Emergency Heat vs Auxiliary Heat

The short answer first

Both labels describe backup heat, but they get started differently. AUX switches on by itself when the heat pump needs help.

EM HEAT is a setting you choose.

With auxiliary heat running, the heat pump usually keeps working while the backup adds capacity. With emergency heat, most systems lock out the outdoor unit entirely and let the backup carry the house alone.

Day to day, leave the thermostat in normal Heat mode and let the controls decide when backup is needed. Save EM HEAT for a heat pump that has stopped working or fallen behind, while you get service arranged.

• AUX is automatic help for the heat pump.
• EM HEAT is a manual setting that may lock out the outdoor unit.
• Normal Heat mode is the everyday choice.
• Emergency heat is a stopgap while you arrange diagnosis.

What auxiliary heat does

In normal Heat mode the heat pump is the main heater. Auxiliary heat, the built-in backup, comes on when the heat pump cannot close the gap on its own.

AUX may appear in very cold weather, during a big thermostat recovery, or while the outdoor unit runs a defrost cycle. A stretch of backup running can simply be part of a normal heating day, not evidence that something broke.

The trigger varies house to house. Controls may watch a balance point, a lockout temperature, an upstage timer, or the gap between room temperature and the set point.

Dual-fuel systems use a furnace as the backup instead of electric resistance heat, so no single outdoor temperature proves when AUX should run.

• The controls bring on backup automatically when they see the need.
• Cold weather, recovery, and defrost can all summon it.
• The exact trigger depends on the equipment and thermostat.
• A brief AUX message is not a failure.

What emergency heat does

Emergency heat, usually shown as EM HEAT or E HEAT, is chosen by you. On most heat pumps it locks out the outdoor unit and runs the backup alone.

It exists for the bad nights: the outdoor unit has an obvious problem, the heat pump has quit, or the house is losing ground. Emergency heat holds temperature while help is on the way.

It is not a way to make a healthy heat pump run hotter. Once the heat pump is fixed, put the thermostat back in normal Heat mode.

• EM HEAT is a manual setting.
• It often runs the backup with the outdoor unit locked out.
• Use it temporarily when the heat pump is not doing its job.
• Return to normal Heat mode after the repair.

When to use each setting

Normal Heat mode covers nearly every winter day. The heat pump leads, and the controls add auxiliary heat when the house needs it.

Cold weather alone is not a reason to switch to emergency heat. Normal mode already lets the system use backup without you locking out the outdoor unit by hand.

Reach for EM HEAT when the outdoor unit is clearly down, the house is not warming, or a serious outdoor problem makes temporary backup the practical call. Then arrange diagnosis rather than leaving the manual setting on until spring.

• Normal Heat is the everyday setting.
• A cold night or a brief AUX message does not call for EM HEAT.
• EM HEAT is for a heat pump that is not delivering heat.
• Get it repaired, then switch back.

Why is AUX heat running so often?

Start with the pattern, not a guessed part. AUX can appear at a cold startup, after a big thermostat change, or during a defrost.

A recovery-minded control may call for backup simply because the room is warming slower than the thermostat wants.

Frequent AUX deserves a closer look when it shows up in mild weather, runs for long stretches, or arrives alongside weak airflow, cool supply air, strange sounds, ice that never clears, or a set point the house cannot reach.

The possible causes run from a dirty filter or restricted airflow to a control or sensor fault, low refrigerant, a thermostat set up wrong, or equipment that needs service. The label on the display cannot name the part.

Before calling, write down the mode, the set point, the room temperature, the weather, how long AUX lasted, whether the outdoor unit ran, and what the supply air felt like.

• AUX during cold recoveries or defrost can be normal.
• AUX in mild weather or for hours at a stretch needs diagnosis.
• Weak airflow, ice, odd sounds, and missed set points add context.
• Write down what the system did before changing anything.

Does emergency heat cost more?

Usually, yes. How much more depends on the backup source, the equipment, the controls, and your rates.

Electric resistance heat generally costs more to run than heat-pump heating, because a heat pump moves heat that already exists while resistance elements manufacture it from electricity.

The bill climbs because emergency heat may run the backup without the heat pump at all. That does not make it the wrong choice when the main system is down.

Keeping a house from getting colder can matter more than avoiding every minute of backup.

When the heat pump works, stay in normal Heat mode. Skip large sudden set-point jumps if recoveries keep dragging AUX on, and replace a visibly loaded filter on the equipment's schedule.

Leave hidden lockout and staging settings alone unless you know the design.

• Backup heat usually costs more to run than the heat pump.
• Emergency heat can cost more because the heat pump is locked out.
• Use the manual setting for the failure at hand, not for the season.
• The right control settings depend on the equipment and its balance point.

What you can safely check at home

Keep your checks on the observation side of the line. Confirm the thermostat is set to Heat, watch whether the room temperature is actually rising, and check the filter if the equipment instructions put replacement in your hands.

Make sure supply registers are open and furniture is not parked on them. From a distance, look for snow, leaves, or ice smothering the outdoor unit.

Note whether the fan or compressor seems to run, without opening panels or touching anything electrical.

Do not pull thermostat wires, move wires between terminals, open the outdoor unit, handle refrigerant, keep resetting a tripping breaker, or work on gas or electrical parts. And if EM HEAT is missing from your thermostat, do not improvise: some systems simply do not have that function, and some need a professional to configure it.

• Confirm the mode and watch whether the room warms.
• Check the filter, registers, returns, and anything piled on the outdoor unit.
• Watch the outdoor unit from a distance, panels closed.
• Wiring, refrigerant, gas, and electrical work belong to a professional.

When to call for heat-pump repair in Frederick

Schedule diagnosis when AUX runs constantly, when EM HEAT is the only setting that produces warm air, when the house misses the set point, when the outdoor unit will not run, or when the system bounces between heating and backup without ever settling.

Any of those patterns can come from airflow, controls, sensors, refrigerant, electrical parts, or sizing. The thermostat display cannot settle it; a technician with meters can.

Tell us which mode was active, how often AUX appeared, whether the outdoor unit ran, whether there was ice, what the supply air felt like, and what the bill did. That timeline separates a setting question from a heat-pump fault.

• Call when AUX runs constantly or shows up in mild weather.
• Call when EM HEAT is the only way to get warm air.
• Call when the outdoor unit quits or the set point is never reached.
• Bring observations; finding the part is our job.

When the thermostat question is a safety question

A carbon monoxide alarm, gas smell, smoke, or a strong burning electrical smell is not a settings question. Leave the house and call emergency services or the utility's emergency line from outside.

Do not heat the house with a gas oven, grill, or portable fuel-burning appliance. Do not keep resetting a breaker that trips on emergency heat.

For a plain heat-pump problem with no safety signal, write down what the thermostat and equipment are doing and arrange service.

The memory device is short: AUX is automatic, EM HEAT is manual. If the heat pump works, let the normal controls run the show.

If it does not, use the temporary backup your system gives you and get the cause checked.

• Gas smell or CO alarm: leave and call from outside.
• Smoke or a burning electrical smell: stop and shut down.
• Never use fuel-burning appliances as space heaters.
• Never keep resetting a breaker that trips on emergency heat.