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Cooling and Air Conditioning
Quick Facts
- Only when correctly engineered and used in conjunction with a tight, insulated building envelope, can very high efficiency levels and can be reached
- Most commonly a part of larger HVAC systems in the form of split air conditioners or electric heat pumps integrated with forced air systems
- Superior efficiency can only be attained with a correctly sized, high efficiency air conditioner used in conjunction with a correctly sized, leak-free duct system
- Higher efficiency alternatives to forced air A/C include mini split ductless A/C, evaporative cooling systems, and air to water heat pumps
- Fresh, outside air can be used with minimal energy loss when the system is combined with an integrated or external heat or energy recovery ventilator
- A whole house fan can complement or even replace traditional A/C in mild, dry climates
- Attic fans are not effective for cooling down a home
Energy Efficiency, Comfort, and Health
Modern high efficiency air conditioners such as energy star units use much less energy than older models; increasingly efficient options are becoming more widely available. When installed in conjunction with a tight building envelope and a well sealed duct system, total system efficiency and total home energy usage decreases since energy is not needlessly wasted. This means a more comfortable home environment while using less energy. Less energy means less CO2 emissions. In addition, a well sealed home can have a recovery ventilator integrated in the HVAC system, meaning that allergens can be filtered out of the home while the air entering the home will fresher and cleaner.
Function and Optimal Operation
Cooling mechanisms work by removing excess heat as well as excess moisture from the air. The human body’s natural mechanism for cooling itself consists of producing moisture on the skin which then cools the body as it evaporates. Excess humidity interferes with this natural mechanism, thereby exacerbating the perceived effects of warm temperatures, and increasing discomfort from heat. All types of air conditioners, which use compressed refrigerant liquid to absorb heat out of ambient air to cool it and then use fans to blow the cooled air into the room being cooled, dehumidify as they cool. This dehumidification contributes substantially to the perceived comfort provided by air conditioners. Evaporative coolers, which are much more energy efficient but which only work in dry climates, remove heat from the air by using a mechanism closer to that of the human body. Whole house fans quickly bring in outside air to cool down a house when it is much cooler outside than inside.
Components and Options
Forced Air A/C | Single Room Window A/C | Mini Split Ductless A/C | Evaporative Cooler | Whole House Fan | Attic Fan
Several cooling options are available on the market with varying energy efficiencies and cooling capabilities. Certain options are more appropriate for certain climates and regions.
Forced Air A/C
These are the air conditioning units commonly seen as large cases outside of homes on the ground or on the roof. The outside unit compresses the refrigerant which is then pumped into the home to the air handler and furnace where the evaporator coil cools and dehumidifies inside air (as the refrigerant absorbs the heat and expands into a gas). The refrigerant then returns to the compressor where it is again compressed into liquid to begin the process over again. The air blowing through the air handler, distributes cool, dry air throughout the home.
Some units can work in reverse and are called heat pumps, meaning they can either cool the home as an air conditioner, or heat the home by drawing heat from the outside into the home. Air to water heat pumps which exhaust heat to a water source (instead of the air) are can be more efficient than air conditioners or air to air heat pumps. Suppliers of air-to-water heat pumps include Altherma.
The R-22 refrigerant traditionally used by forced air (and window or portable) air conditioners is highly damaging to the ozone and is a potent greenhouse gas (with an effect thousand of times that of the same amount of carbon dioxide). As a result is is being phased out and replaced by R410a refrigerant, the new standard which is much more environmentally friendly. Air conditioners are a long term investment, and units with R-22 refrigerant will become increasingly costly and difficult to maintain, due to reduced availability.
Single Room Window A/C
These units work exactly like forced air A/C units, except the compressor and evaporator coils are located in the same case, which is usually hung out of a window, in order to expel heat and humidity to the outside. Cooled air is blown directly into a single room.
Mini Split Ductless A/C
These units still use refrigerant to cool and dehumidify air like forced air and window A/C units, but instead of distributing cooled air throughout the house by a system of ducts (which are prone to inefficiency and air leakage when typically installed) the refrigerant is distributed through the house directly through copper lines to a head unit in each room which contains evaporator coils over which a small fan blows cooled and dehumified air to the room. These units are quite common outside of the US and tend to be much more efficient than ducted systems because they directly cool and dehumidify the air of the room they are in, can be individually adjusted, and are not prone to duct losses.
Examples include:
Evaporative Cooler
Also called “swamp coolers,” evaporative coolers are typically less expensive to install and to operate than cooling systems which use refrigerant. However, they are most effective only in dry climates and care must be taken to properly clean and maintain them. Typically, residential evaporative coolers use direct evaporation and can be described as an enclosed metal or plastic box with vented sides containing:
- A fan or ‘blower’
- An electric motor with pulleys
- A water pump to wet the evaporative cooling pads
- Cooling pads which are kept wet and through which the fan blows outside air (which is cooled as the ambient heat evaporates the water) into the house
The units can be mounted:
- On the roof (down draft, or downflow)
- On exterior walls or windows (side draft, or horizontal flow) of buildings
A new generation of indirect evaporative coolers are coming to market and use less water than other coolers which can use hundreds of gallons a water a day. In particular, the household size coolers from Coolerado use 4 gallons per water (which can still add up if the cooler is runs quite often).
Whole House Fan
With the windows open, a whole-house fan sucks hot air out of a building and forces it into the attic. This displaces the very hot air trapped in the attic (which is pushed out the gable-end or soffit vents). With windows and/or doors open to the outside, the whole-house fan draws cooler outside air into the building to replace the hot air (creating a cooling breeze whilst doing so), and gradually cools off the mass of the house (drywall, framing, furniture, cabinets, etc.) using the “free” and renewable energy source of cool night time air.
Proper operation requires that a few windows be open during use and run times should be long enough (2-6 hours) to allow the mass to cool off, not just flush the hot inside air. The best units operate more quietly and efficiently with the use of multi speed settings. To avoid any air leakage, the fan should be well sealed and insulated when it is not in use.
Examples include:
Attic Fan
An attic fan sits on the highest ridge of the roof and uses small solar panels, or wired electricity, to power a small fan which exhausts the hot air which collects in the attic on hot days. Solar fans function only on sunny days when heat tends to accumulate and cost nothing to operate since they are powered by the sun. Exhausting the hot attic air and replacing it with cooler outside air may help keep the house cool, but is not a very effective measure due to radiant heat gain (from the mass of the roof and attic which absorbs the sun’s heat).
Whole house fans, shading, radiant barriers, air sealing, duct sealing and increased attic insulation are better solutions for low energy home cooling than are attic fans.