- On an all-electric heat pump system, emergency heat shuts the heat pump off and runs only the electric resistance strips in your air handler — which typically costs roughly two to three times as much to run as normal heat pump heating in Louisville winter weather.
- Emergency heat is not the same as auxiliary heat. Auxiliary heat comes on automatically to help the heat pump; emergency heat shuts the heat pump off completely and should only be turned on manually when the heat pump can’t run.
- The two situations that justify switching to emergency heat are a failed compressor or physical damage to the outdoor unit — not cold weather alone.
- If your heat pump is paired with a gas furnace (a dual-fuel system), emergency heat usually runs the furnace instead of electric strips, and the cost picture is different.
- How long you run emergency heat matters more than anything else — a few hours while you wait for a technician is fine; days of it will show up on your LG&E bill.
Switching your thermostat to emergency heat without a real reason is one of the most expensive mistakes a Louisville homeowner with a heat pump can make in January.
Most people discover the emergency heat setting during a cold snap when something seems off — the house isn’t warming up, the outdoor unit sounds different, or the thermostat shows an unfamiliar display. The instinct to flip over to emergency heat is understandable, but it’s often the wrong move. Louisville HVAC Pro’s works on heat pump systems throughout Jefferson and Bullitt County, and here’s how to tell when the setting actually makes sense.
Emergency Heat Costs Two to Three Times More Than Normal Heat Pump Operation
A heat pump moves heat rather than generating it. In normal operation, it pulls heat from the outdoor air and transfers it inside — and standard heat pumps keep producing useful heat well below freezing. Because it moves energy rather than creating it, a heat pump delivers more heat per unit of electricity than straight electric resistance heating can.
On an all-electric system, emergency heat cuts the heat pump out entirely. The only thing producing warmth is the electric resistance strip inside the air handler — essentially a large version of the coil in a toaster. Every unit of heat costs exactly one unit of electricity, with no efficiency multiplier. That’s why the cost difference is so noticeable. Running heat strips as your only heat source for days at a time during a Louisville winter will show up clearly on your LG&E bill.
What Emergency Heat Mode Actually Does
Understanding what’s physically happening inside your system makes it easier to use this setting correctly.
How a Heat Pump Normally Heats Your Home
In heating mode, your outdoor unit runs the compressor to circulate refrigerant. The refrigerant absorbs heat from the outside air at the outdoor coil, gets compressed (which raises its temperature), then releases that heat at the indoor coil in your air handler. The blower pushes your home’s air across that warm coil and through your ductwork. Electric resistance strips may help — but the compressor does the heavy lifting.
What Happens When You Switch to Emergency Heat
When you switch to emergency heat, the thermostat locks out the outdoor unit. The compressor doesn’t run and the outdoor fan doesn’t spin. On an all-electric system, only the indoor blower and the electric resistance strips operate — your system is now working as a straight electric furnace.
That matters for two reasons. First, cost goes up sharply. Second, if the heat pump wasn’t actually broken and you’ve left it locked out for two weeks because you weren’t sure what was wrong, you’ve paid a lot on your electric bill for no reason.
On a dual-fuel system — a heat pump paired with a gas furnace — emergency heat typically runs the gas furnace instead. That’s a perfectly reasonable backup, and the cost comparison depends on gas and electric rates rather than on heat strips. Our comparison of heat pumps and gas furnaces below freezing covers how those systems split the work.
Emergency Heat vs. Auxiliary Heat: They Are Not the Same Thing
This is where most homeowners get confused. Auxiliary heat and emergency heat usually use the same electric resistance strips, but they turn on in completely different ways and for different reasons.
Auxiliary heat — the “AUX” display on your thermostat — comes on automatically. It engages when the heat pump alone can’t keep up: when outdoor temperatures drop low enough that the heat pump’s output falls, when the heat pump is in a defrost cycle, or when you raise the thermostat by more than a couple of degrees at once. The heat pump and the strips run together. Emergency heat is a manual override. You turn it on, and the heat pump shuts off. That distinction is critical for both cost and equipment health.
When You Should Actually Use Emergency Heat Mode
There are really only two legitimate reasons to switch to emergency heat. Outside of these two situations, leaving the system in normal heat — even if something seems slightly off — is almost always the better choice while you wait for a technician.
Your Heat Pump Compressor Has Failed
If the compressor has seized, burned out, or locked out, the outdoor unit may run but produce no useful heat. You might hear the outdoor fan spinning while the house stays cold, or the system may short-cycle. Continuing to call for normal heat just keeps trying to start a broken compressor, which can cause more electrical damage. Switching to emergency heat keeps you warm while you schedule the repair.
Signs that point toward a compressor or refrigerant-side failure rather than normal cold-weather behavior include:
- The outdoor unit running continuously with no improvement in indoor temperature
- A hard clicking, humming, or grinding sound from the outdoor unit that wasn’t there before
- The outdoor unit’s breaker tripping repeatedly
- An error code on the thermostat related to the outdoor unit
- Ice building up on the outdoor unit that doesn’t clear during defrost cycles
None of these alone is definitive proof of compressor failure — several have other causes — but in combination, they’re a strong signal to call for service and switch to emergency heat in the meantime.
The Outdoor Unit Is Physically Damaged
If a falling branch, ice, or impact has physically damaged the outdoor unit — crushed coil fins that block airflow, a damaged refrigerant line, or a broken fan blade — running the heat pump risks further damage to the refrigerant circuit or compressor. Switching to emergency heat while you wait for repairs is the right call.
Louisville’s winters are particularly hard on outdoor units because the weather doesn’t stay cold — it cycles. Temperatures drop below freezing, rise into the 40s and 50s, then drop again. That repeated freeze-thaw stresses coil connections and line joints over time, and meltwater that refreezes around the base can build up if drainage is blocked.
When You Should Not Switch It On
Cold weather alone is not a reason to use emergency heat. If your outdoor unit is running, going through defrost cycles, and the house is holding or slowly gaining temperature — even if it’s taking longer than usual — the system is working. Heat pumps lose capacity as temperatures drop, and on a 20°F night in Louisville, your system will run longer than it does in October. That’s normal. Switching to emergency heat in that situation replaces an efficient, working system with expensive resistance heat.
What Emergency Heat Mode Costs on Your Electric Bill
The cost difference comes down to one number: the coefficient of performance, or COP. A heat pump in typical Louisville winter conditions might deliver around 2 to 3 units of heat for every 1 unit of electricity it uses. An electric resistance strip delivers exactly 1 unit of heat for every 1 unit of electricity. That gap is where your money goes when you run emergency heat unnecessarily.
Louisville homeowners run their heat hardest from roughly late November through early March. Extended cold stretches — overnight lows in the teens for several days — are exactly when people feel the urge to switch to emergency heat, and exactly when running strips continuously produces the biggest jump on the bill.
Why Electric Heat Strips Are So Expensive to Run
A typical residential air handler contains electric resistance heating elements rated in kilowatts — commonly 5, 10, or 15 kW depending on the system. In emergency heat, those strips run at full draw whenever they’re called. A 10 kW strip running for 8 hours uses 80 kilowatt-hours of electricity. A heat pump running at a COP of 2.5 would deliver the same heat using roughly 32 kilowatt-hours. The strips used about 2.5 times as much electricity for the same result.
How Long You Run It Determines the Damage to Your Bill
Running emergency heat for a few hours while you wait for a technician won’t meaningfully change your monthly bill. Running it for four days because you weren’t sure whether to call is a different situation. The cost isn’t a fixed penalty — it scales with every hour the strips run in place of the heat pump.
How Auxiliary Heat Differs in Cost and Activation
Auxiliary heat uses the same strips as emergency heat on most systems, but the heat pump keeps running alongside them. The strips are supplementing the heat pump, not replacing it, so you’re still getting the heat pump’s efficiency on part of the load. Emergency heat gives that up entirely.
Auxiliary Heat Turns On Automatically
Your thermostat or heat pump control watches the gap between the indoor temperature and the setpoint. When that gap is large — commonly around two degrees or more — or when it’s cold enough that the heat pump alone can’t hold the setpoint, it brings on auxiliary heat. Seeing “AUX” on your thermostat on a cold night is not a malfunction. It means the system is doing what it was designed to do.
Tips to Reduce How Often Auxiliary Heat Kicks In
The single most effective thing you can do is avoid large setpoint jumps. When you raise the thermostat five or six degrees at once — coming home to a cold house and bumping it up — the control sees a big gap and immediately calls for auxiliary heat. Raise it gradually instead, a couple of degrees at a time, and give the heat pump time to catch up.
For the same reason, deep overnight or daytime setbacks tend to backfire with heat pumps: recovering from a large setback often runs the strips. Smart and heat-pump-aware thermostats that recover gradually, starting well before you wake up or get home, use far less auxiliary heat than one that gets a sudden five-degree jump.
Filter maintenance matters too. A clogged filter restricts airflow across the indoor coil, so the heat pump delivers less heat and the strips run longer. Check the filter monthly during heating season and replace it every one to three months depending on your household.
How to Manage Your Thermostat to Avoid Unnecessary Costs
Most of the cost problems with emergency and auxiliary heat trace back to thermostat habits. Louisville winters drop sharply overnight and often recover into the 40s or 50s during the day, so a steady, patient approach to the thermostat pays off.
1. Pick a Steady, Moderate Setpoint
A setpoint around 68°F keeps most properly sized heat pump systems in the Louisville area working in a range where they can hold temperature through most winter nights without leaning heavily on the strips. The higher you set it during a cold snap, the harder the system has to work and the more backup heat it uses. If you’re chilly, adding a layer is cheaper than raising the whole-house setpoint by four degrees at midnight.
2. Raise the Temperature a Couple of Degrees at a Time
Asking your heat pump to jump several degrees at once tells the control the heat pump can’t meet the demand alone, and the strips come on. If you raise the thermostat from 65°F to 71°F on a 25°F morning, the strips will likely run the entire time it takes to get there. Move the setpoint up two degrees, let the system reach it, then move it again if needed. Over a winter, that habit adds up on your LG&E bill.
3. Schedule Regular Heat Pump Maintenance
A heat pump with clean coils, the correct refrigerant charge, a working defrost control, and good airflow needs less help from the strips. During a maintenance visit, our technicians at Louisville HVAC Pro’s check refrigerant pressures, inspect the defrost control and sensors, test the reversing valve, and measure airflow. Catching a low charge or a faulty defrost sensor in the fall — before the hard freezes — is far less expensive than running degraded all winter and facing a compressor failure in February.
Use Emergency Heat Only as a Last Resort
Emergency heat exists for a specific purpose: keeping your home warm when the heat pump itself can’t run because of a mechanical failure or physical damage. It’s not a comfort upgrade for cold nights, not a fix for a system that seems slow to heat, and not a response to seeing “AUX” on the thermostat.
If you genuinely suspect your compressor has failed — the outdoor unit isn’t producing useful heat, you’re hearing abnormal sounds, or the breaker keeps tripping — switch to emergency heat to keep the house comfortable, then call Louisville HVAC Pro’s at (502) 553-8534. Running emergency heat for a day or two while a repair is scheduled is exactly the right use of the setting. Running it for two weeks because you weren’t sure what was wrong is what makes it expensive.
Frequently Asked Questions
These questions come up regularly when homeowners are trying to figure out what their thermostat is doing and whether they’re spending more than they need to on heat.
Is it safe to run emergency heat all night?
Yes. Electric resistance strips are a reliable heat source and will keep your home warm overnight. The concern isn’t safety — it’s cost. If your compressor has failed and you’re waiting on a repair, running emergency heat overnight is the right call. If you switched out of caution and the heat pump is actually working, switch back to normal heat and let the system run as designed.
Will my heat pump still work if I leave it on emergency heat?
The compressor and outdoor unit are locked out while emergency heat is on, so the heat pump won’t run. When you switch back to normal heat — assuming the heat pump wasn’t actually broken — it resumes normal operation. Leaving it on emergency heat doesn’t damage the heat pump; you’ve simply given up its efficiency for that period.
How do I know if my heat pump compressor has failed?
The clearest signs are an outdoor unit that runs without improving indoor temperature, a breaker that keeps tripping, new grinding, humming, or hard clicking from the outdoor unit, and outdoor-unit error codes on the thermostat. None is definitive on its own — a low refrigerant charge, a stuck reversing valve, or a failed defrost control can produce similar symptoms — but any of them warrants a service call. A technician can check refrigerant pressures and compressor amperage within minutes to see whether the compressor is actually pumping.
Does emergency heat use more electricity than auxiliary heat?
For the same heat output, yes. With auxiliary heat, the heat pump is still running and handling part of the load efficiently. Emergency heat locks the heat pump out, so all of the heat comes from resistance strips. If you see “AUX” on a cold night, that’s not a reason to switch to emergency heat — auxiliary mode is already the more efficient version of backup heat.
What does the “AUX” display on my thermostat mean?
“AUX” means auxiliary heat is on: the control has decided the heat pump alone can’t meet the current demand and brought on the backup heat to help. It happens automatically and doesn’t mean something is wrong. Common triggers in the Louisville area include outdoor temperatures in the low 20s or teens, defrost cycles, and raising the setpoint more than a couple of degrees at once. If “AUX” is on constantly — even in mild weather with no thermostat changes — that can point to a low refrigerant charge, a defrost problem, or a reversing valve issue, and it’s worth having checked before it leads to a bigger failure.