Auxiliary heat
By the HVAC Responder Editorial Team
Auxiliary heat is a heat pump’s backup heat source — usually electric resistance strips inside the air handler — that switches on when the heat pump alone cannot hold temperature: during deep cold, defrost cycles, or big thermostat setpoint jumps. It heats reliably but costs two to three times more per unit of warmth than the heat pump itself.
Why it matters to a homeowner
Occasional AUX HEAT on the display is designed behavior. Constant aux operation in mild weather is the classic silent bill-inflator, usually traceable to a struggling heat pump, an aggressive thermostat schedule (large setbacks force strip recovery), or a failed outdoor unit quietly leaving the strips to do everything. The winter habit that pays: raise setpoints gradually, and treat an AUX icon that never leaves as a service call, not a personality trait. Emergency heat is the manual version of the same strips — a limp-home switch, not a normal mode.
The strips behind the vents
Inside a heat pump’s air handler sits a rack of electric resistance elements — five to twenty kilowatts of pure toaster. When the thermostat sees the heat pump falling behind, it energizes strips in stages, blending their heat into the airstream. Resistance heat is the honest workhorse of the arrangement: one unit of heat per unit of electricity, every time, at any temperature. The heat pump’s trick is delivering two to three units per unit of electricity — which is precisely why every hour the strips run instead of the pump multiplies that hour’s heating cost.
When aux is supposed to run
Three legitimate triggers: outdoor cold below the system’s balance point, defrost cycles (the strips cover the minutes the pump runs backward), and recovery from a big setpoint jump, because thermostats interpret "three degrees behind" as "send everything." All three are visits, not residencies. The design goal of a properly sized cold-climate system is aux measured in dozens of hours a winter, not hundreds — and the modern generation of low-temperature heat pumps keeps shrinking the honest share of the season the strips deserve.
Reading an aux problem off the thermostat
The diagnostic gold is duration and weather: AUX flickering during a cold snap is design; AUX glowing at 40°F outside is a fault wearing a comfort mask. Usual suspects, in order: refrigerant charge low enough that the pump underdelivers, an outdoor unit not running at all (strips silently carrying the house — the failure nobody notices until the bill), aggressive schedule setbacks forcing daily strip recovery, or a mis-set balance point handing off way too early. A power-bill spike with no thermostat explanation is the same clue arriving by mail.
Emergency heat is a different switch
EM HEAT on the thermostat manually locks the heat pump out and runs strips alone — a limp-home mode for when the outdoor unit is genuinely broken or iced into a sculpture. It is not "extra warm mode," and running it casually double-to-triples the heating bill while masking whatever failed. Use it when the pump is down, book the repair, and switch back after the fix. If you find EM HEAT engaged and cannot remember why, that is this glossary’s politest way of saying: call.
Related terms, defined in brief
Heat Pump — A heat pump is a refrigerant-based system that moves heat rather than generating it: out of the house in summer (exactly like an air conditioner) and into the house in winter, by extracting heat from outdoor air even when that air is cold. Because moving heat takes far less energy than creating it, a heat pump typically delivers two to four units of heat per unit of electricity consumed.
The winter trick is the reversing valve, which flips the refrigerant flow so the outdoor coil absorbs heat and the indoor coil releases it. Modern cold-climate models hold useful capacity below 0°F. Most homes pair the heat pump with backup heat — electric strips or a gas furnace ("dual fuel") — for the coldest tail of the year. Nearly every ductless mini-split is a heat pump too.
Balance Point — A heat pump’s balance point is the outdoor temperature at which its heating output exactly equals the house’s heat loss. Above it, the heat pump carries the load alone; below it, backup heat — electric strips or a furnace — must make up the difference. Typical balance points fall between 25 and 40°F depending on equipment capacity and the house envelope.
This is the setting that quietly decides winter bills on heat pump systems. Configured lazily, auxiliary heat runs during mild weather at triple the cost per BTU; configured well, expensive backup runs only when physics requires it. Insulation upgrades lower the balance point for free, and cold-climate equipment pushes it far down the thermometer. Ask, at commissioning, what yours is set to — and why.
Defrost cycle — The defrost cycle is a heat pump’s self-maintenance routine: in cold, humid weather the outdoor coil ices over, so the system briefly reverses into cooling mode to send hot refrigerant through that coil and melt the frost — producing steam, dripping, a whoosh, and a few minutes of cooler indoor air while auxiliary heat covers the gap.
Homeowners meet defrost as a scare: the outdoor unit smokes (steam), roars, and the vents blow lukewarm. All normal, several times a day in freezing drizzle. The genuine warning signs are different — ice that never fully clears, a unit encased in ice sheets, defrost every few minutes, or water refreezing into a skating rink under the unit. Those point to a failing defrost board, sensor, low charge, or drainage problem, and ice buildup left alone can bend fan blades and crush coil fins. Keep the unit clear of snow and give its drips somewhere to go.
Thermostat — The thermostat is the control that reads room temperature and commands the HVAC equipment: calling for heat, cooling, or fan, and — on multi-stage or heat-pump systems — deciding which stage or backup source runs. Smart thermostats add scheduling, occupancy learning, and remote control, and typically require a C-wire for continuous power.
Thermostats cause a surprising share of "dead furnace" calls: dead batteries, a wire loosened during painting, or a heat-pump thermostat configured wrong so auxiliary heat runs constantly. That last one is expensive and invisible. Smart models help most in homes with real schedules; savings claims of 8–15% assume you actually let the setbacks happen instead of overriding them nightly.
Where you'll meet this term
Contractors reach for "Auxiliary heat" most often during heat pump services visits. If one uses it and the explanation doesn't land, ask them to show the measurement or the part it refers to — every legitimate use of this vocabulary has something physical behind it.
The term in the field: heat pump services
The clearest way to anchor "Auxiliary heat" is the failure calls where it comes up. On heat pump services visits, the surrounding conversation usually starts with symptoms like these:
Electric bills spike in winter
Auxiliary resistance heat running more than it should — controls, balance point, or capacity problem.
All-electric home heated by baseboards or an electric furnace
A heat pump typically delivers the same heat for a half to a third of the electricity.
Chasing utility rebates or the federal credit
Heat pumps carry the largest residential HVAC incentives available — the biggest federal credit in the category plus local stacking.
Considering replacing both furnace and AC at once
One heat pump can replace both — this is exactly the moment the heat-pump math is strongest.
Questions where this vocabulary earns its keep
Can a heat pump reuse my existing ductwork?
Usually, with a caveat: heat pumps move more air at lower temperatures than furnaces, so ducts sized for a furnace sometimes run high static pressure with a heat pump — noise, weak rooms, and efficiency loss. A competent installer measures static pressure and either confirms the ducts or scopes the fixes. Skipping that measurement is how "my new heat pump is loud and the back room is cold" happens.
What does a heat pump cost to run versus a gas furnace?
It hinges on local rates. A heat pump moving 3 units of heat per unit of electricity competes with gas whenever electricity costs less than about 3–4× gas per unit of energy. At typical national averages the heat pump wins in mild and moderate climates and roughly ties in cold ones — where dual-fuel setups capture the best of both. Your utility’s actual rates decide it, not national averages.
Why is there ice on my heat pump — and when is it a problem?
Light frost on the outdoor coil in cold, damp weather is normal, and the unit periodically reverses into defrost to clear it (steam and a whooshing sound — also normal). A solid ice shell, ice that persists through defrost cycles, or fan blades striking ice are service calls: typically defrost controls, a bad sensor, low charge, or blocked drainage under the unit.
Where this term meets a price tag
When "Auxiliary heat" comes up in a quote, the numbers around it are itemized in Heat Pump Installation Cost, Before and After Incentives — national planning ranges, line by line, kept separate from the routing service so you can read any contractor's bid against an independent reference.
Guides where this term does real work
- Heat Pump Not Heating? Normal vs Broken, Sorted — Cool-feeling air, frost, steam clouds — much heat pump “failure” is normal operation. What is actually broken vs physics, and when to call.
Dealing with this in your own system?
An independent local contractor puts a measurement on it — fee quoted up front, findings in writing.
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