It's no illusion: Your air conditioner works harder on hot days. If you've ever heard your air conditioner struggling to cope with a summer heat wave -- and been disappointed in the success of its efforts -- you've probably wondered what you could do to alleviate the machine's suffering -- and yours. You could plant trees and shrubs close to your central air conditioning unit and hope that the consequent lower temperature helped the unit cool your home more effectively. For that to work, the outside temperature would have to somehow be related to the effectiveness of your air conditioner. Luckily for you, it is. Thermodynamics Air conditioners are heat exchangers -- units that transfer heat energy from one place to another. They are limited by the laws of thermodynamics, which are the rules constraining the movement of heat energy. The most common way of seeing this is that two objects of different temperatures put in an environment where they interact will come to equilibrium at some temperature between the two starting temperatures.

Heat transfer also can have other effects, as when you put an ice cube tray full of water in the freezer and the water solidifies. Heat energy flows from the hotter object to the colder object -- that's a statement of the second law of thermodynamics. It's why you need an air conditioner in the first place: Heat will naturally transfer from the hot outdoors to the inside of your home. ac unit for a mobile homeThe Refrigerant Air conditioners work by circulating a refrigerant. best in window ac unitsThat's a fluid that transitions from a liquid to a gas at a relatively low temperature. ac unit model numberWhen the fluid is a gas, its properties are governed by the ideal gas law, which says that the pressure, density and temperature of a gas are all connected.

If you raise the pressure of an enclosed gas, you'll increase its temperature, and if you lower the pressure of a gas, you'll lower its temperature. It takes energy to change a fluid from a liquid to a gas. A refrigerant transitions from a liquid to a gas at high temperature, while at low pressure the transition is at a lower temperature. The exact temperatures, pressures and the energy of the transition are all dependent on the specifics of the refrigerant. The Air Conditioning Cycle Your air conditioner uses the principle of the second law of thermodynamics and the refrigerant's properties to cool your house. Outside your house, the air conditioner compresses the gaseous refrigerant, which -- according to the ideal gas law -- increases its temperature. A fan blows outside air across coils containing the hot, high-pressure refrigerant. The outside air is cooler than the fluid, so -- according to the second law of thermodynamics -- energy will flow from the refrigerant to the outside air.

When the high-temperature gaseous refrigerant gives up energy, it transitions to a liquid. The high-temperature, high-pressure liquid goes through an expander, which converts the refrigerant to a low-temperature, low-pressure liquid as it enters the inside of your house. There another fan blows inside air across the coils, where the warmer air sends heat into the cool coils, converting the liquid to a gas. The gaseous refrigerant enters the compressor and the cycle starts again. Outside Temperature Imagine your refrigerator without the coils that interact with air. It would be a machine that converted a low-temperature, low-pressure fluid into a high-temperature, high-pressure fluid, and back again to the same low-temperature fluid. In real life, inefficiencies heat the fluid, gradually raising the temperature with each cycle. Now add inside cooling coils to your imaginary air conditioner. Those cooling coils pick up heat from the inside air, but they have nowhere to get rid of it, so the temperature of the whole system increases.

So you add an outside heat exchanger to get rid of the extra heat. The amount of heat transferred and the rate at which it's transferred depends upon the temperature difference between the outside air and the refrigerant. The lower the temperature of the outside air, the more cooling is done by the heat exchanger, instead of the compressor. Air Conditioner Efficiency There are a number of different methods for evaluating air conditioner efficiency. The one that is most closely tied to physical principles is the coefficient of performance, or COP. The COP is the ratio of heat removed from a system to the energy required to remove the heat. The theoretical maximum is equal to the coldest temperature of the refrigerant divided by the difference between its coldest and hottest temperatures, where the temperatures are expressed in Kelvin. Even the perfect system decreases in efficiency with increased outside temperature, dropping about 2 percent per degree Celsius (about 1 percent per degree Fahrenheit).

In the real world, the drop in efficiency is even more dramatic; so it's not an illusion when your air conditioner seems to be working harder as the outside temperature increases. References Georgia State University: Second Law of ThermodynamicsWashington University of Saint Louis: Phase Changes and Refrigeration -- Thermochemistry of Heat EnginesNational Institute of Standards and Technology: A Comparison of an R22 and an R410A Air Conditioner Operating at High Ambient Temperatures Photo Credits Comstock/Comstock/Getty ImagesYOU ARE HERE: LAT HomeCollections QUESTION: Our outdoor central air-conditioner compressor unit is located in the hot afternoon sun. Is there any type of cover I can make myself to shade it so it operates more efficiently?ANSWER: Keeping the outdoor compressor unit and the air surrounding it cooler can make your central air conditioner operate more efficiently. Condenser coils can dissipate more heat when the air around them is cooler.In addition to better efficiency, the maximum cooling output from your air conditioner is greater when the air around it is cooler.

This is particularly important on the hottest summer afternoons, when you need the greatest amount of cooling output to keep your house comfortable.Shading the entire side of your house and the outdoor air-conditioner unit is most effective. This can be done with trees or an awning. Particularly with a brick, masonry or stucco house, the house wall acts like a huge solar collector and actually raises the temperature of the air around the air conditioner.You can make a simple cover to shade your air conditioner outdoor unit. Check with your air-conditioning contractor about the proper clearance for air flow. Adequate air flow is essential for high efficiency.One effective air conditioner cover provides both shade and storage. Depending on how large you make this storage/air conditioner cover, you can store your garden tools, fertilizers, etc. Also, the larger you make it, the more shade it provides.It is basically a plywood storage compartment built several feet back (check with your contractor) from the condenser unit.

Build it with a sloped roof (for shade) that extends up over the condenser unit and attaches to your house. The sloped roof gives added height for adequate clearance and it blends better with the lines of most houses.Make the frame for the storage unit/cover with any common lumber. Cover the storage compartment with plywood siding. Install a piano hinge on the top so you can open it.Nail the plywood siding over the rest of the sloped top to cover the condenser unit. Leave the sides by the condenser uncovered to allow for adequate air flow.You can write to me at the address below for Utility Bills Update No. 161 showing a diagram and do-it-yourself instructions for making a storage unit/air-conditioner cover. Please include $1 and a self-addressed stamped business-size envelope.Rectangular Ducts Less Efficient Than RoundQ: I am building a new house and am at the point of installing the heating and air-conditioning duct work. Is it better to use round or rectangular duct work?A: From strictly an energy efficiency standpoint, a round duct is more efficient than a rectangular one.

The air flow is freer and there is less duct surface area for a given duct cross-sectional area. This means less warmth or "coolth" is lost through the duct-work surface before the conditioned air gets to your rooms.Rectangular duct work is more convenient to run between wall studs. Both types of duct are often used in the same system. Round ducts can be run from the air handler to rectangular ducts in the walls. No matter which type of duct is used, be sure to seal carefully all the joints with duct tape. Also, insulate any ducts that run through an unconditioned space.Protect Your Air ConditionerYour air conditioner unit will operate more efficiently if it is shaded from the sun. This do-it-yourself air conditioner cover also provides storage for lawn and patio needs. Adequate clearance House wall Air conditioner Storage compartment Hinge Plywood siding Seizure Led to FloJo's DeathHis 104 scores make his caseRestaurant review: South Beverly GrillBrutal Murder by Teen-Age Girls Adds to Britons' ShockComaneci Confirms Suicide Attempt, Magazine Says