Heat Pump Defrost Mode and Aux Heat: What's Normal in a Wichita Winter

The first truly cold, damp mornings of a Wichita winter bring a familiar round of phone calls from heat pump owners. The outdoor unit is puffing out a cloud of steam. The fan stopped and the unit made a loud whoosh. The thermostat says "AUX HEAT" or "EM HEAT" and the homeowner is worried the system is about to quit. Most of the time, none of this is a problem. It is the heat pump doing exactly what it was designed to do in cold weather.

This guide explains what defrost mode and auxiliary heat are, what normal looks and sounds like, and the specific signs that mean it is time to call a technician.

Why a Heat Pump Needs to Defrost

In heating mode, a heat pump pulls heat out of the outdoor air and moves it inside. To do that, the outdoor coil runs colder than the air around it. When the outdoor temperature drops to roughly freezing and the air carries moisture (fog, drizzle, light snow, or a humid Kansas night), that moisture freezes onto the coil as frost.

A thin layer of frost is normal. A thick layer is a problem, because ice blocks airflow across the coil and the heat pump can no longer pull heat from outside. So the system periodically runs a defrost cycle to clear it.

What happens during a defrost cycle

During defrost, the heat pump briefly reverses itself, running much like it does in cooling mode, so warm refrigerant flows through the outdoor coil and melts the frost. While that is happening:

• The outdoor fan usually stops so the heat stays on the coil.
• You may hear a whoosh or a clunk when the reversing valve shifts at the start and end of the cycle.
• Steam or vapor rises off the outdoor unit as the frost melts. This is water vapor, not smoke.
• Water drips or runs from the base of the unit.
• Indoors, many systems switch on backup heat for a few minutes so you do not feel a blast of cool air from the vents.

A defrost cycle typically lasts only a few minutes and ends on its own once the coil is clear or a time limit is reached. How often it runs depends on the weather. On a cold, wet day it may happen fairly often; on a cold but dry, sunny day it may barely happen at all. Your system's defrost control decides when it is needed.

Auxiliary Heat vs. Emergency Heat

Most heat pumps in the Wichita area are paired with a backup heat source, either electric heat strips in the air handler or a gas furnace (often called a dual-fuel system). Your thermostat may label the backup as "AUX" or "EM." They use the same equipment, but they mean different things.

Auxiliary heat (AUX)

Auxiliary heat turns on automatically when the heat pump needs help. Common reasons include:

Defrost. As described above, backup heat often runs briefly while the outdoor coil is defrosting.

A big temperature jump. If you raise the thermostat several degrees at once, many thermostats bring on auxiliary heat to close the gap faster.

Very cold weather. As outdoor temperatures fall, a heat pump's heating capacity drops while your home's heat loss rises. At some point, often called the balance point, the heat pump alone cannot keep up and backup heat fills in. Where that point falls depends on the equipment, the home's insulation, and how the system was set up.

Seeing "AUX" on the thermostat now and then during a cold snap is normal. Seeing it constantly on a mild 45 degree afternoon is not.

Emergency heat (EM)

Emergency heat is a setting you choose manually. It shuts the heat pump off completely and heats the house with the backup source only. It is meant for true emergencies, such as an outdoor unit that has stopped working or is encased in ice, while you wait for service.

With electric heat strips, running on emergency heat for days can raise your electric bill noticeably, because resistance heat uses far more electricity than the heat pump for the same amount of warmth. Use it as a temporary bridge, not a winter setting.

Tips to Keep Your Heat Pump Running Efficiently This Winter

Make temperature changes gradually. If you use a setback schedule, ask whether your thermostat has a heat pump or "smart recovery" setting. Large, sudden setpoint increases are one of the most common reasons auxiliary heat runs more than it should.

Keep the outdoor unit clear. Leave a couple of feet of clear space around it. Keep leaves, snow drifts, and landscaping away, and make sure meltwater can drain away from the base.

Watch the roofline. Water dripping from a gutter or roof edge onto the top of the unit can freeze into a solid cap of ice that defrost cannot clear. Fixing the gutter is usually the cure.

Change the indoor filter. A clogged filter reduces airflow across the indoor coil, which makes the whole system work harder and can trigger safety limits.

Do not chip ice off the coil. The aluminum fins and refrigerant tubing are easy to damage. If ice is not clearing, switch to emergency heat and call for service.

When Defrost or Aux Heat Means Something Is Wrong

Call a technician if you notice any of the following:

The outdoor coil stays covered in ice. A unit that is solid white or encased in ice for hours, even after defrost cycles, may have a failed defrost sensor, defrost control board, reversing valve, or low refrigerant.

Defrost seems to run constantly, or the unit never seems to defrost at all.

Auxiliary heat runs nearly all the time in moderate weather, or your electric bill jumps well beyond what the cold alone would explain.

The outdoor fan is not spinning outside of defrost, or the unit is making grinding, screeching, or loud banging noises.

Cool air blows from the vents for long stretches, not just a moment during defrost.

Many of these problems start small. A worn defrost sensor or a slow refrigerant leak can go unnoticed until the first hard freeze exposes it, which is exactly when everyone else is calling for heat, too.

Get Ahead of Winter with Midwest Mechanical

A heating tune-up is the best time to catch defrost and backup heat problems before they leave you cold. Our technicians check refrigerant performance, the defrost controls and sensors, the reversing valve, outdoor fan operation, backup heat strips or furnace operation, and your thermostat's heat pump settings.

Our Club Membership makes that easy. For $180 per year per system, members get a spring AC tune-up and a fall furnace tune-up, priority service, and discounts on repairs.

If your heat pump is iced over, running on auxiliary heat around the clock, or just not keeping up, call Midwest Mechanical at (316) 226-8859 or schedule online. We will get your system back to keeping your Wichita home warm.