Heat Pump Compressor / Condenser Fan Defrost Cycle

Outdoor AC unit Defrost Procedure

Why and when a heat pump goes into a defrost cycle when in heating mode

It is normal for the outdoor fan to run while the outdoor unit compressor is running AND the system is in heating mode, not in defrost mode.

When your heat pump is in heat mode, the outdoor unit (compressor condenser) is extracting heat from (even chilly) outdoor air.

The outdoor unit fan runs, air moves across the outdoor coil, heat is absorbed from outdoor air into the coil and into the refrigerant inside the coil where ultimately that heat is delivered indoors through the indoor air handler.

Why does ice form on the outdoor coil?

In heating mode, as heat is extracted from the outdoor coil, its surface becomes colder than the outdoor air temperature and can drop below freezing. Then moisture in outdoor air freezes on the coil surface.

Why is a heat pump defrost cycle needed?

Ice on the outdoor coil blocks air flow across the coil; with no air flow, no heat can be extracted from the outdoor air and worse, the compressor motor itself could be damaged.

So periodically the outdoor unit will go through a defrost cycle. A control board in the outdoor unit is the control over when a defrost cycle occurs.

Just how often a defrost cycle will occur is determined either by a timer on the outdoor unit’s control board (when in heating mode) or by sensors that some heat pumps may use to detect the presence of ice on the coil.

Depending on your heat pump brand and model - thus the heat pump's design - the outdoor unit in heating mode will run a defrost cycle as often as once every 30-35 minutes OR (on some more sophisticated designs), the heat pump may only run a defrost cycle if sensors tell it that the outdoor coil is iced over. 

Heat Pump defrost cycle timer

Typically on the defrost cycle control circuit board, the installer can set a jumper to control the defrost cycle at 30, 60, or 90 minute intervals once the defrost temperature sensor on the outdoor unit senses that outdoor temperature has fallen close to 31°F.

The timer starts when two events have occurred:

1. The outdoor temperature has fallen to about 31°F as reported by a temperature sensor in the outdoor unit (OD unit)
2. The indoor room thermostat is calling for heat

The defrost cycle is initiated at the end of the defrost timer cycle.

The defrost cycle ends when the temperature sensor at the outdoor coil senses the temperature of the coil at about 75 °F or when the circuit board's timer overrides the sensor.

Other more sophisticated defrost control circuit boards monitor other operating details such as refrigerant pressure in order to avoid wasting the energy of a defrost cycle if it's not actually necessary. Fewer defrost cycles - eliminating unnecessary defrost cycles - means a more efficient heat pump system and some energy savings.

Factors that affect the frequency of a heat pump (in heating mode) outdoor defrost cycle include:

• The heating load - amount of heat being delivered indoors
• The compressor and control design (timer versus ice sensors)
• Outdoor temperature (closer to freezing, more frequent defrosting needed). At about 30°F you will usually begin to see ice forming on the outdoor coil or heat exchanger
• Outdoor humidity (more humidity, faster ice formation, more frequent defrost needed)
• On many heat pumps, a defrost cycle control board in the outdoor unit uses a timer and other information to initiate a defrost cycle.
  
  Some heat pumps use only a timer; others use a timer combined with a temperature sensor or thermostat that monitors temperature at the outdoor coil
• On some heat pumps, an air flow sensor detects air flow pressure change across the coil in order to detect icing or frosting of the coil (on some units)

What happens at the outdoor heat pump unit during the heat pump defrost cycle?

Usual design: a control circuit board in the outdoor compressor/condenser unit controls the defrost cycle timing and length.

The heat pump switches from "heating mode" back to "cooling mode" in order to warm the outdoor coil and thus melt frost or ice thereon.

The compressor motor will continue to run - compressing and thus warming refrigerant in the system.

In most heat pump designs, the outdoor fan will stop when a defrost cycle is initiated, and will remain off until the ice has melted away.

In some designs, both the compressor and the fan may stop and electric heaters may be used to perform the necessary defrost.

When does the defrost cycle stop (for a heat pump in heating mode)

The defrost cycle will typically stop after about ten minutes maximum, and often sooner, possibly after as little as one minute of defrost time, depending on weather conditions.

The defrost cycle will stop when temperature sensors at the outdoor unit detect that the coil’s temperature is high enough that no ice would be expected to remain on the unit.

That's a somewhat simplified description. More-sophisticated heat pump control designs add features to protect the compressor motor from damage.

For example, once the heat pump outdoor unit has completed a defrost cycle, if the outdoor unit fan has remained OFF, a timer may prevent the initiation of another defrost cycle even if the outdoor unit's defrost sensor (basically a thermostat) switch closes to try another defrost cycle.

Watch out: do not turn off your heat pump if it’s in the middle of a defrost cycle, or if you must turn it off, leave it off for at least 30 minutes to avoid damaging the compressor motor.

What happens at the indoor air handler during the heat pump defrost cycle?

During a heat pump in heating mode defrost cycle, the indoor air handler will normally stop blowing air into the occupied space - that's because the system is not receiving heat and the designer doesn't want to blow cool or cold air onto occupants. 

Or: the indoor unit will switch into backup heat mode and make use of the alternative heat source such as an electric, oil, or gas furnace.

Indoors at the air handler, or on some systems right at the thermostat, a colored or blinking indicator light may tell occupants that the delivery of heat has stopped for a few minutes to permit a defrost cycle.

Diagnostic note: just as we expect to see about a15-20 degree F temperature drop across the indoor coil when in cooling mode, we expect a similar temperature rise across the indoor heating coil when the heat pump is in heating mode.

If the indoor air through the air handler drops more than just one or two degrees F during the defrost cycle, then no backup heat is operating.

If the indoor air handler is blowing cold air on building occupants during defrost cycle, then the system is not operating properly. The problem could be a bad defrost controller outdoors.

Abnormal Outdoor Heat Pump Unit Fan-On Cycles

Outdoor Heat Pump Fan Runs When it Shouldn't

If your thermostat does not call for heat for an hour or more (perhaps because the indoor thermostat is satisfied), I would not expect the outdoor unit to require more than one additional defrost cycles and I'd not expect the outdoor compressor/condenser fan to continue to cycle.

So if that's happening, I suspect a temperature sensor or a condenser/unit control board error that's worth a service call.

If the outdoor fan runs when the compressor is OFF AND the system is NOT in DEFROST mode, then the repair may be simply to replace a bad fan relay.

Outdoor Heat Pump Unit Defrosts Too Frequently

If the outdoor unit is defrosting too frequently (at intervals of less than 30 minutes), something is wrong.

• If at the same outdoor temperature that frequent defrost cycle has occurred since original installation, the unit may be under-sized and unable to provide the amount of heat being asked of it.
• If the problem is a more recent one, check to confirm that air flow through the outdoor unit is not blocked by snow, brush, leaves, debris.
• If the indoor air handler air filters or blower fan are dirty then the indoor air flow and heat delivery are reduced and the heat pump may be over-taxed (as it can’t satisfy the thermostat).
• If the coil in the indoor air handler is dirty, that too blocks air flow, reduces heat delivery, and over-taxes the heat pump.

Other causes of too-frequent defrost cycles or an outdoor unit stuck in defrost mode include:

• Failed outdoor temperature sensor or poor sensor contact with the refrigerant piping or temperature contact surface
• A failed defrost control board, or one that is mis-wired or a failed defrost control board contactor or relay or timer motor
• Damage to the outdoor coil, such as a large area of bent or smashed fins that block air flow or dirty, debris-blocked outdoor coil
• Low refrigerant - a refrigerant leak - drops the system temperature and increases the defrost cycle rate (until so much refrigerant is lost that the system simply stops working)
• A bad outdoor unit fan motor or fan relay or fan start/run capacitor: fan doesn't run or runs too slowly - the outdoor unit will keep defrost-cycling frequently
• Outdoor fan motor running backwards (possibly after a lightning strike)
• Low voltage causing slow outdoor fan operation

Outdoor Unit Does Not Defrost - No Defrost Cycles

This might be normal, for example if the unit is running in cooling mode, not heating mode, OR if the outdoor temperatures are high enough that no frost or ice is forming on the outdoor coil.

However if temperatures are below 35°F, we expect ice formation (or perhaps you see ice formation) and the unit should be self-defrosting.

If it is not, the temperature sensor could be bad or the defrost cycle control board could be bad.

Or the reversing valve may not be working.

Outdoor Heat Pump Fan Does Not Run when Heat Pump is Running

When the heat pump is operating to deliver either warm or cool air the outdoor compressor/condenser unit fan should be operating.

If the outdoor unit fan does not run when in heating mode, you will see abnormally low discharge temperatures at the outdoor unit, faster icing at the coil, and possible system damage as well as loss of heat delivery indoors.

(If the outdoor unit fan doesn't run when in cooling mode, the system will overheat, will not deliver liquid refrigerant to the expansion valve and cooling coil (indoors) and you will see loss of cool air delivery indoors.)

Also check this article: SHORT CYCLING AC COMPRESSOR

Outdoor Unit Compressor Stops when Defrost Cycle Starts

The outdoor unit compressor normally runs during the defrost cycle in order to compress refrigerant and thus raise the temperature of the outdoor coil.

If the compressor stops, the problem may be

• Defective fan, fan motor, fan start/run capacitor: fan not running
• Defective defrost timer
• Temperature sensor or thermostat defective, loose, not properly installed
• Air pressure differential switch failure (the air pressure sensor confirms that air is flowing through the outdoor coil)
• Defective defrost control circuit board or contactor/relay
• Thermostatic expansion valve (TEV) is jammed or stuck, possibly from debris, or has been improperly adjusted
• Refrigerant leak, low refrigerant