxiliary Heat?
Auxiliary heat is a backup heating system built into heat pumps that activates automatically when outdoor temperatures drop too low for the heat pump to work efficiently on its own. It typically uses electric resistance heating strips to produce immediate warmth. Most homeowners only notice it when the "Aux Heat" light appears on their thermostat during cold weather.
In short: aux heat is your heat pump asking for backup.
It is not a malfunction. It is a feature.
How Does a Heat Pump's Auxiliary Heat Work?
Your heat pump heats your home by pulling warmth from outdoor air and moving it inside. That process works well in mild and cool weather. But when temperatures drop below a certain threshold, usually around 35 to 40 degrees Fahrenheit, the heat pump struggles to extract enough heat to keep up with demand.
That is when auxiliary heat steps in.
The aux heat strips are electric coils inside your air handler. When the thermostat senses that your heat pump alone cannot reach the set temperature within a reasonable time, the auxiliary heat activates automatically. The two systems run together briefly until the indoor temperature catches up.
Key facts about how aux heat operates:
- It activates automatically; you do not control it manually
- It works alongside the heat pump, not instead of it
- It turns off once your home reaches the target temperature
- It is controlled by your thermostat's logic, not a separate switch
What Is the Difference Between Aux Heat and Emergency Heat?
This is one of the most common questions homeowners ask. The two terms sound similar but work very differently.
Feature | Auxiliary Heat | Emergency Heat |
|---|---|---|
How it activates | Automatically | You switch it on manually |
Works with heat pump | Yes, both run together | No, heat pump is bypassed entirely |
When to use | Normal cold weather | Heat pump is broken or damaged |
Energy cost | Moderate | High |
Thermostat indicator | "Aux" light | "Em" or "EM" light |
Auxiliary heat turns on automatically when your heat pump needs a boost. Emergency heat is a manual override that shuts off the heat pump entirely and runs only the backup strips.
Never run emergency heat unless your heat pump is physically damaged or failing. Running emergency heat unnecessarily can double or triple your electric bill.
When Does Auxiliary Heat Kick On?
Aux heat turns on under three specific conditions.
1. Outdoor temperature drops below the heat pump's balance point Most heat pumps begin to struggle when outdoor temps fall below 35 to 40 degrees Fahrenheit. The exact threshold depends on your unit's efficiency rating and age.
2. The temperature gap is too large If your home's indoor temperature is more than 2 to 3 degrees below the thermostat setpoint, the aux heat activates to close that gap faster. This often happens after a long absence or a sharp overnight temperature drop.
3. The defrost cycle is running Heat pumps periodically run a defrost cycle to clear ice buildup from the outdoor coil. During defrost, the system temporarily reverses direction, which means it stops heating the home. Aux heat fills that gap automatically.
Is It Normal for Aux Heat to Run in Texas and Florida?
Yes, though less frequently than in northern states.
Texas winters are mild by most standards, but DFW, Austin, and San Antonio regularly see overnight lows in the 20s and 30s during cold snaps. During events like the February 2021 winter storm, temperatures dropped well below what any heat pump was designed to handle on its own.
For homes in the DFW Metroplex with heat pumps, aux heat should kick on during cold fronts and overnight lows below 35 degrees. Seeing the aux light for an hour or two on a cold morning is completely normal.
In Florida, it is far less common. Orlando and Tampa rarely dip below 40 degrees. If your aux heat is running frequently in Florida, that warrants a closer look at your system.
What is normal vs. what is not:
- Normal: Aux heat runs briefly during cold snaps or early morning warm-ups
- Not normal: Aux heat runs all day, even in mild temperatures
- Not normal: Aux heat runs in weather above 50 degrees
- Not normal: Your home never reaches the set temperature even with aux heat running
Why Is My Aux Heat Running Constantly?
If your aux heat light stays on for hours or runs even on mild days, something is wrong.
Here are the most common causes:
Refrigerant leak: Low refrigerant reduces your heat pump's ability to extract heat from outside air. The system works harder and relies on aux heat more than it should. You may also notice the heat pump running constantly without warming the house.
Dirty air filter: A clogged filter restricts airflow through the system. This forces the heat pump to work longer to reach your set temperature, triggering aux heat more often. Check and replace your filter every 1 to 3 months.
Frozen outdoor unit If the outdoor coil freezes over, the heat pump cannot transfer heat properly. The defrost cycle should handle minor frost, but a fully frozen unit is a sign of a bigger problem, often a refrigerant leak or a failing defrost board.
Failing heat pump compressor The compressor is the heart of the heat pump. If it is failing, the system cannot produce heat effectively and aux heat compensates constantly. This is a repair or replacement situation.
Thermostat set too high for outdoor conditions If it is 25 degrees outside and you set your thermostat to 74, the heat pump may genuinely need to run aux heat for an extended period. Try setting the thermostat no more than 2 to 3 degrees above the current indoor temperature during extreme cold.
Does Auxiliary Heat Cost More to Run?
Yes. Auxiliary heat uses electric resistance strips, which are significantly less efficient than heat pump operation.
Here is a real-world comparison based on typical DFW homes:
Heating Mode | Efficiency (COP) | Estimated Cost Per Hour | Notes |
|---|---|---|---|
Heat pump only | 2.0 to 3.5 | $0.15 to $0.40 | Moves heat, very efficient |
Aux heat + heat pump | 1.2 to 1.8 | $0.50 to $0.90 | Combined operation |
Emergency heat only | ~1.0 | $0.85 to $1.50 | Resistance only, no pump |
Estimates based on average Texas electricity rate of $0.12 to $0.14 per kWh and a typical 3-ton heat pump system.
Running aux heat occasionally during cold snaps has a minor impact on your bill. Running it all day every day during winter will add $60 to $150 or more to your monthly electric bill, depending on your system size and usage.
If your bill has spiked significantly and you have a heat pump, aux heat overuse is worth investigating.
Should I Be Worried If My Aux Heat Light Is On?
Not automatically. Context matters.
Do not worry if:
- It is below 35 degrees outside
- It only runs for 30 to 60 minutes at a time
- It turns off after the house reaches temperature
- It runs during the morning warm-up period
Do call an HVAC technician if:
- It runs continuously all day regardless of outdoor temperature
- Your home never reaches the set temperature
- You see ice buildup on the outdoor unit that does not clear
- Your electric bill has spiked without a clear weather reason
- You notice the aux light on during 50 to 60 degree weather
How Can I Reduce How Often My Aux Heat Runs?
Small adjustments can reduce how hard your system works during cold weather.
- Avoid large temperature jumps. Instead of dropping the thermostat at night and cranking it up in the morning, keep adjustments to 2 degrees or less. Large gaps trigger aux heat.
- Use a programmable or smart thermostat. A gradual ramp-up starting before you wake avoids the sudden demand that triggers extended aux heat runs.
- Replace your air filter regularly. A clean filter keeps airflow strong and your heat pump operating at full capacity.
- Seal drafts and leaks. The less heat your home loses, the less your system has to work to maintain temperature.
- Schedule annual HVAC maintenance. A well-tuned heat pump operates more efficiently and relies on aux heat less. Team Enoch's maintenance visits include refrigerant checks, coil cleaning, and defrost board testing.
When Should You Call an HVAC Professional?
You can monitor your aux heat situation for a day or two during a cold snap before calling. But do not delay if you see any of the following:
- The outdoor unit is completely frozen and not defrosting
- Your heat pump is blowing lukewarm or cool air even with aux heat running
- The aux light is on in temperatures above 50 degrees
- Your electric bill has jumped $80 or more without a clear cause
- You hear unusual noises from the outdoor unit or air handler
Our team at Team Enoch has served DFW, Austin, Houston, San Antonio, and Florida since 2015. We diagnose heat pump issues including refrigerant leaks, failing compressors, defrost board problems, and thermostat calibration. Every technician is NATE-certified and work is backed by our price guarantee.
Frequently Asked Questions About Auxiliary Heat
Is aux heat the same as a furnace? No. Auxiliary heat in a heat pump system uses electric resistance strips, not a gas burner. A furnace is a separate heating appliance that burns fuel. Some homes have a heat pump with a gas backup instead of electric strips, which is called a dual-fuel or hybrid system.
Why does my aux heat come on when I just want to raise the temperature by a degree or two? If your thermostat detects that the heat pump alone cannot close the gap within a set timeframe, it activates aux heat to assist. This is normal. Some thermostats let you adjust the aux heat lockout temperature or the time delay before it activates.
Can I turn off my auxiliary heat? You should not disable auxiliary heat on your own. It is a critical safety and comfort backup. However, if it is running excessively, the cause should be diagnosed and repaired by a technician rather than simply disabling the feature.
What temperature should I set my thermostat to avoid overusing aux heat? During cold weather, keep your thermostat at a steady temperature rather than making large adjustments. Most HVAC professionals recommend not raising the setpoint by more than 2 to 3 degrees at a time. This gives the heat pump time to catch up without triggering extended aux heat runs.
Is it OK to run aux heat all winter? Technically the system can run that way, but it is expensive and often a sign of an underlying problem. If your system needs aux heat to maintain temperature through an entire winter, your heat pump may be undersized, low on refrigerant, or failing. Schedule a diagnostic check.
Does aux heat run during the defrost cycle? Yes. When your heat pump runs its defrost cycle to clear ice from the outdoor coil, aux heat runs automatically to keep your home warm while the outdoor unit temporarily reverses direction. This is normal and usually lasts only 5 to 15 minutes.
Auxiliary heat is a normal, built-in feature of heat pump systems. It exists to fill the gap when cold temperatures push your heat pump past its efficiency limits. Seeing the aux light on your thermostat during a DFW cold snap is not a reason to panic.
What matters is how often it runs and under what conditions.
Brief, weather-appropriate aux heat operation means your system is working exactly as it should. Constant aux heat in mild weather means something needs attention.
If you are unsure whether your system is operating normally, Team Enoch offers diagnostic inspections across DFW, Arlington, Fort Worth, Dallas, Austin, Houston, San Antonio, and Florida. We will tell you exactly what is happening and what, if anything, needs to be done.
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