Heat Pump Defrost Mode: What It Is and What Is Normal

Why frost forms on the outdoor coil

A heat pump heats by pulling heat out of outdoor air and moving it inside. Refrigerant is the fluid that carries that heat, and the outdoor coil is the bank of metal tubes and thin fins where the refrigerant meets the outdoor air. In heating mode, Trane says the outdoor coil is typically 10-20°F colder than the outdoor air. Trane compares the coil to a glass of ice water that collects condensation, and says the condensation freezes when the outdoor temperature is low enough.

HVAC School, a trade-training publisher, says a light layer of frost is standard and isn't a cause for concern. The trouble starts when frost thickens. Lennox says frost buildup acts like insulation, so the coil is blocked and the system works harder for less output. That is the reason a heat pump melts the frost off every so often instead of letting it build.

How the reversing valve melts the ice

The reversing valve is the part that switches the direction of refrigerant flow, which is what lets one machine both heat and cool. Trane says that to melt the ice, the reversing valve reverses the flow of refrigerant to heat up the outdoor coil, essentially putting the heat pump into the cooling cycle for a few minutes. Once the ice is melted, Trane says the heat pump reenters heating mode.

Lennox says sensors and controls watch the outdoor coil's temperature and operating conditions, and the system starts the cycle when frost reaches a certain threshold. HVAC School adds that the outdoor fan stops during defrost, which helps the coil warm up faster. Our reading: because the outdoor coil briefly becomes the warm side, the system is not delivering heat to the house for those minutes.

What you may see, hear and feel

Outside, Trane says you may see a cloud of steam, which it describes as the ice melting and evaporating. Trane also says you may hear a swooshing sound as the refrigerant reverses course, the outdoor fan may stop, and the compressor noise may rise or fall. The compressor is the pump that moves refrigerant through the system.

Inside, Trane says many homeowners will not know anything is happening, while some may notice cool air from the vents instead of warm air. Lennox says many heat pumps switch on backup heat strips during defrost to prevent temperature swings. Trane says you may see AUX heat on the thermostat during defrost, which tells you the electric heat strips are keeping the home warm. The backup strips themselves are explained on the AUX HEAT page, and the manual EM HEAT setting has its own EM HEAT page, so this page does not repeat either.

The melted frost becomes water. ENERGY STAR's guidance on ductless heat pumps says outdoor units release water when they defrost in the winter, which can form icy patches.

How often and how long defrost lasts

The makers describe two ways a heat pump decides to defrost. Timed defrost, Trane says, starts after so many run hours whether the system needs it or not, and lasts a set period. Demand defrost uses sensors, so the system defrosts only when needed and ends the cycle as soon as the ice is clear. Trane says its own cycles are usually 3-5 minutes.

Other sources give longer figures. Lennox says most systems check for frost every 30 to 90 minutes in heating mode. Lennox says a typical cycle runs 5 to 15 minutes. Lennox says on dry or sunny days the cycle may run rarely or not at all. Our reading: the figures differ by maker and model, so the owner's manual is the better guide to what is normal for your unit.

Normal, or a reason to call

The sources agree on what is expected: a light frost, a cloud of steam, a pause in warm air, a stopped fan and a brief change in sound. They differ on when to pick up the phone, and that is worth knowing before you call. HVAC School asks homeowners not to call unless the unit stops heating completely or the coil is covered in a sheet of ice. Trane says to call a local HVAC dealer if the heat pump seems to be constantly switching into defrost mode. Trane gives a second sign: defrost cycles that last longer and longer, with frost and ice starting to cover the unit.

Lennox says possible causes of excessive defrost include dirty or blocked coils, faulty sensors or controls, and low refrigerant charge or other mechanical issues. Our reading: from the yard you cannot tell which of those it is, so ice that stays or defrost that keeps repeating is a question for a technician, not a guess. Lennox says a trained technician can test sensors, verify refrigerant levels and make sure the unit is operating as designed.

Lennox says homeowners can handle some basic checks, such as clearing leaves, snow or debris from around the outdoor unit. Lennox also advises keeping at least two feet of clearance around the unit. Our reading: do not chip or pry ice off the coil, because the fins and tubing are thin and easy to damage. Frost on the indoor coil is a different problem from outdoor defrost, and air conditioner freezing up covers it.

Florida's few cold mornings

Our reading: defrost needs cold, damp air, which Tampa Bay has on only some winter mornings. Trane says a heat pump could easily enter defrost when it is in the low 40s outside, though it is more common when the temperature is in the 20s or 30s. Lennox says some owners are surprised to see defrost in the 40s, because the coil surface can be much colder than the air around it. Our reading: a Tampa Bay owner may see steam and a pause on a cool, damp morning even when the air is above freezing, and may not see it again for weeks.

A heat pump that seems to defrost on warm days, or again and again, is worth raising with a technician. For the wider picture of heat pumps in this climate, see the Florida heat pump guide, and for a heater that seems weak in a cold snap, see the cold-snap heater guide.

Questions to ask a technician

Ask your heat pump's installer or service company these questions, and ask for the answers in writing where you can. Does this heat pump use timed defrost or demand defrost, and where is its defrost sensor? What does the owner's manual give as a normal defrost length and how often it should start? When was airflow around the outdoor unit last checked, and is anything blocking it? Does my thermostat show backup heat during defrost, and how will I know if the backup heat runs longer than it should?