Why Your Heat Pump Switches to Aux Heat on Cold Lexington Mornings

If your thermostat shows AUX HEAT on a January morning in Lexington, that’s heat pump aux heat doing exactly what it was designed to do. If it shows AUX HEAT on a 48-degree afternoon in March, something is probably off. The difference matters for your comfort, and it matters even more for your Kentucky Utilities bill.

This article explains what auxiliary heat is, why Lexington’s winters trigger it so often, how it differs from EM HEAT, and when the pattern points to a repair.

What Heat Pump Aux Heat Actually Is

A heat pump heats your home by pulling heat out of outdoor air and moving it indoors. The U.S. Department of Energy notes that a properly installed air-source heat pump can deliver up to three times as much heat energy as the electricity it uses. That’s the efficient part.

Auxiliary heat is the backup. In most Lexington homes it’s a set of electric resistance heating elements, often called heat strips, inside the indoor air handler. They work like the coils in a toaster: simple, reliable and much more expensive to run than the heat pump itself. In a dual-fuel home, the backup is a gas furnace instead.

Your thermostat brings on aux heat automatically when the heat pump alone can’t keep up, or while the heat pump pauses to defrost.

Why Lexington Mornings Trigger It

Lexington sits in the temperature band where heat pumps work hardest. National Weather Service normals for Blue Grass Airport show about 90 nights a year at or below freezing, including roughly 23 in January and 19 each in December and February. The average January low is 25.4°F.

On those mornings, three things combine:

• Less heat to collect. The colder the outdoor air, the less heat the heat pump can pull from it, while the house is losing heat faster.
• Defrost cycles. Frost forms on the outdoor coil in cold, damp air. The system reverses briefly to melt it, and aux heat runs so you don’t get a blast of cool air.
• Setback recovery. If the thermostat drops the temperature overnight and raises it at 6 a.m., many thermostats call for aux heat to close the gap quickly.

Aux Heat Versus EM HEAT

EM HEAT, the backup-only setting on many thermostats, shuts the heat pump off entirely. Use it only as a stopgap, for example when the outdoor unit is iced solid or damaged, and switch back to normal HEAT once it’s repaired.

What It Does to Your Bill

Heat strips draw a lot of power. A heat pump moving heat at three times the efficiency of resistance heat costs far less per hour than strips producing the same warmth. That’s why a month of heavy aux heat use shows up quickly on a KU bill, even though the house felt fine.

If your winter bills jumped without a cold snap to explain it, aux heat running too often is one of the first things to suspect.

Signs Heat Pump Aux Heat Is Running Too Much

• AUX HEAT stays on for hours when it’s in the 40s outside.
• The outdoor unit isn’t running, or the fan runs but the air from the vents is barely warm, while AUX shows.
• The outdoor unit is covered in a thick block of ice that doesn’t clear.
• Your bill is sharply higher than the same month last year with similar weather.
• A new smart thermostat was installed recently, and the pattern started after that.

The Usual Causes

Low refrigerant. A leak leaves the heat pump with less capacity, so the thermostat leans on backup heat to make up the difference. Refrigerant isn’t used up in normal operation, so low charge means a leak somewhere.

Defrost problems. If the defrost control or its sensor fails, the coil ices over and the heat pump loses most of its output.

Thermostat configuration. Smart thermostats installed on heat pumps need the right settings for the reversing valve, the backup heat stage and, ideally, an outdoor temperature lockout. Wrong settings can call for strips far more than necessary.

An undersized or aging heat pump. Older systems, including many installed in Lexington’s 2000s subdivisions, lose capacity with age. A system that was marginal when new may now rely on strips every cold morning.

Airflow problems. A clogged filter or dirty indoor coil reduces the heat the system can deliver, which can lengthen run times and bring on backup heat.

What You Can Do

1. Replace the indoor filter if it’s dirty.
2. Check that the outdoor unit has clear space around it and isn’t buried in snow or leaves.
3. Look at the thermostat settings. Avoid deep overnight setbacks on a heat pump unless your thermostat is designed to recover without heavy strip use.
4. Watch the outdoor unit for a few minutes while AUX is displayed. Is the fan running? Is there steam (defrost) or a solid block of ice?
5. Compare this month’s KU usage with the same month last year.

If the outdoor unit isn’t running, is iced solid, or the pattern continues on mild days, it’s a sign the system needs a proper diagnosis by a technician, ideally one who confirms the cause with readings before replacing parts.

When Replacement Comes Up

If your heat pump is 15 or more years old, uses R-22 refrigerant and needs a major repair, the conversation may turn to replacement. A properly sized two-stage or variable-speed heat pump typically uses backup heat far less on Lexington’s 20-something-degree mornings. Sizing and ductwork matter as much as the equipment, so a load calculation and a duct check should come before any decision.

Common Questions