The backup-heat setup that looks fine until you realize you’re pulling too much power

Your backup heat is supposed to be the quiet safety net in your home’s comfort system, not the star of the show. Yet in many houses, the setup looks perfectly normal until a cold snap hits, the electric meter spins like a roulette wheel, and you discover that your “backup” is quietly pulling more power than the rest of the house combined. The problem is rarely one dramatic failure and more often a chain of small design, sizing, and maintenance choices that push your system into an expensive, power-hungry mode far more often than it should.

Understanding how that happens, and how to spot it before your next utility bill arrives, means looking closely at how auxiliary and emergency heat are wired into modern heat pumps and packaged units. Once you know what your thermostat is really telling you, and how your equipment behaves in low temperatures, you can keep backup heat in its proper role instead of letting it run your winter budget.

When “Aux” on the thermostat becomes a hidden power hog

On a typical heat pump thermostat, the little “Aux” or “AUX HEAT” indicator is your first clue that backup electric strips are running. In normal operation, that light should appear only when the heat pump alone cannot keep up, such as during a rapid temperature increase or a sharp outdoor temperature drop. Guidance on what auxiliary heat means notes that if you have a heat pump and you suddenly ask for a big jump in your thermostat setting, the system will often call for those backup elements to help. That is by design, but it is also the moment when your power draw can spike, because electric resistance strips convert electricity to heat far less efficiently than the heat pump’s refrigerant cycle.

The trouble starts when that “Aux” label stops being an occasional visitor and becomes a constant companion. Technical advice on AUX operation explains that in normal conditions you might see the AUX light flash briefly, which is completely acceptable, but a steady indicator suggests the system is leaning heavily on the backup heating system. At that point, what looks like a routine winter setting can actually be a high-wattage electric heater running for hours, quietly turning your “efficient” heat pump into one of the biggest loads on your panel.

Why your auxiliary heat runs too much in the first place

If your backup elements are on constantly, the thermostat is usually responding to a deeper problem rather than misbehaving on its own. One common culprit is restricted outdoor airflow, where snow or ice builds up around the unit, plants grow too close, or a damaged fan cannot move enough air across the coil. A breakdown of why auxiliary heat runs too much points directly to snow, plants, and damaged components as triggers that force the system to rely on backup heat more often. In each of those cases, the heat pump’s capacity drops, the thermostat sees the indoor temperature lagging, and the electric strips quietly take over.

Another driver is how you use the thermostat itself. If you regularly set back the temperature overnight and then ask for a large morning increase, the system may default to auxiliary heat to close the gap quickly. Guidance on how to tell if aux is running too much notes that one sign is the AUX indicator staying lit for long stretches, especially when the weather is not extreme. That pattern often reflects thermostat logic that prioritizes speed over efficiency, which might feel comfortable in the moment but can double or triple the power draw compared with letting the heat pump work a bit longer on its own.

Oversized and undersized HVAC systems that trap you in backup mode

Even if your equipment is clean and your thermostat is sensible, the underlying sizing of your system can lock you into a bad relationship with backup heat. An oversized heat pump or furnace tends to cycle on and off frequently, blasting short bursts of heat and then shutting down before the building shell has warmed evenly. Analysis of the oversize trap notes that an oversized HVAC system cycles more often, which leads to uneven temperatures, unexpected breakdowns, and maintenance issues. Those rapid cycles can confuse thermostat algorithms, prompting more frequent calls for auxiliary heat to smooth out the swings, which again means more power consumption than the nameplate efficiency suggests.

Undersizing creates a different but equally expensive problem. If the heat pump is too small for the home’s heat loss, it may run continuously in cold weather and still fall behind, which pushes the thermostat to bring in the backup elements as a permanent crutch. A detailed look at The Dangers of Oversizing and undersizing your HVAC system, framed around what Tony needs to know, explains that when it comes to HVAC systems, both extremes undermine comfort and efficiency. In practice, that means you can end up with a system that either short cycles into backup heat or runs flat out and still needs the electric strips, in both cases pulling far more power than a properly sized design.

When backup heat is a symptom of a deeper failure

Sometimes the backup system is not just overused, it is the only thing keeping the house warm because the primary heat pump has effectively checked out. One diagnostic guide describes a scenario where, if your heat pump compressor fails and your backup heat takes over, the heat pump fan never shuts off and six weeks later you get a shocking electric bill. That warning comes from a DIY test that helps you determine if your heat pump is causing a high electric bill by checking whether the compressor is actually running when the system is calling for heat. If the outdoor unit is silent while the indoor air handler hums along, you are likely heating the house entirely with resistance strips, which is one of the most power-intensive ways to run.

Another red flag is what some technicians call “zombie” operation, where the system appears to be heating but the outdoor compressor is not participating. A breakdown of zombie heat pumps notes that if you notice that your system is heating but the outdoor compressor is not running, or if the emergency heat light is on, you are likely running on backup heat alone. In that state, the equipment may still respond to thermostat changes and blow warm air, so it feels “fine” from the couch, but behind the scenes it is drawing a continuous high load that can overwhelm both your budget and, in some cases, the capacity of an older electrical service.

Emergency heat: last resort, not everyday setting

Most modern thermostats include an “EM HEAT” or “Emergency Heat” mode that bypasses the heat pump entirely and runs only the backup elements. That setting exists for a narrow purpose, such as when the outdoor unit is iced over or mechanically failed and you need temporary heat until a repair. A troubleshooting guide titled When the Cold Sets In explains that understanding emergency heat failures starts with confirming that the emergency heat switch is in the “on” position only when truly needed. If you leave that mode engaged out of habit, or flip it on whenever the weather feels harsh, you are effectively choosing the most power-hungry configuration your system can offer.

Packaged terminal air conditioners and heat pumps, the kind you see in hotels and some apartments, add another wrinkle. Many of these units include a 5 kW electric heater that kicks in as backup, and guidance on understanding backup heat notes that when the thermostat frequently shows “Aux” and the unit switches to 5 kW electric heat, it can waste energy and money long term. In a small space, that kind of resistance heater can draw a large share of the available circuit capacity, so if you are also running space heaters, hair dryers, or other high-wattage devices, you may find yourself tripping breakers or stressing wiring that was never intended to handle all of those loads simultaneously.

Airflow, coils, and the physics that push you into backup

Even with the right size equipment and careful thermostat use, poor airflow can quietly sabotage your heat pump and push it toward backup heat. In heating mode, the indoor coil becomes the condenser, and if airflow across that coil is too low, the system’s internal pressures climb. A technical explainer that begins with the phrase Because the indoor coil becomes the condenser in heat mode notes that low indoor airflow can cause really high head pressure, compressor strain, and other issues tied to duct problems or dirty filters. When that happens, the heat pump may shut down on safety controls or simply deliver less heat, prompting the thermostat to call for auxiliary elements more often to maintain the setpoint.

Real-world complaints from homeowners often trace back to this kind of subtle performance loss. In one discussion, a user describes a heat pump using a lot of electricity and wonders how to know if it is tuned correctly, and a reply points out that if your heat is on but your heat pump fan is not, you should assume your heat is not coming from the heat pump. That advice, captured in an Oct thread, is a reminder that simple checks, like listening for the outdoor fan or feeling for airflow at vents, can reveal when the system has quietly shifted into a backup-heavy pattern. If you ignore those clues, the physics of restricted airflow and high head pressure will keep nudging the system toward resistance heat, and your power draw will follow.

How to spot a backup-heat problem before the bill arrives

Catching a runaway backup system early is as much about paying attention to patterns as it is about watching any single indicator. One practical approach is to treat your thermostat as a dashboard and note how often the AUX or EM HEAT labels appear during typical winter days. Guidance on how to tell if aux is running too much emphasizes that a constantly lit AUX indicator, especially in mild weather, is a clear sign that the backup elements are doing more than their share. Combine that with a mental note of how often you hear the outdoor unit running, and you can quickly tell whether the heat pump is carrying the load or the strips are quietly in charge.

It also helps to think about your home’s electrical behavior as a whole. If you notice lights dimming slightly when the heat kicks on, or breakers tripping when the system runs alongside other large appliances, that can indicate that the backup elements are drawing a large chunk of your panel’s capacity. In the same online discussion about high electric use, another comment suggests that if your heat is on but your heat pump fan is not, you should check whether you have electric backup heat and confirm if that is the case. That reminder, captured in a second Oct reply, underlines how often homeowners underestimate the impact of those strips. By pairing simple observations with a basic understanding of how your system is wired, you can spot a backup-heat problem while it is still a matter of thermostat settings and maintenance, not a surprise four-figure bill.

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*This article was developed with AI-powered tools and has been carefully reviewed by our editors.