Air conditioning and heat pump systems all work by moving heat, yet very few people can say which part of the system actually does the moving. Two components carry most of that work: the evaporator coil and the condenser coil. They look almost identical, a block of tubes threaded through thin metal fins, and on a spec sheet they are often listed one after the other as if they were interchangeable. They are not. They sit at opposite ends of the system, run at very different pressures and temperatures, fail for different reasons, and need different care.
This guide explains what each coil does inside the refrigeration cycle, the differences that actually matter in operation, why each coil corrodes in its own way, and how to tell which one is causing a problem without taking the unit apart. It also covers something most comparisons leave out: in a heat pump, the two coils swap jobs depending on the season, which changes how you should think about both of them.
The Refrigeration Cycle in Four Steps
An HVAC system does not make cold air. It removes heat from one place and dumps it in another, and refrigerant is the medium that carries it. The refrigerant changes between liquid and vapour as it travels around a closed loop, and each change of state either absorbs or releases a large amount of heat. That is the whole trick, and it explains why the system needs four components rather than two. The two coils are where heat is exchanged, but they only work because of the two components between them.
The loop runs like this. The compressor takes low-pressure refrigerant vapour and squeezes it to high pressure, which also makes it hot. That hot vapour goes to the condenser coil, where it gives up heat to the outdoor air and condenses into a liquid. The high-pressure liquid then passes through an expansion device, either a thermostatic or electronic expansion valve or a fixed orifice, where its pressure drops sharply and part of it flashes to vapour, which chills the rest. The cold, low-pressure mixture enters the evaporator coil, absorbs heat from the indoor air, finishes boiling into vapour, and returns to the compressor to start again.
One consequence of this layout is often missed. The condenser has to reject the heat picked up indoors plus the energy the compressor added while squeezing the vapour. So the condenser always handles more heat than the evaporator, typically somewhere around a quarter to a third more on a comfort cooling system. That is why outdoor coils are usually physically larger than indoor ones, and why a condenser that loses even part of its capacity to dirt affects the whole system so quickly.
What the Evaporator Coil Does
The evaporator coil sits on the indoor side, inside the air handler, the furnace cabinet on a ducted system, or the wall unit of a split air conditioner. Refrigerant arrives at low pressure and low temperature, usually with a saturation temperature of roughly 4 to 7 °C in comfort cooling. Warm room air is blown across the fins, heat moves into the refrigerant, and the refrigerant boils as it travels through the tubes. By the time it leaves the coil it should be fully vapour and a few degrees warmer than its boiling point. That small margin is called superheat, and it protects the compressor from receiving liquid it cannot compress.
Sensible and Latent Cooling
The evaporator does two jobs at once, and the second one is the reason people feel more comfortable in an air conditioned room than the thermometer alone would suggest. The first job is sensible cooling, which lowers the air temperature. The second is latent cooling, which removes moisture. Because the coil surface is colder than the dew point of the room air, water vapour condenses on the fins exactly as it does on a cold glass. That water runs down into a drain pan and out through a condensate line.
In humid climates a large share of the evaporator’s capacity goes into this moisture removal rather than temperature reduction. This is also why the condensate path matters so much. A blocked drain line does not reduce cooling at first, but it fills the pan, overflows into the ceiling or down the wall, and keeps the coil sitting in standing water. For a broader look at moisture control across the whole system, see our guide to HVAC humidity control.
What the Condenser Coil Does
The condenser coil is the evaporator’s mirror image, and on a split system it lives in the outdoor unit next to the compressor. Hot, high-pressure vapour arrives from the compressor, a fan pulls outdoor air across the fins, and the refrigerant gives up its heat. To push heat into outdoor air, the refrigerant has to be hotter than that air, so the condensing temperature normally sits around 10 to 15 K above the outdoor temperature on a typical unit, and somewhat less on high-efficiency units with larger coils. On a 35 °C afternoon that means refrigerant condensing at close to 50 °C.
Three Zones Inside One Coil
It helps to think of the condenser as three sections rather than one. In the first part, the very hot vapour cools down to its condensing temperature, which is called desuperheating. In the middle, the vapour condenses into liquid at a roughly constant temperature, and this is where most of the heat leaves. In the last section, the liquid cools a few degrees below its condensing point, and that margin is called subcooling.
Subcooling matters because it guarantees that what reaches the expansion device is solid liquid rather than a mix of liquid and bubbles. A technician reading subcooling together with superheat can tell a surprising amount about the system’s condition, which we come back to below. If you want the wider context of how efficiency is measured across the cycle, our article on coefficient of performance covers it.
Evaporator Coil vs Condenser Coil at a Glance
| Feature | Evaporator coil | Condenser coil |
|---|---|---|
| Location in cooling mode | Indoors, in the air handler or indoor unit | Outdoors, in the condensing unit |
| What it does | Absorbs heat and moisture from indoor air | Rejects heat to outdoor air |
| Refrigerant pressure | Low | High |
| Refrigerant state | Enters as a cold two-phase mixture, leaves as vapour | Enters as hot vapour, leaves as liquid |
| Heat handled | Room heat load only | Room heat load plus compressor work |
| Surface condition | Cold and wet with condensate | Hot and dry |
| Typical fouling | Fine dust bonded to wet fins, biofilm, mould | Leaves, grass, pollen, cottonwood, salt |
| Typical corrosion | Formicary corrosion of copper tubes | Galvanic and salt corrosion of fins |
| Diagnostic reading | Superheat, temperature split across the coil | Subcooling, condensing temperature over ambient |
Materials and Corrosion: Why Each Coil Fails Differently
Most coils in residential and light commercial equipment are built the same way: copper tubes carrying the refrigerant, pressed through thin aluminium fins that spread the heat transfer area. The two metals are chosen for good reasons, since copper is easy to braze and repair while aluminium is light and conducts heat well. The problem is that putting two different metals in contact creates a galvanic pair. Where moisture containing salts bridges the joint, the aluminium slowly corrodes away from the copper, the contact loosens, and heat transfer drops long before anything leaks.
That galvanic attack is mostly a condenser problem, because the outdoor coil is the one exposed to rain, fertiliser, pool chemicals and, most seriously, salt air. Units within a few kilometres of the sea can lose fin integrity within a few years if they are uncoated. Coastal installations often specify coated fins, such as epoxy or the gold or blue protective coatings common on Asian-built outdoor units. Another answer is the all-aluminium microchannel coil, which removes the copper and aluminium junction entirely. Microchannel coils resist this kind of corrosion well, but they are harder to repair, and a leak frequently means replacing the coil rather than brazing it.
The evaporator has a different and sneakier enemy. Formicary corrosion, sometimes called ant-nest corrosion because of the branching tunnels it leaves inside the copper wall, is caused by weak organic acids such as formic and acetic acid. Those acids form from volatile organic compounds in indoor air, including formaldehyde from furniture and building materials, some adhesives, and certain household cleaners. The result is pinhole leaks that are very difficult to find and that often appear on fairly new coils. It is one of the less obvious links between indoor air quality and equipment life, and it is a good reason to take sources of formaldehyde seriously, as we discuss in how to remove formaldehyde from indoor air.
Indoor coils in many split units also carry a hydrophilic fin coating, often blue, which helps condensate spread into a thin film and drain away instead of sitting in droplets. That coating improves drainage and reduces water being blown off the coil into the room. It does not stop formicary corrosion, which attacks from inside the tube wall. When a refrigerant leak is suspected on either coil, the methods in our guide to HVAC leak detection apply.
How Tropical Humidity Affects the Evaporator Coil
In hot and humid climates such as Southeast Asia, air conditioners often run for most of the day, and the evaporator is almost never dry. That changes how quickly it gets dirty. Dust that would pass over a dry fin sticks to a wet one, and once a thin layer has built up, it holds moisture and nutrients that let bacteria and mould colonise the coil. The result is a slimy biofilm that restricts airflow, smells musty when the unit starts, and can release spores into the room.
The drain pan and condensate line suffer in the same conditions. Algae and sludge grow readily in the warm, constantly wet pan, and in a split wall unit the drain hose is narrow enough to block easily. When it does, water drips out of the indoor unit, which is the most common complaint about split air conditioners in tropical homes. None of this is solved by more frequent outdoor cleaning. It is solved by good filtration, keeping the drain clear, and periodic cleaning of the indoor coil with a cleaner suited to it.
Good filtration is the evaporator’s real first line of defence. A filter that is clogged or missing lets dust straight onto the wet fins, and a filter that is too restrictive cuts airflow and makes the coil run colder. The right answer is a filter matched to what the fan can handle, changed on schedule, and supported where needed by room-level air cleaning rather than relying on the coil to collect what the filter missed.
Does Efficiency Drop the Moment a Coil Gets Dirty?
Not immediately, and that is precisely why dirty coils are such a common and expensive problem. Fouling builds up gradually, and the system compensates for a while by running longer. Nobody notices the change from one week to the next. What they notice is the end point, which usually arrives on the hottest days of the year.
On the condenser, dirt acts like a blanket. The coil cannot reject heat as easily, so the condensing temperature and pressure climb. The compressor then works against a higher pressure ratio, drawing more power while delivering less cooling. A commonly used rule of thumb puts the penalty at roughly 2 to 4 percent more compressor energy for each kelvin rise in condensing temperature. On a heatwave afternoon a badly fouled condenser can push head pressure high enough to trip the high-pressure safety switch, and the system shuts down exactly when it is needed most.
On the evaporator, dirt reduces both airflow and heat transfer. Less heat reaches the refrigerant, so its evaporating temperature falls. If it drops below 0 °C, the condensate on the fins freezes. Ice blocks more airflow, which lowers the temperature further, and within hours the coil can become a solid block of ice with almost no air passing through it. When that ice melts, it often overwhelms the drain pan. So the honest answer is that efficiency erodes slowly, then capacity fails suddenly. For a wider set of measures, see our guide to reducing HVAC energy consumption.
Telling Which Coil Is the Problem Without Dismantling the Unit
A surprising amount can be learned from the outside, starting with things any owner can check. Look at the filter first, because a clogged filter produces exactly the same symptoms as a dirty evaporator. Then look at the larger, insulated copper pipe running to the outdoor unit, which is the suction line. Frost or ice on that line, or on the indoor unit itself, points to the evaporator side. Water leaking from the indoor unit points to the drain. A visibly matted outdoor coil, or one blocked by plants, a fence or stored items, points to the condenser.
A thermometer adds one more useful check. Measure the air going into the indoor unit and the air coming out. On a system working normally, that temperature split across the evaporator is usually somewhere around 8 to 12 K, a little lower in very humid weather because more of the capacity is going into moisture removal. A split well above that range often means too little airflow across the coil. A split well below it suggests the system is not moving enough heat, which might be a refrigerant problem or a condenser problem.
A technician with gauges can separate the causes much more precisely, and here superheat and subcooling earn their place. A dirty evaporator and a low refrigerant charge both cause low suction pressure and icing, so they are easily confused. The difference is superheat. With low charge, the evaporator is starved and superheat runs high. With an airflow problem across a dirty coil, the refrigerant does not finish boiling and superheat runs low. A dirty condenser shows up differently again: high head pressure and a condensing temperature sitting much further above outdoor air temperature than the design allows. Getting this diagnosis right matters, because adding refrigerant to a system whose real problem is a dirty coil overcharges it and creates a second fault on top of the first. Our guide to HVAC tools and equipment covers the instruments involved.
Cleaning Each Coil Correctly
The two coils need different handling because they are built and exposed differently. For the condenser, switch the unit off at the outdoor isolator before touching it, clear leaves and debris by hand, and rinse with a garden hose at normal mains pressure. Spray in the opposite direction to the normal airflow where you can reach, so dirt is pushed out of the coil rather than deeper into it. Never use a pressure washer, since it flattens the thin aluminium fins, and use only a cleaner that is rated for aluminium. Bent fins can be straightened with an inexpensive fin comb.
The evaporator is harder to reach, is surrounded by electrical components and the drain pan, and in ducted systems is often sealed inside the air handler. For most owners that is a job for a technician, who will use a no-rinse or rinse cleaner suitable for indoor coils, clear the drain line, and check the pan at the same time. Our separate article on the importance of HVAC coil cleaning goes into methods and frequency in more detail, and the HVAC maintenance checklist places coil care alongside the rest of the routine.
In a Heat Pump, the Coils Swap Roles
Everything above describes a system in cooling mode. A heat pump uses the same four components but adds a reversing valve that changes the direction of refrigerant flow. In heating mode, the outdoor coil becomes the evaporator, absorbing heat from cold outdoor air, while the indoor side becomes the condenser and releases that heat into the building. The names “evaporator” and “condenser” describe what a coil is doing at a given moment, not where it is installed, and on a heat pump each coil does both jobs over the year.
That swap has practical consequences. In winter the outdoor coil runs below the dew point and often below freezing, so frost builds on its fins in cool, damp weather. The heat pump periodically runs a defrost cycle, briefly reversing to warm the coil and melt the frost. A dirty outdoor coil frosts faster and defrosts less effectively, which means a heat pump owner has a reason to keep that coil clean in winter as well as in summer.
On an air-to-water heat pump, the type used with underfloor heating and fan coil systems, the indoor side is not an air coil at all. The refrigerant gives up its heat to water through a plate heat exchanger, and that water is then circulated to the emitters. Inside each fan coil unit there is still a finned coil, but it carries water rather than refrigerant, and its output is controlled by a water valve rather than by the refrigeration circuit. This arrangement keeps refrigerant out of the occupied space entirely, which is one reason it is gaining ground as lower GWP refrigerants come in. Our articles on heat pump components and air-to-water heat pumps explain the wider system, and heat pump vs air conditioner sets out the comparison with a cooling-only unit.
Frequently Asked Questions
What is the main difference between an evaporator coil and a condenser coil?
The evaporator coil absorbs heat from indoor air at low pressure, turning refrigerant from a cold mixture into vapour. The condenser coil releases that heat, plus the energy added by the compressor, to outdoor air at high pressure, turning the vapour back into liquid. In cooling mode the evaporator is indoors and the condenser is outdoors.
Why is the condenser coil usually bigger?
Because it has to reject more heat than the evaporator absorbs. It handles the indoor heat load plus the work done by the compressor, which is typically around a quarter to a third more on a comfort cooling system.
Why does an evaporator coil freeze?
Most often because airflow across it is too low, from a clogged filter, a dirty coil or a fan problem, or because the system is low on refrigerant. Both lower the coil temperature below 0 °C, so the condensate freezes. A technician separates the two by measuring superheat.
Can I clean my condenser coil with a garden hose?
Yes, with the power switched off at the isolator, at normal mains pressure, and spraying against the direction of airflow. Do not use a pressure washer, and use only cleaners rated for aluminium fins.
Why do evaporator coils develop pinhole leaks?
Formicary corrosion is a common cause. Organic acids formed from indoor chemicals such as formaldehyde attack the copper tube wall from the outside, leaving fine tunnels that eventually leak. It can affect quite new coils.
Does a heat pump have an evaporator and a condenser?
Yes, but each coil plays both roles. A reversing valve changes the refrigerant direction, so in heating mode the outdoor coil becomes the evaporator and the indoor side becomes the condenser.
Heat Pumps and Hydronic Components from Legom
Jiaxing Legom Technology Co., Ltd. manufactures heat pumps, hydronic valves and indoor air quality products in Jiaxing, Zhejiang, supplying distributors and brands in more than 90 countries. Our air-to-water heat pumps, from compact 6 kW units up to the 16 kW model, are built around the reversing cycle described above, with the outdoor coil working as the evaporator in heating mode and a plate heat exchanger delivering heat to the water circuit. That water side then connects to underfloor heating or fan coil systems, where our HVAC valves and fan coil valves control flow to each water coil.
Because the evaporator coil of any air conditioner depends heavily on clean air reaching it, we also make air purification products designed to work alongside cooling equipment, including the KJF-B01 air conditioner companion and our wider air purifier range. They do not replace the filter in the air handler, and they do not clean a coil that is already fouled. What they do is reduce the dust and airborne load in the room, which is part of the same picture.
For distributors and brands, our OEM and ODM service covers branding, specification and market documentation, and our guide to HS codes for HVAC products covers classification for import.
Reviewed by Maggie Shen, Director at Legom, on November 3, 2026. This article was reviewed for technical accuracy, including the four-stage vapour compression cycle, the additional heat rejected at the condenser from compressor work, refrigerant state at each coil, superheat and subcooling, typical evaporating and condensing temperatures, galvanic and formicary corrosion mechanisms, condensate and drain pan behaviour in humid climates, coil cleaning methods, and coil role reversal and defrost in heat pumps.