Mechanical Cooling Systems

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By Energy Circle Staff - August 18th, 2009

Sometimes even a well-shaded, super-tight, energy-efficient house won't keep you cool without some help, which is why we turn to a mechanical cooling system to help keep us comfortable.

Sometimes even a well-shaded, super-tight, energy-efficient house won't keep you cool without some help, which is why we often turn to some sort of mechanical cooling system.

Mechanical cooling could be something as simple as fan-powered ventilation (which, in turn, could be anything from a table fan to a whole-house ventilation system), an evaporative cooling system (which blows cooled, humidified air into a house, and cools much more efficiently than A/C), an air conditioning system (which physically removes heat from indoor air - effective but inefficient), or a combination.  This article provides an introduction to each, and gives a quick summary of their ups and downs, with special attention to the types of climate in which each is most useful.  We've also done a quick back-of-the-envelope calculation of what it would cost to run any of these systems 24/7 all summer long - an excercise, of course, purely for comparison, as it would be pretty silly to run any of these non-stop all summer.

Ventilation Cooling:

In a well-shaded home in the northern U.S., fan-powered ventilation can be an adequate source of cooling.  In warmer climes, where an air conditioner or an evaporative cooler might be necessary to stay comfortable, ventilation can help cooled air circulate throughout the house, and keep indoor air fresh and healthy.  In either case, something as simple as a small table fan can be one of the most cost-effective cooling devices available.

U.S. Climate Zones in Cooling Degree Days

Ventilation creates a wind-chill effect, which lowers perceived temperature even while air temperature remains constant.  A simple draft in a warm house, achievable by anything from a whole-house ventilation system to a table fan and a cracked window - even a bathroom exhaust fan and a cracked window, for that matter - can lower perceived temperature by 4-8 degrees.  Circulating air always feels cooler than stagnant air, so this is true both for circulating outside air into a house, and for circulating indoor air within a closed, air conditioned house.

Fan-powered ventilation can also assist and limit the use of energy in an air conditioned house.  An efficient fan can distribute cooled air throughout the house more efficiently than an air conditioner by itself, and so cut back on the time the air conditioner needs to run in order to be effective.

Ventilation also serves to replace warm indoor air with cool outdoor air.  In milder climates, where the air is warm during the day and cool at night, or in a well-shaded area where the outside air stays relatively cool, opening windows and using running a fan (either in the form of a whole-house fan that draws air through windows and exhausts it through attic vents, an exhaust fan like a bathroom fan or range hood, or a ceiling or table fan) will flush out warm indoor air with cool outdoor air. In the absence of air conditioning, indoor air may be warmer than outdoor air because of radiant heat gain coming from the roof (which is probably dark-colored and exposed to the sun).  A building will retain heat at night, as well, when hot air outside has cooled down; so it's often good practice to open windows at night and close them during the day, especially in regions with high temperature swings between day and night.

An energy efficient fan might use around 50 watts, which at 10 cents per kilowatt-hour translates to about half a penny an hour (or, if you were to run it 24/7 all summer, about 10 bucks).

When you break out your fans for the summer, be sure to give them a good scrub, so you're not blowing around basement-mold-dust, etc., all around your freshly spring-cleaned house, where your children play and your baby sleeps.  For more information about the health benefits of ventilation, see our article on indoor air quality.

Evaporative Cooling Systems:

Evaporative cooling systems are an efficient, inexpensive alternative to air conditioners in hot, dry climates.  They cool indoor air by drawing in outside air and running it through water-soaked pads before blowing it into the house, cooling it and infusing it with moisture.  Evaporative coolers should only be used in regions where the relative humidity stays below 50% in the summer (because they add moisture to the air, which in humid climates makes the air feel sticky and uncomfortable). They can either direct air straight into a room, or be connected to a duct system for whole-house distribution.

Evaporative coolers use only 400-1800 watts (or from 4-18 cents per hour on average), compared to 2000-5000 watts (20-50 cents/hour) to run an air conditioner, which makes them an economical cooling option in the right climate.  A back-of-the-envelope calculation based on average electricity consumption / average electricity cost brings us to just under $240 to run an evaporative cooler 24/7, all summer long.  We like evaporative coolers, too, because they continuously draw fresh air into the house (as older, stale air is exhausted through either an exhaust fan or a couple open windows), which helps ensure good indoor air quality.

Air conditioners:

Air conditioners work by physically removing the heat from warm indoor air using a cold coil called an evaporator, which then transfers that heat to an outside coil called a condenser and sends it off into the wind.  Air conditioners do a great job removing heat, which is why, at some point or another, we've all thanked some greater power for their existence.

Air conditioners are, however, energy hogs, indisputably.  For that reason, we consider them a last resort, after steps to cut back on heat gain and ventilation options have been exhausted.

Within the wide world of necessary A/C, however, there are some distinctions to be made, as well as steps to increase the efficiency of any air conditioning method.

Single-room air conditioners are ideal in milder climates where a home needs to be cooled less often, and in smaller homes where a central A/C is not necessary.  They use a fraction of the energy of whole-house systems, because they generally cool only occupied portions of a house.  A room air conditioner could use between 500 and 3000 watts (5-30 cents / hour).  Running one of these puppies continuously all summer could cost anywhere from $100-600 over the course of the summer.

A central air conditioner supplies the whole house with cooled air through a duct system that returns to a central air handler.  Although there are steps to increase the efficiency of a central A/C system - using a programmable thermostat, air sealing, making sure ducts are insulated and have no leaks, for example - the nature of a central A/C system is to heat a bunch of empty rooms.  As such, it's probably the least energy efficient cooling option out there, using between 2000 and 5000 watts, (20-50 cents / hour), which would cost between $400-1100 to run all summer long.

An alternative to central A/C is a mini-split air conditioner, which combines aspects of room and central A/C systems.  A mini-split consists of an outdoor unit (housing the condensing coil to release the heat collected indoors), and one or more indoor units, each housing a fan and an evaporator coil.  Although it's possible to connect a mini-split air conditioner to a duct system, their energy saving advantage comes from their ability to connect multiple indoor units - each controlled individually - to a single outdoor unit.  This eliminates wasted energy through leaky ducts, and allows spot-cooling through a single system - so you don't waste energy cooling empty rooms.  Although mini-splits currently hold less than 3% of the North American air conditioning market, they account for more than 50% of the air conditioning market worldwide.  This is a rapidly advancing technology that presents a large potential for energy savings.

When using any type of air conditioning, it's never efficient to cool the great outdoors.  Proper air sealing is equally as important in the summer months as in the winter for that reason, as well as the protection it affords against poor indoor air quality.  As good as ventilation and air circulation both are, you want to know where all that air is coming from - and you don't want it to be from a neglected crack in a moldy basement.

Keeping the windows shut in cooled areas (with the shades down during the day, to keep out the sun) and making sure your insulation is up to snuff will also take a big bite out of your electricity bill during the cooling season.



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Air Cooling and Ventilation System

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