If you’ve ever glanced at your thermostat on a cold morning and noticed a little “Aux Heat” light glowing back at you, you’re not alone in wondering what it means—and whether it’s something to worry about. The good news is that “Aux Heat” is usually a normal part of how many heat pump systems work. The less-good news is that if it’s on too often (or at the wrong times), it can be a sign your system is working harder than it should, which can bump up your energy bill fast.
Heat pumps are popular because they can both heat and cool your home using the same core technology. But the settings and terminology can feel a bit cryptic: “Heat,” “Emergency Heat,” “Aux,” “Stage 2,” “Balance point,” “Defrost.” If you’re a student renting your first place, a newcomer adjusting to a new climate, or just someone who wants to keep costs predictable, understanding these settings can make a real difference.
Let’s unpack what the Aux Heat light actually means, how heat pump settings work, when auxiliary heat is expected, and when it’s a signal to troubleshoot. Along the way, you’ll get practical tips for comfort, efficiency, and knowing when it’s time to call in a pro.
Heat pumps in plain language: what they’re doing behind the scenes
A heat pump doesn’t “make” heat the same way a furnace does. Instead, it moves heat from one place to another. In summer, it moves heat out of your home (cooling). In winter, it moves heat into your home (heating). That sounds like magic, but it’s really just refrigeration physics working in reverse.
When it’s mildly cold outside, a heat pump can usually extract enough heat from outdoor air to warm your home efficiently. As outdoor temperatures drop, there’s less usable heat available, and the system has to work harder to pull it in. At some point, it may need help—this is where auxiliary heat comes in.
Most commonly, auxiliary heat is electric resistance heat strips inside the indoor air handler. Think of them like a giant toaster element that warms air quickly. They’re effective, but they’re also energy-hungry compared to the heat pump’s normal operation.
What the “Aux Heat” light actually means
The “Aux Heat” indicator means your thermostat is calling for auxiliary (supplemental) heat in addition to the heat pump. In other words, the system is using its backup heating source to help meet the temperature you’ve requested.
This usually happens for one of two reasons: either the heat pump can’t keep up with demand (often due to cold outdoor temps), or your thermostat settings and temperature changes are prompting a faster warm-up than the heat pump alone can deliver.
It’s important to note that “Aux Heat” is not the same thing as “Emergency Heat.” Aux is automatic assistance. Emergency Heat is a manual mode you choose when the heat pump is not working properly and you want to run only the backup heat source.
Aux Heat vs. Emergency Heat: similar words, very different outcomes
Aux Heat is automatic support
Aux Heat is your system’s way of saying, “I can get you there, but I need a little help.” The heat pump continues running, and the auxiliary heat kicks in as needed. Many thermostats will show “Aux,” “Aux Heat,” or “Stage 2 Heat” depending on the model.
Because it’s automatic, you might see it come on during cold snaps, after big temperature setbacks, or during defrost cycles (more on those soon). Seeing it occasionally is normal in many climates and homes.
The key is frequency and duration. A brief Aux cycle is one thing. Aux running for hours every day is another—and it can be a sign something is off.
Emergency Heat is a manual override
Emergency Heat (often shown as “EM Heat”) is when you tell the thermostat to stop using the heat pump entirely and rely only on the backup heat. This is typically used if the outdoor unit is damaged, frozen, or otherwise not functioning.
Emergency Heat is usually more expensive to run, especially if your backup is electric resistance. It’s meant as a temporary “get me through today” setting while you arrange repairs, not as a normal winter mode.
If you’re ever tempted to switch to Emergency Heat just because it “feels warmer,” pause. It might warm faster, but it usually costs significantly more—and it can mask an underlying issue that deserves attention.
Common reasons the Aux Heat light turns on
Outdoor temperatures drop below the heat pump’s sweet spot
Heat pumps are most efficient when the temperature difference between inside and outside isn’t extreme. As outdoor air gets colder, the heat pump’s capacity drops. Many systems are designed to call for Aux Heat when the outdoor temperature falls below a certain threshold (sometimes called the balance point).
This balance point varies based on your heat pump model, your home’s insulation, wind exposure, and how warm you keep the thermostat. A well-sealed home might stay comfortable longer without Aux. A drafty home may trigger Aux earlier and more often.
If you notice Aux Heat mainly during the coldest nights and it turns off again during daytime warming, that’s often just the system doing what it’s designed to do.
You raised the thermostat by several degrees at once
Many “smart” thermostats and heat pump thermostats are programmed to bring the temperature up quickly if you jump the setpoint. For example, if you wake up and change 66°F to 72°F, the thermostat may decide the heat pump alone will take too long and will engage Aux Heat to speed things up.
This is one of the most common “surprise Aux” moments. People see the light, assume something is broken, but it’s just the thermostat trying to satisfy your request faster.
If you want to reduce Aux usage, try smaller adjustments (1–2 degrees at a time) or use a schedule that gradually warms the home before you wake up.
Defrost cycles temporarily reduce heating output
In cold, humid conditions, frost can build up on the outdoor coil. Heat pumps periodically run a defrost cycle to melt that frost. During defrost, the system briefly reverses operation, which can feel like it’s blowing cooler air inside.
To keep you comfortable during this process, many systems engage Aux Heat while the defrost cycle runs. This prevents cold drafts and helps maintain indoor temperature.
If Aux Heat turns on for short periods during damp, chilly weather and then shuts off, defrost support is a likely explanation.
Airflow issues make it hard to deliver heat
Even if the heat pump is producing heat, your home may not feel warm if airflow is restricted. Dirty filters, blocked return vents, closed registers, or duct problems can reduce the amount of warm air reaching your rooms.
When the thermostat senses the temperature isn’t rising fast enough, it may call for Aux Heat. In that case, Aux is compensating for a delivery problem rather than an outdoor temperature problem.
Checking filters and vents is one of the easiest DIY steps to reduce unnecessary Aux operation.
Refrigerant or mechanical problems reduce heat pump capacity
A heat pump that’s low on refrigerant, has a failing compressor, or has sensor/control issues may struggle to heat properly. The thermostat sees the setpoint isn’t being met and triggers Aux Heat to fill the gap.
This is where the Aux Heat light becomes a useful clue. If it’s on frequently during mild weather—when the heat pump should be able to handle heating on its own—there’s a higher chance something needs professional attention.
In these cases, Aux Heat isn’t the problem; it’s the symptom. The underlying issue is that the heat pump isn’t delivering its normal capacity.
Heat pump thermostat settings that affect Aux Heat
Heat mode vs. Auto mode
Many thermostats offer “Heat,” “Cool,” and “Auto.” Auto automatically switches between heating and cooling depending on indoor temperature. That sounds convenient, but in shoulder seasons (spring/fall), it can cause more switching than you expect.
Frequent mode switching can lead to comfort swings and sometimes unnecessary staging (including Aux) if the thermostat is trying to correct quickly after a swing. If you’re noticing odd behavior, try using “Heat” in winter and “Cool” in summer instead of “Auto.”
This is especially helpful in climates where days are warm but nights are cold, because Auto can bounce back and forth more than you’d like.
Staging and “second-stage heat”
Some thermostats show “Stage 1” and “Stage 2” heating. Stage 1 is typically the heat pump alone. Stage 2 is either a higher-capacity compressor stage (for two-stage heat pumps) or auxiliary heat strips, depending on the system.
With a two-stage or variable-speed heat pump, Stage 2 might still be the heat pump—just running harder. In that case, the Aux Heat light might not mean electric strips; it depends on how your thermostat labels stages and how it’s wired.
If you’re unsure what “Aux” means in your specific setup, the equipment model numbers (indoor air handler and outdoor unit) can clarify whether you have electric heat strips, a dual-fuel setup, or a multi-stage compressor.
Temperature swing (differential) settings
Some thermostats allow you to adjust how sensitive the system is before it calls for more heating. A tighter differential may keep your temperature more consistent, but it can also increase cycling and trigger Aux more readily.
A wider differential can reduce cycling but may allow the temperature to drift more before it kicks in. There’s no one perfect setting—it depends on comfort preference, insulation, and how quickly your system responds.
If you rent or live in student housing, you may not have access to these advanced settings. Still, it’s useful to know they exist because they influence when Aux engages.
Compressor lockout and outdoor temperature sensors
Some systems use an outdoor temperature sensor to decide when the compressor should run versus when auxiliary heat should take over. This is called compressor lockout. If the lockout temperature is set too high, your system may rely on Aux more often than necessary.
On the other hand, if it’s set too low, the heat pump may struggle in extreme cold, potentially leading to discomfort or excessive runtime.
These settings are usually configured during installation or service. If Aux seems excessive compared to your neighbors with similar homes, lockout settings are one possible factor.
When Aux Heat is normal—and when it’s a red flag
Normal patterns you can feel good about
Aux Heat is generally normal if it appears during very cold weather, after a big thermostat increase, or briefly during defrost cycles. You might also see it more in older homes or homes with less insulation, where heat loss is higher.
Another normal scenario: you come home after being away, the house is chilly, and you crank the thermostat up. Aux comes on to help recover temperature faster. If you let the system warm gradually instead, Aux might not be needed.
If your home reaches the set temperature without running Aux for long stretches, your heat pump is likely doing its job.
Warning signs that deserve a closer look
Aux Heat can be a red flag if it’s on when outdoor temperatures are mild (for example, 45–60°F / 7–16°C), or if it runs almost continuously just to maintain a modest indoor temperature.
It’s also worth investigating if your vents blow lukewarm air for long periods, rooms heat unevenly, or your electric bill spikes suddenly without a clear weather-related reason.
Finally, if you see “Aux Heat” and your home still feels cold, that suggests the system is struggling overall—either with heat production, airflow, or thermostat control.
Practical ways to reduce Aux Heat usage without freezing
Use smaller temperature changes (or schedule them)
If you tend to make big jumps in temperature, you’re basically inviting Aux Heat to join the party. Instead of going from 66°F to 72°F in one step, try 66°F to 68°F and give it time.
If your thermostat supports scheduling, set it to start warming the home earlier at a slower pace. A gradual ramp-up often keeps the system in efficient heat-pump mode longer.
This can be especially helpful for students with early classes or anyone with a consistent morning routine.
Keep airflow simple and steady
Replace or clean your air filter on schedule. A clogged filter is one of the most common causes of poor airflow, and it can make your system run longer and trigger Aux more often.
Also, avoid closing too many supply vents. It seems logical to “force” air into the rooms you use, but closing vents can increase static pressure in the ductwork and reduce overall system efficiency.
Make sure return vents aren’t blocked by furniture, laundry piles, or curtains. Returns are how air gets back to the system; blocking them can make the whole setup underperform.
Seal the easy leaks first
If your home is losing heat quickly, the thermostat will keep calling for more heat—and Aux may kick in to help. Quick wins include weatherstripping around doors, using draft stoppers, and sealing obvious gaps around window frames.
Thick curtains can help at night, especially on older windows. If you’re renting, you can still do a lot without making permanent changes.
Even small improvements to heat retention can reduce how often your system needs that expensive backup heat.
Pay attention to humidity and comfort
Dry air can make a room feel cooler than it is. If indoor humidity is very low in winter, you might be tempted to raise the thermostat higher, which can trigger Aux more often.
If you can safely add a bit of humidity (for example, with a portable humidifier), you may feel comfortable at a slightly lower temperature. That can reduce heating demand overall.
Comfort is not just temperature—it’s also humidity, airflow, and radiant heat loss. Tweaking those can sometimes save money without sacrificing how you feel.
How to tell if your system uses electric strips, gas backup, or something else
Electric resistance strips (most common “Aux Heat” setup)
In many all-electric homes and apartments, auxiliary heat is provided by electric heat strips inside the air handler. When they run, you may notice the air feels hotter at the vents, and your electricity usage can jump.
These strips are simple and reliable, but they’re typically the most expensive way to heat compared to a heat pump operating efficiently.
If your utility bills climb sharply whenever Aux appears, electric strips are a likely culprit.
Dual-fuel systems (heat pump + gas furnace)
Some homes use a heat pump for mild winter weather and a gas furnace for colder conditions. In these setups, “Aux” might mean the furnace is helping or taking over when it’s more cost-effective.
Dual-fuel can be efficient, but it needs correct thermostat configuration—especially the outdoor temperature “changeover” point. If that point is set poorly, you may burn more fuel than necessary.
If you have gas service and a furnace cabinet indoors, you might be in a dual-fuel arrangement.
Heat pump type matters: single-stage, two-stage, variable-speed
A single-stage heat pump is either on or off. Two-stage and variable-speed systems can run at lower output most of the time, which often improves comfort and efficiency.
With more advanced systems, the thermostat may show different stages of heating before it ever calls for Aux. That can mean fewer hours of expensive backup heat, especially in moderately cold climates.
If you’re considering an upgrade someday, system staging is a big part of why newer heat pumps can feel more consistent and cost less to run.
Thermostat messages that often get confused with Aux Heat
“Heat On” doesn’t mean Aux
Many thermostats simply show “Heat On” whenever the system is heating. That could be the heat pump alone, or it could be heat pump + Aux. The dedicated “Aux” indicator is what tells you auxiliary heat is active.
If your thermostat doesn’t show an Aux indicator at all, you may need to check the manual or app to see whether it can display staging details.
Some smart thermostats log runtime by stage, which is a great way to see how often Aux is actually used.
“Recovery” or “Smart Recovery” can trigger Aux
Smart thermostats often learn how long your home takes to warm up and may start heating early to hit your scheduled temperature on time. In some cases, they’ll use Aux to ensure they meet the schedule.
If you notice Aux turning on at predictable times (like right before you wake up), your thermostat’s recovery feature may be the reason.
You can often adjust this behavior by changing schedule targets, reducing the temperature difference between night and morning, or tweaking thermostat settings if available.
“Aux” can appear during normal maintenance alerts
Some systems display maintenance reminders or alerts that coincide with higher Aux usage—like “Filter” reminders during winter. It’s easy to connect the two and assume something broke.
Sometimes the fix really is as simple as a filter change, but sometimes the timing is coincidence. The best approach is to check basics first (filter, vents, thermostat settings) and then monitor.
If Aux remains frequent after you’ve handled the basics, that’s when deeper troubleshooting makes sense.
Energy costs: why Aux Heat can change your bill so quickly
Heat pumps are efficient because they move heat
In normal operation, a heat pump can deliver more heat energy than the electrical energy it consumes. That efficiency is often described as a coefficient of performance (COP). A COP above 1 means you’re getting “more heat than electricity,” thanks to heat transfer.
Aux heat strips, on the other hand, are basically 1:1—one unit of electricity becomes one unit of heat. That’s not “bad” in terms of physics, but it’s usually expensive in terms of utility costs.
So when Aux runs often, your system shifts from “high efficiency heat transfer” to “straight electric heating,” and your bill reflects that.
Small behavior changes can matter
If Aux triggers because of big thermostat jumps, changing that habit can reduce the number of hours those heat strips run. Even shaving off a little Aux runtime each day can add up across a month.
Likewise, improving airflow and sealing drafts can reduce the demand that causes Aux to engage. You don’t necessarily need a major renovation to see a difference.
If you like data, try tracking when Aux turns on (time of day, outdoor temperature, thermostat changes). Patterns become obvious quickly.
What to check yourself before calling for help
Filter, vents, and thermostat basics
Start with the simplest items: replace the air filter (or clean it if it’s reusable), confirm supply vents are open, and ensure return vents aren’t blocked. If you have multiple returns, check them all.
Next, look at thermostat mode and fan settings. “Fan On” runs the blower continuously, which can feel drafty in winter and may make you think the system isn’t heating well. “Auto” is usually best unless you have a specific reason to run the fan nonstop.
Also check if someone accidentally set the system to Emergency Heat. It happens more often than you’d think, especially in shared housing.
Outdoor unit conditions (without getting risky)
Take a look at the outdoor unit. Make sure it isn’t buried in snow, surrounded by leaves, or blocked by objects. Airflow around the unit matters for heat extraction.
In winter, it’s normal to see some frost, but thick ice buildup that doesn’t clear can indicate a defrost problem or airflow issue. Don’t chip ice off with tools—you can damage the coil.
If you suspect severe icing, it’s a good reason to get professional service rather than forcing the system to run harder.
Listen for changes and track runtimes
Heat pumps sound a little different than furnaces. You may hear the outdoor unit running more steadily, and you may hear whooshing during defrost. What you’re listening for is a big change from normal: grinding, squealing, loud buzzing, or short cycling (turning on/off frequently).
If your thermostat or app shows runtime by stage, check how often Aux is engaged. If it’s dominating your heating hours, you’re likely paying for backup heat more than you should.
Having a few notes—outdoor temp, thermostat setpoint, and when Aux appears—can help a technician diagnose faster.
When it’s time to bring in a professional (and what to ask)
If Aux Heat is running constantly, if your home can’t reach set temperature, or if you suspect refrigerant/defrost issues, it’s smart to schedule service. A technician can check refrigerant charge, electrical components, defrost controls, airflow, duct static pressure, and thermostat configuration.
If you’re also managing cooling season comfort, it’s helpful to work with a team that understands both sides of the system. For example, if you’re comparing options or learning what’s involved, you can look at air conditioning services to get a sense of what comprehensive support typically includes—maintenance, diagnostics, repairs, and efficiency improvements.
On the heating side, you can ask specifically about heat pump staging, auxiliary heat settings, and whether your system is sized and configured correctly. If you want to see what heating-focused support often covers, heating system services can give you a good overview of the kinds of issues pros look for beyond the obvious symptoms.
Heat pump comfort tips for students, renters, and shared households
Agree on temperature rules (seriously)
In shared housing, Aux Heat often becomes a “who touched the thermostat?” problem. One person cranks it up, Aux kicks in, the bill rises, and everyone’s confused. A simple household agreement can prevent that.
Try setting a reasonable range (like 68–70°F / 20–21°C when home, a bit cooler at night) and avoid big jumps. If you want it warmer, add layers or use a small space heater cautiously and safely—but note that space heaters can also raise costs quickly.
If you’re renting and utilities are included, you still want to avoid stressing the system. Constant Aux use can lead to wear, and you don’t want to be without heat during exams because something failed.
Use doors and airflow to your advantage
If your home has rooms that heat unevenly, use interior doors strategically. Keeping doors open can help air circulate, but if one room is always cold, it might benefit from being closed and heated more directly—depending on vent placement.
Ceiling fans on low speed (clockwise in winter) can help push warm air down from the ceiling, making rooms feel more even without raising the thermostat.
These small comfort tweaks can reduce the urge to crank the temperature and trigger Aux.
Know what you’re responsible for vs. the landlord
In many rentals, you’re responsible for basic filter changes, while the landlord is responsible for equipment repairs and major maintenance. If Aux is constantly on and you’ve handled the basics, document what you’re seeing and report it.
Sharing specifics helps: “Aux Heat is on most of the day when it’s 50°F outside,” or “system runs continuously but indoor temp won’t rise above 67°F.” That’s more actionable than “it’s not warm.”
If you’re new to a region with colder winters than you’re used to, it’s also worth asking what type of heating system the building uses and what “normal” looks like for that setup.
FAQ-style answers to common Aux Heat questions
Is it bad if Aux Heat comes on?
No—Aux Heat is a designed feature. It’s there to keep you comfortable when the heat pump alone can’t keep up or when the thermostat wants faster heating.
It becomes a problem mainly when it’s frequent during mild weather or when it runs for long periods, because that can signal inefficiency or a system issue.
Think of Aux as a safety net: great to have, not ideal to rely on constantly.
Should I turn off Aux Heat to save money?
Most thermostats don’t let you simply “turn off” Aux without changing system configuration, and it’s not usually recommended. If the heat pump can’t keep up, disabling Aux could leave you uncomfortable or cause the system to run nonstop.
A better approach is to reduce the triggers: avoid big temperature jumps, improve airflow, and keep up with maintenance.
If you suspect the thermostat is using Aux too aggressively, a technician can often adjust staging settings safely.
Why does my heat pump blow cool air sometimes?
Heat pumps often deliver air that feels cooler than furnace air, even when heating properly. A furnace might supply air at 120–140°F, while a heat pump might supply air closer to 85–100°F depending on conditions.
During defrost cycles, you might feel a cooler draft briefly, and Aux may come on to offset that. That’s normal.
If the air is truly cold for extended periods or the home isn’t warming, that’s when you should investigate.
How do I know if my thermostat is compatible with a heat pump?
Heat pumps typically require a thermostat designed for heat pump wiring and staging (often with terminals like O/B for reversing valve and AUX/W2 for auxiliary heat). Using the wrong thermostat can cause odd behavior, including unnecessary Aux operation.
If you’ve recently replaced a thermostat and noticed Aux Heat more often, compatibility and wiring are worth double-checking.
In rentals, thermostat swaps may not be allowed, but you can still report issues if the system’s behavior changed after a device replacement.
Choosing the right help when you need it
When you do need professional support, it helps to work with a team that regularly deals with heat pump controls, staging, and efficiency—not just quick fixes. Heat pumps are straightforward once you understand them, but they’re also sensitive to airflow, refrigerant charge, and thermostat configuration.
If you’re in a region where heat pumps are common (or you’re relocating and comparing service options), looking at a provider’s local experience can be useful. For instance, seeing how an HVAC company in Austin describes their service area and offerings can give you a feel for whether they’re set up to handle heat pump-specific troubleshooting, maintenance, and system optimization.
Wherever you live, the best service calls are the ones where you can clearly explain what you’re seeing: when Aux appears, how long it runs, what the weather is like, and whether the home is reaching temperature. That turns the Aux Heat light from a mystery into a helpful diagnostic clue.
A quick mental checklist the next time you see “Aux Heat”
If you notice the Aux Heat light again, run through this quick list. Did the temperature outside just drop sharply? Did you raise the thermostat by a lot? Is it damp or icy outside (defrost conditions)? Are your filters clean and vents open? If the answer to one of those is “yes,” Aux may be doing something totally normal.
If none of those fit—and Aux is on a lot during mild weather—take it as a nudge to investigate further. Heat pumps are efficient when everything is dialed in, and small issues can quietly push them into expensive backup mode.
Once you understand what Aux Heat is and what triggers it, you’re in a much better position to stay comfortable, keep costs under control, and know when it’s time to ask for help.
