Aux heat vs emergency heat: what your Florida heat pump is telling you

The call usually comes in the second week of December: "My thermostat keeps saying Aux Heat, and the power bill doubled." Or the opposite: "It got cold, so I switched it to Emergency Heat to be safe." Both settings run the same electric strips in your air handler. They are not the same thing, and mixing them up is one of the more expensive mistakes a Florida homeowner can make with a heat pump.

Aux heat is automatic, and a little of it is normal

Auxiliary heat is the thermostat bringing the electric heat strips in to help the heat pump. It does that on its own, in three situations:

• During a defrost cycle. On a cold, damp morning the outdoor coil frosts up and the unit reverses for a few minutes to melt it. The strips come on so the air at your vents does not go cold while that happens.
• When you raise the setpoint by more than two or three degrees at once. Most thermostats read a big gap as "the heat pump will take too long" and add the strips to close it.
• When it is too cold outside for the heat pump to keep up alone. In Orlando, that is a few hours a year.

Seeing Aux light up for five or ten minutes at a time is the system doing its job. Seeing it lit for hours is not.

Emergency heat is a manual override, and you will rarely need it here

Emergency Heat (Em Heat on most thermostats) shuts the outdoor unit off completely and heats the house with the strips alone. It exists for one situation: the heat pump outside is broken. Maybe it is iced solid, a limb came down on it, or it is making a noise you do not like. Em Heat keeps the house warm until it is repaired.

It is not a "stronger" heat setting. In Aux mode the heat pump and the strips run together. In Em Heat only the strips run, so you get less total heat, at the highest cost per degree the system can produce. If you switched to Em Heat during last winter's cold snap, check that it is not still there.

Why all-day Aux heat is a red flag in Florida

This part is specific to Florida. Heat pumps here are sized for the cooling load, and an Orlando cooling load is large. A heat pump's heating output at 47°F is roughly equal to its rated cooling capacity. Meanwhile, a house loses far less heat on a 45-degree night than it gains on a 95-degree afternoon. So a heat pump that cools your house properly in August has heat to spare on almost any Orlando winter night.

A heat pump that is working should carry the house on its own down into the mid-30s, with the strips stepping in only for defrost. If Aux is on for hours when it is 50°F outside, the heat pump is not pulling its weight, and the thermostat is quietly covering for it with the most expensive heat in the house.

What the difference costs

At 15¢ per kWh, rounded. Your rate and equipment will differ. The figure that matters is per unit of heat delivered: strips cost about three times as much as a working heat pump. We break the numbers down further in what it costs to heat an Orlando home.

What makes Aux run when it should not

Here is what we usually find, starting with the most common:

1. 1The thermostat is on Em Heat. Someone switched it during a cold snap and nobody switched it back.
2. 2Big temperature swings. If you turn the heat off at night and back up to 72°F at 6am, the strips do the morning recovery. See the best thermostat setting for a heat pump.
3. 3The outdoor unit is not running at all. A failed capacitor or contactor stops the compressor, and the thermostat cannot tell. It only knows the house is not warming. In Central Florida, a contactor packed with fire ants is a regular find in late autumn.
4. 4Low refrigerant charge. The heat pump runs but only produces lukewarm air, so the strips keep getting called in.
5. 5A defrost fault. The unit is stuck in defrost, or defrosting constantly. See ice on your heat pump.
6. 6A thermostat set up wrong. This is common after a DIY smart-thermostat swap, where staging, the system type or the O/B setting were left at the factory defaults.
7. 7A clogged filter. Restricted airflow drops the heat pump's output enough that the thermostat gives up waiting.

A three-minute check you can do tonight

1. 1Set the thermostat to Heat, not Em Heat, and raise it two degrees.
2. 2Within five minutes, go outside. The outdoor fan should be spinning and the compressor humming. The air coming off the top of the unit should feel cooler than the outdoor air, because the unit is pulling heat out of it.
3. 3Back inside, after ten minutes the air at a supply register should be around 90–100°F. That is warm, not hot.

If the outdoor unit is not running, switch to Em Heat to stay warm and call us. That is exactly the situation Em Heat is for. The heating diagnostic is $89 and credited to the repair, and the first cold week of the year is when it pays to call early. Every company in Orlando gets busy at once. More on what we test is on our heating repair page.