It happens every year right around now. The first real cold snap arrives, your thermostat flashes a little indicator that says “Aux Heat” or “Auxiliary Heat” — and a few weeks later, an electric bill shows up that makes you wonder if something is broken. Meanwhile, out at the outdoor unit, your heat pump occasionally stops, hisses, and sends up a cloud of steam like it’s about to launch.
None of that is necessarily a problem. Auxiliary heat, emergency heat, and the defrost cycle are three of the most misunderstood features of heat pump ownership, and the confusion costs homeowners real money — in panicked service calls for a system working exactly as designed, or in quietly inflated bills from backup heat running far more than it should. This guide explains what each one does, when it should (and shouldn’t) run, how electric heat kits are sized, and how to keep expensive backup heat to a minimum all winter.
Table of Contents
- Aux heat, emergency heat, and defrost: quick definitions
- How a heat pump heats — and why it needs backup at all
- What auxiliary heat is and when it should come on
- Emergency heat: the switch you should almost never touch
- The defrost cycle: why your heat pump steams, hisses, and pauses
- Electric heat kits explained: sizing, kW ratings, and breakers
- Why backup heat costs so much to run
- How to keep aux heat use (and your bill) down
- When aux heat running is a red flag
- Dual fuel: when a furnace is the backup instead
- FAQ
Aux Heat, Emergency Heat, and Defrost: Quick Definitions
If you only read one section, make it this one:
Auxiliary heat (aux heat) is supplemental heat — almost always electric resistance heat strips inside your air handler — that turns on automatically alongside the heat pump when the heat pump alone can’t keep up, or during defrost. The heat pump keeps running; the strips just help.
Emergency heat (EM heat) is a manual thermostat setting that shuts the heat pump off entirely and runs the backup heat strips alone. It exists for genuine emergencies — typically a failed outdoor unit — not for cold days.
Defrost mode is a normal, automatic cycle in which the heat pump briefly reverses itself to melt frost off the outdoor coil. The steam, gurgling, and temporary pause you see on cold mornings are usually this, not a malfunction.
Same equipment, three very different behaviors. Now the details.
How a Heat Pump Heats — and Why It Needs Backup at All
A heat pump doesn’t create heat the way a gas furnace does. It moves heat, extracting thermal energy from outdoor air (yes, even cold air contains heat energy) and transferring it indoors through the refrigeration cycle. That’s why it’s so efficient: for every unit of electricity consumed, it typically delivers two to four units of heat, depending on conditions and equipment.
The catch is that a heat pump’s capacity falls as the outdoor temperature falls — right as your home’s heating demand rises. At some outdoor temperature those two lines cross: the house loses heat faster than the heat pump can supply it. That crossover is the balance point, and for many standard heat pumps in a typically insulated home it lands somewhere in the 25–40°F range. Below it, the heat pump still produces useful heat — it just needs help. That help is auxiliary heat.
Modern cold-climate inverter heat pumps push the balance point dramatically lower and can carry many homes with little or no backup heat even in single-digit temperatures — we covered that in depth in our cold-climate heat pump guide. But most installed systems, and most new budget-friendly ones, still rely on electric heat strips as the safety net.
What Auxiliary Heat Is and When It Should Come On
In a typical ducted heat pump system, auxiliary heat means an electric heat kit: coils of resistance wire (think of a giant toaster element) mounted in the air handler, downstream of the indoor coil. When energized, air passing over the elements picks up heat before entering your ducts.
Your thermostat decides when to call for it. The three legitimate triggers:
1. The temperature gap is too large
Most thermostats stage aux heat on when indoor temperature falls roughly 2°F or more below the setpoint and the heat pump alone isn’t closing the gap. This is why a big setback recovery — bumping the thermostat from 62°F to 70°F on a cold morning — almost guarantees aux heat: the thermostat sees a large gap and throws everything at it.
2. It’s below the balance point outside
During a genuine cold snap, the heat pump simply can’t carry the whole load, and the strips make up the difference. This is the system working as designed — and as we covered in our winter 2026–27 outlook, backup heat hours are worth planning for.
3. The defrost cycle is running
During defrost, the heat pump temporarily works in reverse, which would blow cool air into the house. Most systems energize the heat strips during defrost to temper that air. Brief aux heat during defrost is normal.
What’s not normal: aux heat glowing on your thermostat for hours in mild 45–50°F weather. That usually points to a problem — more on red flags below.
Emergency Heat: The Switch You Should Almost Never Touch
Here’s the rule of thumb that saves homeowners the most money: emergency heat is for emergencies, not for cold weather.
Switching your thermostat to EM heat locks out the heat pump and runs your home entirely on electric resistance strips. Since the strips deliver only one unit of heat per unit of electricity — versus two to four from the heat pump — you’ve just made your heating two to four times more expensive per hour of runtime, while your perfectly functional heat pump sits idle.
Use emergency heat when:
- The outdoor unit has failed or is making alarming noises (grinding, screeching) and you’re waiting on service.
- The outdoor unit is encased in ice that defrost can’t clear, or has been physically damaged (fallen limb, flood).
- A technician tells you to.
If you’ve had the system on EM heat “because it got cold,” switch it back to normal heat mode. The system will bring aux heat in automatically whenever it’s genuinely needed — that’s the whole point of automatic staging.
The Defrost Cycle: Why Your Heat Pump Steams, Hisses, and Pauses
When a heat pump runs in heating mode, its outdoor coil gets colder than the outdoor air. In temperatures roughly below 40°F with humidity in the air, moisture condenses and freezes on the coil. A light frost coating is normal; left unchecked, it would insulate the coil and choke off airflow, strangling the system’s capacity.
So the system defrosts itself. The control board — using coil temperature sensors, timers, or (on newer “demand defrost” boards) smarter logic that defrosts only when needed — periodically flips the reversing valve. For a few minutes, the heat pump runs in air-conditioning mode, pumping hot refrigerant through the outdoor coil to melt the frost. The outdoor fan stops, the strips typically fire indoors to cover the temporary cooling, and the melting frost flashes into the dramatic steam cloud that generates so many worried phone calls.
Normal defrost behavior: cycles lasting a few minutes, occurring occasionally during frosty conditions, with steam, dripping water, a whooshing or swooshing sound when the reversing valve shifts, and sometimes a brief louder hiss.
Worth a service call: the outdoor unit staying caked in thick ice for hours, defrost seemingly running constantly, ice sheets on top of the unit or encasing the coil, or the unit sitting in a frozen puddle because the base can’t drain. Chronic icing can indicate low refrigerant charge, a failing defrost sensor or board, a stuck reversing valve, or simple drainage/clearance problems around the unit.
One easy preventive step: keep the outdoor unit clear. Leaves, grass clippings, mulch piled against the cabinet, and later in the season snow drifts all restrict airflow and make frost worse. Our fall heat pump prep guide walks through the full pre-season checklist.
Electric Heat Kits Explained: Sizing, kW Ratings, and Breakers
This section is for homeowners who want to understand what’s in their air handler — and for contractors speccing equipment.
Electric heat kits are rated in kilowatts (kW), and commonly come in sizes such as 5, 8, 10, 15, and 20 kW depending on the air handler line. Converting to the BTU numbers used elsewhere in HVAC: 1 kW of resistance heat produces about 3,412 BTU/h. So a 10 kW kit delivers roughly 34,000 BTU/h — a meaningful chunk of heat for most homes.
How much heat kit do you need?
The right answer comes from a heat-load calculation, not a guess. The backup heat generally needs to cover the gap between the home’s heat loss at design temperature and what the heat pump produces at that temperature — and in many jurisdictions and utility programs, oversizing strips is actively discouraged because it inflates electrical service requirements and demand. General guidance:
- Milder climates with high balance points may do fine with 5–8 kW, mainly covering defrost tempering and rare cold snaps.
- Cold climates often call for 10–20 kW, especially when the strips must be able to carry the whole house if the heat pump fails.
- Larger kits require more electrical capacity — bigger breakers and conductors, and larger kits are typically split into stages and often include circuit breakers built into the kit. Your electrician and your panel capacity have a vote here.
Heat kits are matched to the specific air handler model — they’re not universal. If you’re configuring a system, start with a proper load estimate using our free HVAC system sizing tool, and see our air handler guide for how heat kits integrate with the blower and coil. If you’re unsure which kit pairs with which air handler, call us at (877) 843-4822 — matching equipment correctly is exactly what our team does all day.
Why Backup Heat Costs So Much to Run
Electric resistance heat is 100% efficient — every watt in becomes heat — and it’s still the most expensive common way to heat a home, because 100% is its ceiling while a heat pump routinely operates at an effective 200–400% by moving heat rather than generating it. Every hour your heat strips run, you’re paying roughly two to four times more for the same warmth than the heat pump would charge you. A 10 kW strip bank draws 10 kilowatt-hours every hour it runs steadily; a cold month with heavy aux use shows up on the bill fast. That’s not a reason to fear aux heat — it’s the reason to manage it.
How to Keep Aux Heat Use (and Your Bill) Down
Set it and (mostly) leave it
The classic furnace strategy of deep nighttime setbacks backfires with heat pumps. Each large recovery triggers aux heat, and the strips can erase the savings of the setback. With a standard heat pump, keep setbacks small — a degree or two — or skip them. If you use a smart thermostat, choose one with heat-pump-aware recovery that ramps the temperature back gradually using the compressor instead of slamming on the strips.
Check your thermostat’s aux lockout settings
Many thermostats let you set an outdoor temperature above which aux heat is locked out (requires an outdoor sensor or internet weather data). Setting a sensible lockout — tuned to your system’s balance point — stops the strips from piling on during mild weather when the heat pump could have handled it alone.
Keep airflow healthy
A clogged filter or blocked returns starve the system of airflow, dragging down heat pump output and inviting the thermostat to call for help. Fresh filters are the cheapest heating upgrade there is — we stock them along with thermostats and heat kits in our HVAC parts and supplies department. Duct restrictions matter too; our recent static pressure and airflow guide covers how to find and fix them.
Tighten the envelope
Every bit of air sealing and insulation raises your effective balance point — meaning the heat pump alone can carry the house deeper into winter before the strips ever energize.
Upgrade the equipment when the time comes
If your aux heat runs constantly every winter, the long-term fix may be a heat pump with more low-temperature capacity. Today’s inverter-driven and cold-climate models hold their output far better in the cold than older single-stage units. Buy Comfort Direct carries a full range of central heat pumps — including Goodman, Daikin, and Solace systems — at wholesale-direct pricing, with free shipping on orders over $99 and 0% APR financing available through Affirm.
When Aux Heat Running Is a Red Flag
Call a technician (or call us for parts and guidance) when you see:
- Aux heat in mild weather. Strips running at 45–55°F outdoors usually mean a heat pump that’s underperforming — low charge, a failing compressor or reversing valve, dirty coils — or a thermostat misconfiguration.
- Aux heat on, but lukewarm air. Could be a failed heat strip element, a tripped limit switch, a blown sequencer or relay, or a breaker on the heat kit that’s tripped.
- The “Aux Heat” indicator never turns off. If it runs for many hours straight even in moderate cold, something in the staging logic or the heat pump itself deserves a look.
- Burning smells beyond the first use of the season. A brief dusty smell the first time the strips fire each fall is normal (dust burning off the elements). Persistent burning odors are not — shut the system down and get it inspected.
Contractors: sequencers, relays, limit switches, defrost boards, and complete heat kits are stocked in our parts department, and our price match guarantee applies. If a part isn’t listed, contact us or email Info@BuyComfortDirect.com.
Dual Fuel: When a Furnace Is the Backup Instead
Electric strips aren’t the only backup option. In a dual fuel system, a gas furnace sits where the air handler would be, and the controls hand off from heat pump to furnace below a chosen outdoor temperature. In regions with cold winters and reasonable gas prices, that handoff can beat electric strips decisively on operating cost. We ran the numbers in our complete dual fuel guide; if your aux heat bills sting every January and you have gas service, it’s worth a read.
FAQ
Is it bad that my aux heat is on?
Not by itself. In genuinely cold weather, during defrost, or while recovering from a setback, aux heat is normal. It’s a concern when it runs in mild weather or essentially never shuts off.
Should I switch to emergency heat when it’s really cold?
No. Normal heat mode already brings in the backup strips automatically while keeping the efficient heat pump running. EM heat shuts the heat pump off and costs more. Reserve it for an actual outdoor-unit failure.
My heat pump is smoking — is it on fire?
Almost certainly not. A cloud of water vapor during or right after a defrost cycle is normal. Smoke with a burning smell from the indoor unit is a different matter — shut down and investigate.
Why does cool air blow for a few minutes on cold mornings?
Defrost mode temporarily reverses the system. The strips usually temper the air; if the air is outright cold for more than roughly ten minutes, have the defrost controls checked.
The Bottom Line
Auxiliary heat is your heat pump’s teammate; emergency heat is its substitute; defrost is its self-maintenance. Understand the difference and you’ll stop worrying about steam clouds, stop paying for EM heat you didn’t need, and start trimming the single most expensive component of winter heat pump operation. Keep setbacks gentle, keep filters fresh, set a smart aux lockout, and let the automatic staging do its job.
And when it’s time for a heat kit, a thermostat, replacement parts, or a whole new system, Buy Comfort Direct ships nationwide with free shipping on orders over $99, 0% APR financing through Affirm, and real HVAC people on the phone at (877) 843-4822 to make sure every component matches before it ships.