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Musings of an Energy Nerd

Air Conditioner Basics

Answers to common cooling equipment questions

Moving heat. As the circulating refrigerant turns from a liquid to a vapor and then back to a liquid again, it absorbs heat from the indoor air and dumps the heat into your back yard.
Image Credit: Martin Holladay

What does a Vermonter know about air conditioning? I live so close to the Canadian border that half of the radio stations are in French. If my house needs cooling, I just let the fire in the wood stove die down.

When I first began reporting on air conditioning topics over a decade ago, I felt out of my element. Impelled by the certainty that there’s no such thing as a dumb question, I’ve managed over the years to badger a few air-conditioner experts, all of whom contributed to my education. So now I finally know the difference between an evaporator coil and a condenser coil.

In no particular order, here are the answers to a few air-conditioning questions.

Divorce decree

Q. What’s a “split” air conditioner?

A. A split air conditioner is the typical central air conditioner found in many U.S. homes. It consists of an outdoor unit that sits on a small concrete pad and an indoor coil located in the furnace plenum or the air handler. The term “split air conditioner” distinguishes this type of cooling system from other types of air conditioners, including window units.

Two thousand pounds of … I dunno

Q. What’s a ton of cooling?

A. In the old days, people used to buy ice to keep cool. A “ton” of cooling capacity is based on the amount of heat absorbed by one ton of ice melting over 24 hours. One ton of cooling capacity is equal to 12,000 Btu/h.

Maybe “Dental X-Rays”?

Q. What does “DX” stand for?

A. DX stands for Direct eXpansion — the standard refrigeration cycle used in most American air conditioners. (There are other refrigeration cycles — for example, the ammonia absorption cycle used in propane-fueled refrigerators.)

Calculating efficiency

Q. What’s a SEER rating?

A. SEER stands for Seasonal Energy Efficiency Ratio. An air conditioner’s SEER rating is its total cooling output in BTU during a “normal” annual cooling season divided by the unit’s total energy input in watt-hours.

The EER (energy efficiency ratio) is a similar rating that is based on the unit’s performance during peak conditions (typically 95°F).

The higher a unit’s SEER or EER, the more efficient the unit. The minimum permissible SEER for residential split-system air conditioners is 13.0.

Condensed soup

Q. What’s the difference between the condenser and the compressor?

A. Most people use the term “condenser” to refer to the outdoor unit sitting on the concrete pad. The condenser has several components, including the compressor (a pump used to compress the refrigerant), the condenser coil (also known as the outdoor coil), and the condenser fan. The condenser fan blows outdoor air over the condenser coil to cool the refrigerant in the coil.

Shifting gears

Q. What’s a two-speed condenser?

A. A two-speed condenser operates at a low setting (for energy efficiency) in moderate weather and a high setting in very hot weather. A two-speed condenser usually has a two-speed compressor as well as a two-speed condenser fan motor.

Nuts and bolts

Q. How does an air conditioner work?

A. The refrigerant cycle depends on five components: the compressor, the refrigerant, the condenser, the expansion valve, and the evaporator coil. The compressor is the pump that moves the refrigerant. When the refrigerant passes through the condenser, it starts out as a vapor. As the refrigerant’s heat is extracted by the air flowing over the outdoor coil, the refrigerant cools and condenses back into a liquid.

After the refrigerant leaves the condensing coil, it passes through the expansion valve, which is a valve with a very small opening. While the entry side of the expansion valve is at a high pressure, the far side of the valve is at a much lower pressure. Gases cool as they expand; the refrigerant expands suddenly as it finds itself in a low-pressure environment and therefore cools. The valve turns the refrigerant into a cold, fast-moving mist.

The cold refrigerant circulates through the indoor evaporator coil located in the furnace plenum, where the furnace fan blows indoor air over the coil. The evaporation of the refrigerant in the indoor coil occurs at a very low temperature — this low boiling point is a characteristic of the fluid chosen as a refrigerant — and (as you may remember from high school) the evaporation process draws heat to the boiling refrigerant from its surroundings. (As a liquid evaporates, it absorbs heat.) The warm air blowing over the coil is cooled as the refrigerant warms up.

The refrigerant leaves the coil as a warm vapor; it then cycles back to the outdoor condenser.

It’s reversible

Q. What’s a heat pump?

A. An air conditioner removes heat from a house and dumps it in the back yard. If you reverse its operation during the winter, the same equipment can be used to remove heat from the back yard and dump it indoors. Such a reversible air conditioner is called a heat pump.

Some people wonder how a heat pump can scavenge any useful heat from the outdoor air when it’s only 35°F out. The answer is that, through the magic (well, the physics) of the refrigeration cycle, as long as you can lower the temperature of the outdoor air, you can extract useful heat. So, if you lower the temperature of a volume of outdoor air from 35°F to 27°F, you can extract heat from the outdoor air and dump the heat indoors — raising the indoor temperature from, say, 69°F to 71°F.

Wringing the air dry

Q. How does an air conditioner help to dehumidify a house?

A. When warm, humid indoor air blows over the indoor evaporator coil, some of the moisture in the air condenses on the cold coil. After a while, the coil begins to drip. A pan under the coil collects the dripping condensate; after several minutes, the water begins to accumulate in the pan and eventually goes down the drain.

If the air conditioner short cycles — that is, if the home’s thermostat is quickly satisfied because the air conditioner is too big — then the air conditioner may shut off before the condensed moisture has time to find its way down the drain. If that happens, all of the moisture on the cold coil and in the pan re-evaporates, and the moisture stays in the house.

The moral of the story: a small air conditioner with long duty cycles can do a better job of dehumidification than a large air conditioner with short duty cycles.

Not overt but latent

Q. What is latent cooling?

A. An air conditioner provides both sensible cooling — that is, it lowers the temperature of the air flowing through the air handler — and latent cooling — that is, it dehumidifies the air flowing through the air handler.

Dehumidification is a type of cooling because the process of dehumidification adds heat to the air. Water has a latent heat capacity of about 1,000 BTU per pound. If you’re dehumidifying indoor air, you need to use some of your equipment’s cooling capacity — even if don’t end up lowering the indoor air temperature at all.

The ratio between a building’s sensible heat load and its total heat load (including the latent load) is called the sensible heat ratio. The sum of sensible energy and latent energy is called enthalpy. (“ERV” stands for “enthalpy-recovery ventilator.” Since few people understand the meaning of the word “enthalpy,” ERV manufacturers decided to start telling their customers that ERV stands for “energy-recovery ventilator.” They figured that lying about the acronym’s origin was easier than explaining what “enthalpy” means.)

Under peak conditions, the typical residential air conditioner provides about 70% of its cooling capacity as sensible cooling and 30% as latent cooling. In a hot, humid climate, however, it’s possible for 40% or 50% of a home’s cooling load to be a latent load. That’s why many energy-efficient houses in Houston require supplemental dehumidification — often provided by a stand-alone dehumidifier.

Air flow adjustments

Q. If I want more moisture removal — more latent cooling — do I want more air flow or less air flow over the indoor coil?

A. As a rule of thumb, air conditioner manufacturers recommend that a residential cooling system should provide 400 cfm of air flow per ton of cooling capacity. However, lower air flows (less than 350 cfm per ton) will provide more latent cooling.

Reducing the air flow to improve a unit’s latent capacity only works up to a point, however. If you slow the air flow too much, the coil can freeze.

Last week’s blog: “Duct Leakage Testing.”


  1. Michael Schonlau | | #1

    How about mini-splits & high-velocity systems?

    Can you expand your explanation to mini-split heat pumps and high-velocity mini-duct systems?


  2. Doug McEvers | | #2

    High velocity
    High velocity systems seemed to be the rage about 5 years ago when I attended the 4B Conference in Green Bay, not heard much about them since.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Minisplits and high-velocity systems
    I'm not sure what you need to know, but here are a few thoughts:

    1. High-velocity air conditioning systems have few benefits except for the fact they they use ducts with a small diameter. Such small diameter ducts are useful in retrofit situations. But these air conditioners are less efficient than other options, so they are best avoided unless you need to snake ducts in tight quarters.

    For more information on high-velocity systems, see

    2. Ductless minisplit air conditioners are the standard air conditioner in Asia and Europe. In the US, they cost more than standard split-system air conditioners; since Americans aren't used to seeing the wall-mounted fan-coil units indoors, most Americans prefer the systems they are used to.

    People who can afford ductless minisplits (and who don't mind the appearance of the wall-mounted indoor units) are usually delighted with their performance.

    Some ductless minisplits are used for cooling only; others are reversible heat pumps than can provide both heating and cooling. In recent years, Japanese and Korean manufacturers have come out with very efficient ductless minisplits capable of providing space heating at cold temperatures -- even -17°F. They are beginning to be used for all-electric homes in Vermont, and are especially appropriate for Passivhaus buildings or net-zero-energy homes equipped with PV.

    For more information on heating with a ductless minisplit, see "Heating a Tight, Well-Insulated House."

  4. Dick Russell | | #4

    Refrigeration cycle
    Martin, your refrigeration cycle diagram is correct, but in your explanation of the cycle you have the compressor moving the liquid refrigerant. The compressor sucks in somewhat warmed but vaporized refrigerant from the evaporator coil and pumps it up to some high pressure. That compression makes the vapor hot, so that heat can flow out of it to air moving past the condenser coil.

    As an experiment, have someone work the handle of a tire pump like mad, with the hose connected to a tire, and feel how warm the bottom of the pump gets just above where the hose comes out. That heat doesn't come from friction of the piston in the cylinder. The heat represents part of the mechanical energy expended on the gas by the compressor (eg. the guy pumping like mad). The temperature rise can be calculated readily from the properties of the vapor and the compression ratio.

    When the warm, high pressure liquid refrigerant crosses the expansion valve, it undergoes what's called a "constant enthalpy" flash, meaning no heat is added or removed (there). Since the low pressure in the evaporator coil is below the pressure that would be needed to keep the refrigerant in a liquid state, some vaporization occurs. Since vaporization requires energy (the vapor refrigerant has a higher heat content than does the liquid form at the same temperature), the temperature of the resulting vapor/liquid mixture must drop, to keep the total heat content of the mixture constant. This is "autorefrigeration." The fluid actually entering the evaporator coils already is partially vaporized (you did say it is a mist). As more heat is added by the air moving past the evaporator coil, vaporization continues to completion, after which the vapor just gets warmer.

    The efficiency of the unit, determined in part by the surface area of the evaporator coil, affects the temperature of the vaporized refrigerant leaving the coil and entering the compressor. That temperature is "warm" only in the relative sense, and can't be any warmer than that of the air moving past the evaporator coil.

    All refrigeration cycles are "heat pumps," absorbing heat from some low temperature and rejecting it at some higher temperature. Proper selection of fluid to use as refrigerant depends on those "low" and "high" temperatures, whatever they may be. In natural gas processing, to take an extreme example, ethylene can be used as a refrigerant to suck heat out of liquid methane at -260 F, and reject the heat to the evaporator side of a second refrigeration loop, using propylene refrigerant vaporizing at a relatively balmy -50 F. The propylene loop rejects heat to cooling water.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Thanks for fleshing out the explanation
    Thanks for your in-depth explanation of the refrigeration cycle. I have corrected the text by removing the word "liquid" in the sentence that discussed the compressor.

  6. Dick Russell | | #6

    Lower limit for ethylene as refrigerant
    I need to make a correction of my own. I had a different separation in mind when I said ethylene could be used as a refrigerant to suck heat out of liquid methane at -260 F. Actually, the lowest heat source temperature for realistic use of ethylene would be about -150 F, to keep the ethylene vapor to the suction side of the compressor above atmospheric pressure. If you go vacuum, slight air leakage into the system over time runs the risk of an explosive mixture.

  7. User avater
    Tristan Roberts | | #7

    ductless mini-splits and their kin
    I took a pretty detailed look at mini-splits in this article from Environmental Building News, which people might be interested in checking out:

    Ductless Mini-Splits and Their Kin: The Revolution in Variable-Refrigerant-Flow Air Conditioning

    Great article, Martin—I love the sense of humor.

    – Tristan Roberts
    Editor, LEEDuser

  8. anny | | #8

    air conditioner
    how to clean a/c in my room?

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Anny
    I assume you are asking about a window air conditioner. Here's an article to explain how to clean the filter and indoor evaporator coil of a window-mounted air conditioner:

  10. anthony | | #10

    split a/c suction side high pressor
    split ac suction side slowley incressing high pressor and h.p switch getting cut off. I have changed new compressor also. but it same.suction charged pressor is 65 psi.

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