TL;DR: Auxiliary heat is the electric heat strip backup built into most heat pumps, and it kicks in automatically during cold snaps or rapid temperature jumps. In Central Florida it should run rarely, so if AUX is showing up often on your thermostat in Tavares, Clarcona, Astor, Paisley, Grand Island, or Zellwood, it usually points to a heat pump that needs service rather than normal winter weather.
If you’ve ever glanced at your thermostat on a cold January morning and seen the letters AUX light up, you’ve probably wondered what it means — and whether it’s costing you money. For homeowners around Tavares, Clarcona, Astor, Paisley, Grand Island, and Zellwood, understanding auxiliary heat can help you spot a real problem before it shows up as a sky-high electric bill.
Here’s a straightforward explainer on what auxiliary heat is, how it works, and why it matters more in Central Florida than most homeowners realize.
What Is Auxiliary Heat?
Auxiliary heat, often labeled AUX on thermostats, is a secondary heating source built into a heat pump system.
It automatically activates when outdoor temperatures drop below freezing or when the heat pump cannot efficiently reach your thermostat’s set temperature alone.
In most residential systems, this backup takes the form of electric resistance coils, often called heat strips, installed inside the air handler.
Auxiliary heat refers to the electric heat strips inside your heat pump system air handler, and when the heat pump can’t satisfy your thermostat setting on its own, it will activate these heat strips to ensure your home stays warm.
The heat strips warm up like a toaster when electricity is applied to them, and the warmth transfers to the air flowing through the air handler.
How Auxiliary Heat Actually Works
A heat pump doesn’t generate heat the way a furnace does.
In the winter, it extracts heat from the outside air, even in cold temperatures, and transfers it indoors.
This process is very efficient until the outside air gets too cold for the heat pump to work effectively on its own.
That’s where the backup comes in.
Auxiliary heat is a supplemental heating system that provides additional heat when the primary heat pump system cannot maintain or achieve the thermostat’s setting due to low outdoor temperatures, working with the heat pump to warm the home’s air.
Your thermostat is the brain of the operation:
it signals either the heat pump or the backup system to provide heat for your home, and the majority of the time the heat pump is the primary source, with the thermostat choosing the most energy-efficient heat source at any given time.
The Two Triggers for Auxiliary Heat
- Cold outdoor temperatures below the system’s “balance point.”
At some temperature, the heat pump’s capacity is just equal to the home’s heating load — a point called the balance point. Below that, the load is bigger than the capacity and you need some type of supplementary heat. - A big, fast jump in the thermostat setting.
Auxiliary heat helps raise the indoor temperature quickly if you increase your thermostat setting by more than 2-3 degrees at once.
Auxiliary heat also assists briefly during the defrost cycle.
It assists during the defrost cycle to prevent cold air from blowing out of your vents while the outdoor unit de-ices itself.
Auxiliary Heat vs. Emergency Heat
These two terms get confused constantly, but they’re not the same thing.
Auxiliary heat (AUX) is automatic — it runs alongside your heat pump to boost heating power, and this is normal operation in cold weather.
Emergency heat, by contrast, is a manual setting you select when the heat pump itself has failed and the electric strips are doing all the work alone — something that should prompt a service call, not routine winter use.
Why This Matters More Than You’d Think in Lake County
Central Florida doesn’t get the brutal winters that make auxiliary heat a daily necessity up north. Local climate data shows just how mild our cold season really is:
January has the coldest nighttime temperatures for Tavares, with an average of 45.6°F, and there are only about 6.2 days annually when the nighttime low falls below freezing.
That’s important context. Industry guidance notes that
auxiliary heat lockout should typically be set to 35°F or lower
, and that
if a system isn’t sized correctly, you run into comfort issues when strip heat is locked out, since the balance point can be no greater than 30°F
on a properly matched system. In other words, a correctly sized heat pump in Tavares, Clarcona, Astor, Paisley, Grand Island, or Zellwood should rarely dip below freezing often enough to need much strip heat at all.
So if your thermostat shows AUX running for hours at a time, or every single morning, that’s usually not “just winter” here — it’s a sign something else is going on.
The Pros and Cons of Auxiliary Heat
Pros
- It keeps your home warm during the rare hard freeze or cold snap that a heat pump alone can’t handle.
- It responds instantly, which matters if you’ve bumped the thermostat up several degrees at once after returning from a trip.
- It protects the system during defrost cycles so you don’t feel a blast of cold air.
Cons
- It’s expensive to run.
A hidden factor that can drive up winter power bills is strip heating, which might show up on your thermostat as Auxiliary Heat or Emergency Heat, and on your bill it shows up as expensive, potentially increasing heating costs by hundreds of dollars. - Electric resistance heat is far less efficient than the heat pump’s normal compressor-driven operation.
Heat strips convert electricity directly into heat with nearly 100% efficiency at the point of use, but because they do not move heat like a heat pump, they use more electricity to deliver the same comfort. - Frequent or constant AUX activation in a mild climate like ours usually signals an underlying problem, not normal operation.
When Auxiliary Heat Signals a Problem (Not Just Cold Weather)
In Tavares and the surrounding Lake County communities, auxiliary heat should be the exception, not the rule. If you’re seeing AUX on frequently, consider these common causes:
- Low refrigerant charge. A heat pump that’s lost refrigerant can’t move heat efficiently and will lean on the strips to compensate.
- A failing or miswired thermostat. Incorrect staging settings can cause auxiliary heat to engage too early or stay locked on.
- An undersized or oversized system for your home’s actual heating and cooling load.
- A dirty filter or blocked airflow that’s forcing the system to work harder than it should.
- An aging compressor that can no longer keep up with demand, even on a mild Florida morning.
This is closely related to another issue we cover in our guide on why your AC might be blowing warm air — many of the same root causes (refrigerant, airflow, electrical) affect both cooling and heating performance on a heat pump.
Heat Pumps Make Sense in Florida — With the Right Setup
Despite the auxiliary heat conversation, heat pumps remain one of the smartest heating and cooling choices for our climate because they do double duty.
Heat pumps are helpful year-round because they provide cooling in the summer as well as heating in the winter.
The key is proper sizing, charge, and thermostat configuration from the start — which is exactly why we walk homeowners through our heat pump vs. straight cool comparison before any installation.
Efficiency also plays a role in how often auxiliary heat kicks in. A higher-efficiency system holds its heating capacity longer as temperatures drop, which is part of why we walk customers through what SEER2 ratings mean for their electric bill when comparing equipment options.
What to Do If Auxiliary Heat Runs Too Often
- Check your thermostat settings first. Avoid large, sudden temperature jumps, which can trigger unnecessary AUX activation.
- Check or replace your air filter. Restricted airflow is one of the simplest, most common culprits.
- Schedule a professional tune-up. A trained technician can check refrigerant charge, electrical connections, and thermostat staging to confirm everything is calibrated correctly for our climate.
- Ask about your system’s balance point. If your heat pump is undersized for your home, auxiliary heat may be compensating for a mismatch that a proper load calculation would catch.
A precision tune-up is often the fastest way to find out whether auxiliary heat is doing its job occasionally, as designed, or constantly, as a symptom. Keeping up with seasonal maintenance through a Preventative Maintenance Agreement also helps catch these issues before a cold snap exposes them. You can check current offers on our seasonal specials page.
The Bottom Line for Lake and Orange County Homeowners
Auxiliary heat is a normal, built-in safety net for heat pumps everywhere — but in Tavares, Clarcona, Astor, Paisley, Grand Island, and Zellwood, our mild winters mean it should only show up occasionally. If AUX is running often, it’s worth a professional look rather than just accepting a higher power bill as the cost of a cold morning.
Recommended Reads
- Landing Page for Seasonal Specials – Online Booking – Lucas Air Conditioning and Heating
- Landing Page for Seasonal Specials – Lucas Air Conditioning and Heating
- Why Is My AC Blowing Warm Air? 6 Common Causes Explained
