Heat Pump Defrost: Why the Coil Ices and What to Check

The short version

Frost on the outdoor coil in heating weather is normal, and the defrost cycle is what the equipment does about it. The trap is treating every iced coil as a failed defrost board, because the board is only the first of three things that have to work. The control has to start a defrost, the defrost has to clear the coil, and the meltwater has to leave the unit. Swapping the board can only change what the control does, and it leaves the refrigerant circuit and the drainage path exactly as it found them.

The customer pays for that twice. The unit is still iced when you leave, so the compressor carries less of the house, and on a house with electric strips those run longer to make up the difference. That is the winter the customer sees on the power bill, on top of the part they already paid for.

Force a defrost with the model’s own procedure before you condemn anything. Every manufacturer has its own method, so work from that unit’s service literature rather than a jumper position that worked on something else. A commanded defrost then shows you, inside a cycle Lennox puts at 5 to 15 minutes, whether the control can start one, whether the reversing valve shifts, whether the outdoor fan stops, whether any backup heat the house has comes on, and whether the ice actually melts. Those five observations split the fault into a control problem, a refrigerant circuit problem, or a drainage problem. No single pressure reading sorts it that way.

Why frost forms on the outdoor coil

In heating mode the outdoor coil is the evaporator, so the refrigerant inside it has to run colder than the outdoor air for heat to move from the air into the refrigerant. Energy Vanguard puts the usual difference at “10 to 20 degrees Fahrenheit colder than the outdoor air.” That is one publication’s figure rather than a specification, and the real difference moves with the equipment and the conditions. Either way, the coil is running below the weather, which is why a heat pump can grow frost on a day nobody would call cold.

Two conditions have to be true at the same time for frost rather than water. Oak Ridge National Laboratory states it in its review of defrost technologies: “When the operating conditions are extremely cold and humid and the surface temperature of the heat exchanger well below the freezing point (lower than the dew point temperature of the air) moisture from the air stream will freeze on the surface.” The dew point is the temperature at which the air gives up its water vapor as liquid. A surface below the dew point pulls moisture out of the air, and a surface also below freezing keeps it there as frost.

That pairing explains why frosting is worst in the mild part of the heating season rather than the coldest part. Energy Vanguard gives the typical conditions as an outdoor air temperature below 40°F with relative humidity around 70 percent or higher. Very cold air holds little moisture, so there is less water available to deposit on the coil, though the coil is certainly cold enough.

Frost on the fins does two things to the machine, and they compound. It blocks the air path through the coil, so less air reaches the surface that is supposed to give up its heat. It also sits between the air and the metal as a layer of poor conductor. Less heat entering the refrigerant means the coil runs colder still, which collects frost faster. Left alone the coil ends up icebound and the heat pump ends up making very little heat, which is the condition every defrost strategy exists to prevent.

What happens during a defrost cycle

A defrost is a short, deliberate run in cooling. Trane describes its own equipment plainly: “the reversing valve reverses the flow of refrigerant to heat up the outdoor coil.” Hot discharge gas from the compressor goes out to the frosted coil and melts the ice from the inside of the tubing outward. HVAC School describes the same sequence at the wires: “When the system goes into defrost, a 24v signal goes to the reversing valve to switch the cycle from heating to cooling, the outdoor fan motor shuts off, and a 24v signal goes out on the white wire to bring on the backup heat indoors.”

On most machines the outdoor fan stops on purpose. Writing in Contracting Business, Bryan Orr gives the reason: “The condenser fan is shut off to increase the speed of defrosting.” A fan pulling 35°F air across a coil you are trying to heat is working against the cycle. Trane words it as “The outdoor fan may stop,” which is worth keeping as written, since not every machine handles the fan the same way.

Backup heat covers the indoor side on the systems that have it. While the outdoor coil is being heated, the indoor coil is the evaporator, so the refrigerant side is pulling heat back out of the house the system is there to heat. Energy Vanguard describes one common solution: “One solution is to turn on the auxiliary heat when the defrost cycle starts.” Trane says the same thing about its equipment, with the qualifier that it depends on the house having strips: “Backup heat comes on (assuming the homeowner has backup electric heat strips, and most do).” Lennox scopes it the same way, saying “Many heat pumps automatically switch on backup or auxiliary heat strips during defrost to prevent temperature swings inside.” Without strips, what the customer feels comes down to the blower. Trane says those homeowners “may notice cool air coming from their air vents instead of warm air,” which is normal rather than a fault, while Energy Vanguard’s own Mitsubishi has no auxiliary heat and stops the blower during defrost instead.

Which way the reversing valve is driven is a property of the unit rather than of the brand. HVAC Know It All states the common case: “Most manufacturers today default their reversing valves to heating mode, meaning the O terminal must be energized to switch to cooling.” The same article lists the manufacturers it says go the other way and energize for heating: “Rheem, Ruud, Weathermaker, Ameristar, Bosch Air Source.” Do not work a unit off that list. HVAC School separately agrees on two of them, writing that “Ruud and Rheem heat pumps utilize the B terminal instead of the O terminal, and they energize the reversing valve in heating mode,” but a Carrier WeatherMaker package unit manual gives the opposite for its own equipment: “Reversing valves are energized during the cooling stages and the defrost cycle and de-energized during heating cycles.” HVAC Know It All says as much itself, telling readers to verify the control designation on the equipment in front of them and never assume it from the brand. On a unit you have verified as energized for heating, the solenoid is energized during normal heating and drops out for defrost. If you are about to judge a defrost by whether 24 volts appears at the valve, get the designation off that unit’s wiring diagram first.

Steam off the outdoor unit during a defrost is normal. Lennox tells customers, “You may notice steam rising from the outdoor unit. No need to worry, this is water vapor and it’s normal during the process.” Trane describes “a cloud of steam coming from your outdoor unit, which is the ice melting and evaporating,” along with “a swooshing sound as the refrigerant reverses course.” Two manufacturers publishing the same thing is worth knowing on a January call, because the customer reporting smoke may be describing the steam off a defrost. Both manufacturers scope it to the cycle, so confirm the unit is actually in defrost, and rule out real smoke, before you tell the customer that is what they saw.

How the control decides to start and stop a defrost

Two strategies cover most of the residential equipment on the road, and they fail in different ways.

Time and temperature. The control watches the outdoor coil temperature and a clock, and both have to be met before it acts. HVAC School describes it as, “Time and temperature (T/T) defrost requires the coil sensor (thermostat) to close, which then starts a timer,” and puts the interval at 30, 60, or 90 minutes of run time, with a defrost initiating only if “the defrost sensor or thermostat remains closed.” HVAC Training Solutions, writing separately, states the same requirement as a rule: “Both conditions, time and temperature must be met to initiate a defrost.”

The temperature half is often a bimetal snap disc clamped to the coil. HVAC School gives those temperatures as examples rather than as a rule, describing a White-Rodgers universal control whose disc “will snap shut at 32 degrees Fahrenheit (0 Celsius) to initiate defrost” and comes “out of defrost at 65 degrees Fahrenheit,” and giving the same pair elsewhere as Carrier defrost thermostat values. Bryan Orr publishes 30°F and about 65°F for a Carrier line thermostat in Contracting Business. Treat those as the common design rather than a specification, get the thresholds for the board in front of you, and note that Contracting Business and HVAC School are one source here rather than two, because the same author writes both.

Termination has two exits. The cycle normally ends on coil temperature, and it ends on the clock when the temperature never arrives. HVAC Training Solutions states the logic: “Either condition, time or temperature independent of each other, will terminate the defrost cycle,” and gives the usual ceiling as, “Normally the maximum amount of time for defrost is 10 minutes.” A unit that runs the full time limit every cycle and still comes out iced is telling you the control never registered its termination temperature, which is a finding rather than a nuisance. Whether the coil itself never got there, or the sensor, its wiring, or the board never registered it, is the next question.

Demand defrost. Newer controls decide from the sensors themselves, and they keep a maximum time between defrosts in reserve. HVAC School describes the method: “Demand defrost checks the coil temperature sensor and outdoor air temperature sensors at all times,” and says such a control “only goes into defrost when the temperature difference between the sensors exceeds a set difference” rather than defrosting on a fixed 30 to 90 minute run time. Trane puts a defrost on its demand system at “usually 3-5 minutes.”

The reason the industry moved is that time and temperature does not account for the variable that actually grows frost. Energy Vanguard is blunt about it: “This system is based only on temperature and time. Apparently, heat pump manufacturers forgot that frost forming on a surface also depends on the amount of moisture in the air and this thing called the dew point.” Energy Vanguard also published one homeowner’s measurements from a custom demand controller on a single system, which came out at roughly 150 kWh saved per heating season against a two hour defrost interval and roughly 400 kWh against a half hour interval. That is one system rather than a study, and it gives a sense of what unnecessary defrosts cost.

Bryan Orr states the target both strategies are aiming at: “The ultimate goal of defrost is to prevent the coil from becoming icebound with the fewest and shortest defrosts possible.” Both halves of that sentence matter on a service call. Too few defrosts leaves an iced coil, and too many waste compressor runtime and, on the houses whose strips come on with the cycle, burn resistance heat for nothing.

What you find	Suspect	Confirm with
Light frost, unit heating normally	Normal operation	Watch for a defrost on that control’s own logic, a 30, 60, or 90 minute interval on time and temperature, a sensor difference on demand
Coil iced over, no defrost ever starts	Coil sensor, board, or wiring	Model-specific forced defrost, then the reversing valve command that unit’s wiring diagram calls for and any W output the house has
Defrost starts but the ice stays	Reversing valve not shifting, low charge, or weak compressor	Line temperatures at the valve body, charge against manufacturer data
Defrost every few minutes, cycles cut short	Sensor out of calibration or mounted wrong	Thermistor resistance against the manufacturer’s chart at a known temperature, or a snap disc against its closing and opening temperatures
Ice building upward from the base pan	Blocked drainage or water falling on the unit	Base pan drain openings, the roofline and gutter above the unit
Coil ices in mild weather, suction pressure low	Low charge or restricted outdoor airflow	Superheat and subcooling against manufacturer data, coil and fan condition

A row is a hypothesis rather than a diagnosis. Two of these faults can be present at once, and low charge in particular can produce ice while the defrost control does exactly what it was designed to do.

Normal ice and ice that is a fault

Carrier draws the line in one sentence: “During winter, it’s normal for a little frost to develop on the outdoor coil, but a coating of ice indicates an issue.” Interplay Learning’s technician training puts the same threshold in field terms, calling a thin layer of frost over the coils “not cause for alarm” and stating that “light frost is normal.”

What separates the two is whether the ice clears. Frost that appears between defrosts and disappears during them is the system working. Trane names the pattern that is not working: a unit that “seems to be constantly switching into defrost mode or the defrost cycles last longer and longer with frost and ice starting to cover the unit.” A solid sheet across the face of the coil, ice blocking the fan blade or stopping it from turning, or a block growing up out of the base pan is a fault, and it is a fault with a cause that a defrost cycle by itself will not remove.

Ice that grows from the bottom up is its own category and it is frequently missed. A defrost that melts accumulated frost produces liquid water that has to leave the base pan. If the drain openings are blocked with leaves, dirt, or ice, or if a gutter or roof valley above the unit is dropping water onto it, the next defrost adds to a block instead of clearing one. Interplay Learning lists “infiltration of water from a gutter or the roof, which then freezes” among the causes of a frozen unit. In that case the control, the valve, and the charge can all be correct and the unit will still be encased by February.

How to work an iced coil

Work the three questions in order, because each one costs less than the one after it and each one rules out a different subsystem.

Question one: does a defrost start?

Walk the unit, then read the board. HVAC School puts those in that order: “After you’ve done a visual inspection of the entire system, the first place to start in the ‘narrow’ part of your diagnostics is by looking at the fault code.” Most defrost boards flash a code or hold a fault in memory, and a stored fault about a coil sensor saves you an hour.

Then command one. Boards provide a way to do this, and it is board-specific: some have a speedup function, and some are jumpered across test pins. HVAC School describes the accelerated test on one common universal control, where “The speedup function on the White-Rodgers universal defrost control allows you to bypass the short-cycle delay or initiate defrost to test the board.” Use the instructions for the board you are holding rather than a jumper position you remember from a different brand. It also helps to have frost on the coil to melt, and HVAC School suggests making some: “It’s a good idea to run the unit in heat mode with the fan off to allow a little bit of frost to build up on the coil.”

With the defrost running, take the two voltage readings HVAC School names: “Voltage between O (reversing valve) and Common = 24v” and “Voltage between W (backup heat) and Common = 24v.” Confirm the outdoor fan stopped. These readings are taken on an energized control, and The Occupational Safety and Health Administration standard OSHA 1910.333(c)(2) limits that work to qualified people: “Only qualified persons may work on electric circuit parts or equipment that have not been deenergized under the procedures of paragraph (b) of this section. Such persons shall be capable of working safely on energized circuits and shall be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools.”

Before you reach into the fan section or handle a component, disconnect the equipment from every source of electric energy, release any stored energy, lock and tag each disconnecting means, and have a qualified person test the parts you will be exposed to for voltage, including induced voltage and unrelated backfeed. OSHA 1910.333 requires that “A lock and a tag shall be placed on each disconnecting means used to deenergize circuits and equipment on which work is to be performed, except as provided in paragraphs (b)(2)(iii)(C) and (b)(2)(iii)(E) of this section,” and that “A qualified person shall use test equipment to test the circuit elements and electrical parts of equipment to which employees will be exposed and shall verify that the circuit elements and equipment parts are deenergized.”

If no defrost starts, check what the board needs before you suspect the board. HVAC School points at the R leg first: “The board will run without R because there will be a Y call, but defrost can’t occur because R redirects on O and W, and your heat strips and reversing valve can’t work if there isn’t 24v on R.” With the calls, the power, and the board’s own configuration confirmed, the fault is in the coil sensor, the board, or the wiring between them. HVAC School narrows it the same way: “Defrost issues can be narrowed down to just a few possible problematic parts. These parts are the circuit board, temperature sensors, and wiring or harnesses.” Test a thermistor against the manufacturer’s chart rather than against a resistance you expect. Bryan Orr’s procedure in Contracting Business is to consult the factory data: “Thermistors can be tested by consulting factory charts for either resistances or voltage drops at particular temperatures,” with the coil temperature itself established by measurement, since he attaches “a K-type thermocouple or line temperature clamp near the defrost thermostat to monitor the temperature.” For a snap disc, HVAC School gives the goal as confirming “that they close at the defrost control’s set point.”

Question two: does the defrost clear the coil?

A defrost that starts and leaves ice behind is usually the refrigerant circuit or a cycle that ended too early, and the reversing valve is the first thing to look at. Watch the refrigerant line temperatures at the valve body change when the board calls for the shift. Nothing changing means the slide did not move.

Do not condemn the valve on that alone. Bryan Orr explains what actually moves the slide: “The compressor creates the pressure differential when it pumps vapor. If you have a weak or faulty compressor, it might not create a strong enough pressure differential to switch between heating and cooling mode effectively.” A discharge line that never builds pressure does the same thing from a different direction, since “If the vapor pressure is too low in the discharge line, it may not be strong enough to push the slide in either direction.” A valve that will not shift is a symptom shared by a failed valve, a solenoid that never got its command, and anything that keeps the two sides of the system from separating enough to move the slide, which Danfoss describes as “the pressure differential between the high-pressure and low-pressure sides of the refrigeration plant.”

Separating them takes both the electrical side and the refrigerant side. Check the board output and the voltage at the solenoid coil, since Danfoss notes that the valve “possesses a solenoid pilot valve with a coil that can be powered to change the direction of flow of the refrigerant,” and take pressures, superheat, and subcooling against the manufacturer’s heating mode data for that unit.

After a defrost that shifts correctly and still leaves ice, the charge and compressor questions are next, since there may not have been enough heat available to melt it. Carrier states the charge half plainly: “If your system is low on refrigerant, it can’t produce enough heat to melt ice buildup during winter.” Do not clear the control yet either. A snap disc has to “open at the temperature needed to terminate defrost,” as HVAC School puts it, and one that opens early ends the cycle with ice still on the coil.

Question three: does the water get out?

Look at the base pan drain openings and clear them. Look above the unit for a gutter, a roof valley, or a condensate line dropping water onto it. Look at what the unit is sitting on, because a pad at grade in a snow region gives meltwater nowhere to go. Cold climate equipment is frequently sold with a base pan heater for this reason, and the manufacturer’s installation instructions are what apply to mounting height and clearances for the model you are working on.

Common mistakes

Walk the whole unit, then read the board’s fault code or light emitting diode (LED) pattern before you take a single reading, since HVAC School puts the visual inspection first and the fault code first in the narrow part of defrost diagnostics.
Force a defrost with the board’s own speedup or test procedure and confirm the outputs that unit actually has, meaning the reversing valve command, the outdoor fan stopping, and backup heat on the systems that carry it, which Lennox describes as “Many heat pumps” rather than all of them.
Watch whether the ice actually melts during that commanded defrost, because a defrost that starts and does not clear the coil points at the valve, the charge, the compressor, or a cycle that terminated early.
Check the base pan drain openings and whatever sits above the unit before you order a part, since meltwater that cannot leave builds ice from the bottom up.
Test a coil sensor against the manufacturer’s resistance chart at a measured temperature rather than against what the resistance looks like to you.
Tell the customer that steam off the unit during defrost is normal, since Lennox and Trane both publish exactly that, and that the water underneath the unit is melted frost on its way out of the base pan.

Don’t

Replace a defrost board because the coil is iced, because starting a defrost is one of three things that has to work and the other two leave the same ice.
Chip ice off the coil with a screwdriver or pour hot water on the unit, since Lennox warns that tools damage the coils, fins, and fan blades and that sudden temperature change can crack components.
Apply a torch or any open flame to an iced coil, because the coil is full of refrigerant under pressure. Copeland’s guidance for systems holding flammable refrigerant is written about service cuts rather than about ice, and it is still blunt: “Never use a torch to remove compressor components or tubing.”
Condemn a reversing valve that did not shift until charge and compressor performance have been checked, since the compressor is what creates the pressure differential that moves the slide.
Assume the reversing valve is energized for cooling on every unit, or take the designation off a brand list, since HVAC Know It All lists Rheem, Ruud, Weathermaker, Ameristar, and Bosch Air Source as energizing for heating while a Carrier WeatherMaker package unit manual gives its own valves as energized in cooling and in defrost.
Read light frost on a coil in heating weather as a fault, since Carrier calls a little frost normal and reserves the concern for a coating of ice.

Frequently asked questions

Is ice on a heat pump normal in winter?

Light frost on the outdoor coil is normal in heating weather, and a coating of ice is not. Carrier states it as “During winter, it’s normal for a little frost to develop on the outdoor coil, but a coating of ice indicates an issue.” The test in the field is whether the ice clears. Frost that appears between defrost cycles and disappears during them is the system doing its job, while a solid sheet across the coil face or a block growing out of the base pan is a fault with a cause behind it.

How often does a heat pump defrost?

It depends on which control the equipment uses. On time and temperature controls the board checks at a set interval, usually every 30, 60, or 90 minutes of run time according to HVAC School, and it defrosts only if the coil sensor also reports frosting conditions. Lennox describes its systems as programmed “to check for frost every 30 to 90 minutes during heating mode.” Demand defrost controls do not initiate on that interval, and HVAC School describes them as going into defrost “only when the temperature difference between the sensors exceeds a set difference.” They are not off the clock entirely, though, because the same article notes that “there is usually a maximum time between defrosts; six hours is a common timeframe.”

How long does a defrost cycle last?

Published figures vary by equipment and by control type, so use the thresholds for the unit in front of you. Lennox puts the whole process at “between 5 and 15 minutes, depending on weather and frost levels,” while Trane gives “usually 3-5 minutes” for its demand defrost. Many boards also carry a hard ceiling, and HVAC Training Solutions gives the common one as “Normally the maximum amount of time for defrost is 10 minutes.” A unit that hits that ceiling on every cycle and still comes out iced never registered its termination temperature, which leaves the coil temperature itself, the sensor, its wiring, and the board to check.

Why does the heat pump blow cold air during defrost?

During defrost the system is running in cooling, so the indoor coil is absorbing heat from the house rather than adding it. Many systems cover that with backup heat, and Energy Vanguard describes one common solution as turning “on the auxiliary heat when the defrost cycle starts.” Trane says backup heat comes on during defrost, with the qualifier that this assumes the house has electric heat strips, and Lennox writes that “Many heat pumps automatically switch on backup or auxiliary heat strips during defrost.” On a system without backup heat that keeps the blower running, Trane says the homeowner “may notice cool air coming from their air vents instead of warm air,” which is normal rather than a fault. Other systems handle the indoor side differently, and Energy Vanguard’s own Mitsubishi stops the blower for the length of the defrost instead.

Is the steam coming off my heat pump smoke?

Steam off the outdoor unit during a defrost is water vapor from the melting ice. Lennox tells customers, “You may notice steam rising from the outdoor unit. No need to worry, this is water vapor and it’s normal during the process,” and Trane describes “a cloud of steam coming from your outdoor unit, which is the ice melting and evaporating.”

Do not write off a report of smoke on a smell test. Confirm the unit is in defrost, and look for what a smell will not tell you, meaning visible smoke, heat where there should be none, arcing, and scorched wiring or components. If any of that is there, stop the equipment and get everyone clear before you diagnose anything further, because the US Fire Administration’s instruction on seeing smoke is to “get out right away.” It is equally blunt about why: “Smoke and toxic gases kill more people than flames do,” and “Residents could have less than 2 minutes to escape a home fire once the smoke alarm sounds.” Once the equipment is ruled out, explain the defrost cycle to the customer so the next one is not another call.

What makes a heat pump stop defrosting?

The fault is usually in the coil temperature sensor, the defrost board, or the wiring between them, which is how HVAC School narrows it: “Defrost issues can be narrowed down to just a few possible problematic parts. These parts are the circuit board, temperature sensors, and wiring or harnesses.” A separate group of faults allows the defrost to start and prevents it from clearing the coil, and those are a reversing valve that does not shift, a low charge, and a compressor that is not pumping well. Blocked base pan drainage produces ice that no defrost cycle can remove.

Glossary

Contracting Business: a trade publication cited here on defrost control settings.
Defrost board: the control that decides when to start and end a defrost cycle.
Defrost cycle: the reversing valve shifts to cooling so hot gas warms the outdoor coil and clears frost.
Demand defrost: a control that starts defrost from sensor readings rather than a fixed clock interval.
Energy Vanguard: a building science publication cited here on coil temperature.
HVAC School: a practitioner training and reference organization cited here.
HVAC Training Solutions: a training organization cited here on defrost termination logic.
LED: light emitting diode, the indicator a board flashes a fault code on.
OSHA: Occupational Safety and Health Administration, whose 1910.333 limits energized work.
Reversing valve: the valve that swaps the coils between heating and cooling.
Termination: the condition, by time or temperature, that ends a defrost cycle.
Thermistor: a temperature sensor whose resistance changes, read against the maker’s chart.
Time and temperature defrost: a control that starts defrost on a clock and ends it on coil temperature.
White-Rodgers: a control manufacturer whose universal defrost board is cited here.

Drafted with AI assistance and reviewed by the author.

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