Key Takeaways
- Auxiliary heat is the automatic backup heat source in a heat pump. This includes its electric resistance strips or secondary fuel furnace that kicks in when a heat pump can’t keep up. You can apply this by monitoring your thermostat symbols and owner manual to see when aux heat is active.
- Common triggers are extremely low outdoor temperatures, defrost cycles, and oversized or aggressive thermostat increases. Minimize unnecessary activation by making smart, gradual temperature adjustments and scheduling.
- Auxiliary heat is less energy efficient and increases operating costs during extended use. Keep an eye on energy consumption and insulate well to reduce dependence and costs.
- Proper maintenance and sizing limits aux heat use and prolongs equipment life, so schedule yearly checkups and demand a load calculation prior to installing a system.
- Emergency heat is different from auxiliary heat in that it’s a manual thermostat setting that bypasses the heat pump. Utilize emergency mode solely for genuine heat pump malfunctions in order to prevent increased energy consumption.
- Intermittent auxiliary heat running is okay and not a failure. Call a reputable HVAC technician only if auxiliary heat runs twenty-four hours a day, seven days a week with no obvious trigger.
Auxiliary heat on a heat pump is a secondary electric or gas heat source that backs up the pump when the outdoor temperature falls too low or when quick heating is required. It comes on automatically when the heat pump by itself cannot reach the thermostat setpoint.
Common varieties include electric strip heaters and gas furnaces that warm the inside temperature more quickly but consume a lot of energy.
The main body details how it operates, expenses, and timing to modify settings.
Auxiliary Heat Explained
Auxiliary heat is the backup heating unit in a heat pump system that runs when the heat pump alone can’t keep up. Typically, it utilizes electric resistance heat strips or a separate fuel-fired unit, such as a gas or oil furnace, to bolster the heat pump. Auxiliary heat comes on during deep cold, quick temperature swings, or defrost cycles so indoor comfort keeps going without missing a beat. This is common for a lot of newer HVAC systems and doesn’t indicate that the system is breaking down.
1. The Backup System
Auxiliary heat is the backup source that kicks on when the heat pump can’t keep up. It’s dependable in freezing weather or when your outdoor unit dips in performance. Systems usually employ electric resistance coils in the air handler or a tied-in gas or oil furnace depending on the installation.
The backup system maintains room comfort even under conditions where the heat pump alone would come up short.
2. The Activation Triggers
Typical causes are outdoor temperature falling below a limit, frequently about 4 °C (40 °F), quick thermostat changes, or the heat pump going into defrost. Most thermostats will default to auxiliary heat if indoor temperature is 3 to 5 °C (3 to 5 °F) below the set point.
If the heat pump runs for 15 to 30 minutes without increasing the indoor temperature, the thermostat will generally indicate to switch to auxiliary heat. Make up an easy table of triggers on various thermostat models to compare exact thresholds and timers.
3. The Technology
The source in most configurations is electric resistance heat strips or a backup fuel system. New heat pumps and smart thermostats utilize sensors and control boards to determine when auxiliary heat should initiate.
That aux mode is integrated into the central heating control so both systems work seamlessly together. New technology has made these decisions quicker and the switching more streamlined, lessening extraneous run time.
4. The Thermostat Signal
Your thermostat triggers it to come on when it senses that the temperature in the home is not increasing fast enough. Most digital thermostats indicate “AUX” or “EM HEAT” when auxiliary heat is active.
Your thermostat controls both the heat pump and the backup system and toggles between the two automatically. Know your thermostat’s controls so you understand when auxiliary heat is activated and if that light staying lit for an hour during mild outdoor temps deserves a checkup.
5. The Efficiency Contrast
Heat pumps tend to be more efficient than auxiliary heat. The backup consumes more electricity or fuel and increases costs when used extensively.
Heat pumps are most efficient when used in mild climates. Auxiliary heat is mandatory in colder climates or extremes. A few reduce aux reliance by smarter thermostat schedules, better insulation, and timely maintenance.
Activation Scenarios
Auxiliary heat is a backup source that kicks in when the heat pump by itself is unable to keep up with the home’s heating load. It’s transient and based on outside temperature, system efficiency, thermostat settings, and operating modes. Here are the typical scenarios that activate auxiliary heat and what they imply for comfort, efficiency, and wear on your system.
- Outdoor temperature drops below about 1–4°C (35–40°F)
- Prolonged cold snaps or extended heating demand
- Heat pump defrost cycles to remove outdoor coil ice
- Fast or big thermostat setpoint raises greater than 1.5C or 3F.
- Recovery after a large nighttime setback
- Thermostat programming or staging that forces backup heat
- System performance issues causing insufficient heat delivery
Cold Weather
Auxiliary heat typically engages when outdoor temperatures are below the heat pump’s efficient range. Most heat pumps begin to lose efficiency near 1 to 4 degrees Celsius as there is less heat available in the outside air to extract. In those scenarios, the additional electric strips or auxiliary furnace provide extra warmth so indoor temperatures remain consistent.
Cold-climate heat pumps are built to minimize auxiliary heat with enhanced compressors and refrigerants, but even those appliances employ backup heat during severe cold. Homeowners in northern climate zones should anticipate greater auxiliary operation during extended winter cold snaps and schedule energy budgets accordingly.
Defrost Cycle
When the outdoor coil gets iced up, the heat pump goes into a defrost cycle to melt the ice and bring the airflow back. During that short window, the outdoor unit can switch to reverse or have a time period where it is actually pausing heating and counting on the heat strips or backup furnace to keep air inside the house warm.
Defrost cycles are automatic, required for optimal operation in cold, damp weather, and usually last a few minutes at a time. An abnormally high number of defrosts, in excess of normal cycles, can be indicative of airflow or refrigerant issues, so observe cycle frequency and consult a technician if defrosts appear excessive.
Rapid Heating
Aux heat loves rapid thermostat changes. If you turn the thermostat up more than roughly 1.5°C (3°F) above what it is currently inside, the system may trigger auxiliary heat to achieve the setpoint more quickly. It ranges from turning up the thermostat in the morning after a significant nocturnal lapse.
Auxiliary heat assists in satisfying the immediate need, but constant use drives up consumption. Activation scenarios such as gradual setpoint changes and smart thermostats that stage heat pump and backup operation can cut down on unnecessary auxiliary cycles and reduce costs while still maintaining comfort.
Cost and Efficiency
Auxiliary heat is a fallback that ensures a home stays warm when the heat pump falls short of demand. In this section, we analyze the impact that backup has on power consumption, monthly expenses, and equipment durability. We also provide actionable guidelines to minimize wasteful costs while maintaining convenience.
Energy Consumption
Auxiliary heat, especially electric resistance strips, consumes significantly more energy than the heat pump’s standard performance and is frequently multiple times inefficient.
- Heat pump mode vs. auxiliary strips: Heat pumps move heat and can have coefficients of performance (COP) above 2.0 to 4.0 in favorable conditions, meaning they deliver 2 to 4 units of heat per unit of electricity. Electric resistance strips are close to 1.0 COP, so they consume much more electricity to provide the same heat.
- Fuel-based backups: Backup furnaces burning gas or oil may be more or less efficient than electric strips depending on model and fuel price. They still typically cost more per unit of delivered heat than a well-performing heat pump.
- Cold-weather impact: When outdoor temperatures fall below about 2 to 3 degrees Celsius (35 degrees Fahrenheit), the heat pump’s ability to extract heat drops and auxiliary systems run more. This shift raises energy consumption.
- Real-world behavior: If auxiliary heat runs often, monthly electricity bills can spike. Track with smart thermostats or utility data to identify when strips power on and how much additional energy they draw.
Programmable thermostats can cut out unneeded auxiliary run-time. Steer clear of large night setbacks that demand the backup to bounce back fast. Instead, apply modest night drops and maintain indoor temperatures around 68°F (20 °C) for reduced total consumption.
Financial Impact
Auxiliary heat contributes to bills when used as auxiliaries due to lower efficiency than the primary heat pump. Frequent firing during extended cold spells or inadequate sizing manifests as surprise cost spikes in winter.
Compare bills year-to-year and watch for sudden peaks. Regarding cost and efficiency, turning your thermostat down, caulking leaks, and insulating will decrease the run time for additional systems.
Consider upgrades: modern cold-climate heat pumps maintain higher efficiency at lower outdoor temperatures and can cut reliance on auxiliary heat. Balance equipment cost with savings over the long term. In milder climates, a high-efficiency heat pump can nearly eliminate all auxiliary use and reduce lifetime costs.
Install smart thermostats to identify patterns and adjust schedules. Small behavior changes and better controls frequently cut bills more inexpensively than equipment replacement!
System Lifespan
If you’re relying on auxiliary heat too much, it is causing unnecessary wear and tear on your electric strips or backup furnace.
Balanced operation allowing the heat pump to carry as much load as possible decreases cycling and strain on the auxiliaries, elongating their lifespans. Regular maintenance keeps both systems running efficiently: clean coils, check refrigerant charge, and test auxiliary elements.
Proper sizing and quality installation matter. An undersized main unit forces more auxiliary use and shortens overall system life.
System Optimization
System optimization describes the practical measures that maintain an efficient, reliable, and cost-effective heat pump and auxiliary heat operation. It details how thermostat strategy, routine maintenance, and improved shell performance combine to minimize avoidable auxiliary heat use, increase comfort, and reduce bills.
Thermostat Settings
Define a reasonable, consistent temperature to prevent tripping auxiliary heat unnecessarily. Big jumps, boosting heat more than 2 to 3 degrees Celsius or punching the setpoint up in the morning, can cause the system to demand auxiliary strips or a backup furnace.
Aim for smaller changes: increase by 1 to 2 degrees Fahrenheit or 0.5 to 1 degree Celsius steps so the heat pump can keep up before backup engages. Overnight lapses require attention. A very low night setpoint can generate a huge morning load and fire the aux stage.
Instead, take a small stumble and schedule a slow ramp up. Smart or programmable thermostats can take care of this automatically, and we’re seeing them get smarter with learning patterns to minimize auxiliary cycles.
Smart thermostats provide staging and learning functions to reduce auxiliary heat. They can sense when the heat pump alone will satisfy the load and defer or restrict backup initiation.
Reference your thermostat manual to verify heat pump, auxiliary or emergency heat modes and apply manufacturer recommendations to properly adjust staging and recovery times.
Proper Maintenance
Consistent HVAC tune-up eliminates the conditions that cause auxiliary heat to run more frequently. Technicians will check refrigerant levels, reversing valves and test the auxiliary heat strips or backup furnace for proper operation.
Debris, low refrigerant or faulty controls increase heat pump strain and can force backup heating. Change air filters often to maintain airflow. Limited airflow makes the heat pump work harder and can even trip auxiliary stages.
Change or clean filters based on your usage and local air quality. Annual inspections by seasoned pros catch control faults and wear before they are calls to emergency.
Regular check-ups minimize the likelihood of expensive repairs and confirm the system cycles between stages as intended.
Home Insulation
Better insulation keeps the heat in, reducing demand on both the heat pump and its auxiliary system. Optimize your system. Upgrade attic, wall, and floor insulation to reduce heat loss.
Even modest improvements result in measurable reductions in run time. Seal windows, doors, and ducts against drafts and heat leakage. Closing blinds on cold nights and opening them on sunny days utilizes passive solar gain and minimizes tertiary demand.
Improved insulation and air sealing make it simpler to maintain a consistent temperature, so the heat pump can take care of the majority of heating without any backup.
Dual-fuel systems can still bring benefits in really cold climates by switching to a gas furnace when the heat pump’s coefficient of performance dips.
Common Misconceptions
Auxiliary heat tends to elicit strong reactions because it pops up on the thermostat when people don’t expect it. It encompasses such a range of behaviors and controls that readers enjoy a nice baseline before the specific myths are tackled. Beneath, common misunderstandings surrounding auxiliary heat are demystified, clarified, and debunked with real world examples and action items.
It’s Always Bad
Auxiliary heat isn’t evil or wasteful. It’s a backup source designed to maintain indoor temperature when the heat pump can’t reach the set point on its own. For instance, during a sharp temperature dip far below a system’s balance point, generally around 0 to 2 °C, the compressor can lag or falter and the auxiliary strips or coils inject heat in order to stave off a room going chilly.
Sporadic aux ‘kicking on’ overnight or a front roll through is normal and to be expected in the majority of climates. Dramatically dropping the thermostat at night to save energy can backfire, triggering more aux heat and raising bills. Small setbacks or using a programmable schedule that keeps big swings in check is smarter.
Knowing when auxiliary heat runs helps decide if further action is needed. Frequent, unexplained use could mean the heat pump is undersized or needs service, while short bursts during extreme cold are normal.
Emergency Heat is Different
Emergency heat is a manual setting on the thermostat that runs the backup heat source and disables the heat pump. It’s used when the heat pump has gone out or needs to be down. Auxiliary heat, on the other hand, is an automatic assist that the system kicks in as necessary.
Running emergency heat as a regular mode is more costly because you’re short-circuiting the heat pump’s otherwise efficient operation. Emergency heat should be used for obvious failures or service-specific situations. A simple comparison is that auxiliary heat provides an automatic boost when needed and emergency heat is a manual pump-bypassing mode for emergencies.
- Auxiliary heat does not always mean failure.
- Aux heat can run in milder climates if the system is undersized.
- Big thermostat setbacks can bring auxiliary heat into play and increase energy consumption.
- Turning the thermostat way up won’t make it warm any faster and could trigger aux heat.
- Aux and the heat pump can often complement each other.
My Pump is Broken
Turning on emergency heat doesn’t necessarily mean you have a broken heat pump. Aux heat can come on for routine reasons: defrost cycles, quick outdoor temperature drops, or when the heat pump hits its balance point.
See continuous aux operation with no apparent reason; that’s when to call in an HVAC pro. Test system lights or logs or thermostat history first. They typically indicate if aux is cycling as normal or if it’s running nonstop.
The truth is, in many situations, a good read through the manual or a quick diagnostic check saves an unnecessary service call.
The Unseen Influences
Auxiliary heat on a heat pump performs more than just filling a momentary void. It answers to an array of seen and unseen forces that sculpt comfort, expense, and system longevity. These unseen influences range from technical decisions like sizing and installation to more general factors like local weather, household habits, and even the intangible environmental energies that shift a space’s energy and the system’s reaction to it.
The next few subsections decompose those drivers and illustrate why they’re relevant when selecting or operating a heat pump system.
System Sizing
Correctly sizing the heat pump and the aux unit is key to reliable performance. A unit sized to the true heat loss of the building runs steady and hits setpoints with minimal backup, while an undersized heat pump makes the aux heater run more frequently, driving energy consumption and bills through the roof.
Request that your HVAC contractor perform a load calculation grounded in the building envelope, orientation, window styles, and occupant habits. This provides a solid foundation for appropriately sizing your heat pump capacity and backup requirements.
Oversized systems cause short cycling. The equipment turns on and off frequently, lowering efficiency and wearing components faster. Short cycling decreases dehumidification and causes whipsaw temperature swings that feel ‘wrong’, connecting back to invisible forces such as occupant comfort and perceived warmth.
The properly sized backup element should only kick in when the main heat source can’t meet demand.
Installation Quality
A clean design and pro installation tie the system components together so they work as designed. Improper wiring, misconfigured control boards, or badly located sensors can trigger the aux heat unnecessarily.
When sensors measure incorrect temperatures because they hang around drafts or radiators, the system makes an error in estimating demand and wastes energy. Select licensed techs with residential brands under their belt and make sure controllers, staging, and thermostats are all set up for your model.
When integrated well, it minimizes surprise auxiliary cycles and equipment wear. Request wiring, sensor, and control setting documentation post-install.
Climate Adaptation
Local climate determines the frequency with which auxiliary heat will assume control. In cold climates, a heat pump’s efficiency decreases with outdoor temperature, so having reliable backup or a cold-climate heat pump is wise.
In more temperate climates, supplemental heat might barely come on, but insulation, thermostat tactics and occupant behavior still influence consumption. Environmental factors affect comfort in less direct ways: cooler rooms can make people feel sluggish and push them to raise thermostats, prompting more auxiliary use.
Think region-specific fixes—improved insulation, custom thermostat schedules and climate-informed equipment decisions—that minimize backup heat reliance while maintaining comfort.
Conclusion
Auxiliary heat assists a heat pump in maintaining a warm home when the pump can’t meet the load. It kicks in at low outdoor temperatures, during defrost cycles, or when the system needs a quick boost. Homes that are poorly insulated have the feature working more. Electric strip heat is quick, but expensive. Gas or dual-fuel options reduce bill spikes in cold weather. Easy solutions such as sealing, insulating, and thermostat adjustment provide less of a need for auxiliary heat. Regular tune-ups keep it tight and more efficient. Read meter data or a smart thermostat to identify additional runtime and high costs. Need assistance selecting the optimal system or locating discounts? Contact me for a quick review and definitive next steps.
Frequently Asked Questions
What is auxiliary heat on a heat pump?
Auxiliary heat is a secondary heating source, typically electric resistance or gas, that operates when the heat pump is unable to satisfy the thermostat setting or in extremely cold weather.
When does auxiliary heat turn on?
It kicks in when the heat pump is too cold, is defrosting, or the thermostat demands quick recovery from a significant setback.
Does auxiliary heat cost more to run?
Yes. Auxiliary heat is generally less efficient and more costly per unit of heat than the heat pump, so costs increase when it runs a lot.
How can I reduce auxiliary heat usage?
Crank up the thermostat a few degrees, don’t make big setbacks, insulate, and keep the heat pump maintained so it can do most of the heating without backup.
Will auxiliary heat damage my heat pump?
No. Auxiliary heat is meant to assist the system. It stops the equipment from working overtime and keeps everything safe.
How does a smart thermostat affect auxiliary heat?
Smart thermostats can restrict unnecessary auxiliary heat by applying optimized recovery and staging. This reduces runtime and saves you money.
Is auxiliary heat the same as emergency heat?
No. Emergency heat skips the heat pump altogether and just uses the backup heater. Auxiliary heat works in tandem with the heat pump as necessary.