By Mike McClintock
Special to The Washington Post
Thursday, May 26, 2005
Once the really swampy weather arrives, it's hard to make a case for anything but an overwhelming supply of central air conditioning. But portables have a place -- and if you have central air already, so do zone controls.
Both deliver cool, dry air where you need it, when you need it. That's more economical than unrestricted central air that cools bedrooms as well as stairwells and closets that people use but don't really live in.
People-targeted air conditioning is worth considering because it's less expensive and more energy-efficient than a typical central system. Here is a closer look at the two main ways to arrange it.
Portable air conditioning
Among hundreds of portables on the market, the main distinctions are installation method and cooling capacity.
You could use a true portable and mount it in a window, or a basically permanent portable and mount it in a wall. The second option is obviously more complicated and requires making a hole in your house. The first option is easier but blocks a window. And once installed, seasonal removal often slides to the bottom of the to-do list and window units stay in place year round.
Small portables are easy to install yourself. Typically, it's a question of basic measuring, leveling and mounting a couple of supports on siding under the window. Bear in mind, though, that large models can weigh 100 pounds or so, which can make these basic operations quite difficult.
Through-wall installations involve some carpentry, drywall patching, trim work and more. But whether you try it yourself or hire a contractor, installing the unit directly under a window often makes the job easier. Typically, wall studs there are just fillers and not load bearing.
It also makes sense to pick a section of wall that is free of pipes, wires and ducts. If you have a choice, for a wall or window mount, choose a location near an electrical outlet so you won't need any extra wiring work. It also helps to select the shady side instead of the sun struck side of a room. The cooler it is outside, the easier it is for the appliance to give up heat it has collected inside the house.
Estimating cooling needs
Many retailers and installers base cooling capacity on the no-complaints rule: No matter how hot and muggy it gets, your huge machine will always have more than enough capacity.
But bigger (not to mention more expensive) isn't always better. You'll be deluged with cold air on hot days to be sure. But the Washington-based Association of Home Appliance Manufacturers (AHAM) says an oversize unit can cool a room so quickly that it cycles off before lowering the humidity.
There are several ways to estimate capacity, expressed in BTUs (British Thermal Units), though contractors sometimes talk about "tons" of cooling. One ton translates to a capacity of 12,000 BTUs per hour and is used in a very basic formula of one ton per 500 square feet of floor space.
But there are many variables, such as the amount of insulation in the walls and the number of windows in the room. To match capacity to your situation, most contractors use a complicated worksheet called the Cooling Load Estimate Form. But there is a simplified, consumer-friendly version posted on the new AHAM Web site, http://www.cooloff.org .
First, you work through a checklist to reach a capacity estimate for the room or area you want to cool. Then you move to an extensive list of manufacturers and models that fill the bill.
At that point you'll also want to consider the appliance's efficiency. For room air conditioners, it's expressed as an Energy Efficiency Rating (EER). The higher the number, the more efficient the unit -- and its price, of course -- though you should save money in the long run.
Zoned central air conditioning
You can also build people-targeted air conditioning into an existing central system (or a new system, of course) with zone controls. They use the same central supply system and blower, but split the delivery ducts into sections.
Most floor plans benefit from at least two zones -- one for sleeping areas and another for living areas, or one for each floor of the house.
The guts of the system are motorized, thermostat-controlled dampers installed at key locations in the ducts. They swivel to reduce or close off the flow to one section and send more cool air to another.
Each zone has its own setback thermostat. That means you can save money by raising the setting in sleeping areas during the day, for instance, and lowering it in living areas where the people are.
Before bedtime, the thermostats can automatically reverse the delivery and save money by allowing the temperature to rise downstairs, say, while everyone is upstairs for the night. In most cases, this targeted efficiency will pay for the zoning upgrade in a few years.
Several zone equipment manufacturers say the systems can cut cooling costs from 10 to 30 percent. But if you have air ducts (97 percent of all new single- and multi-family homes are built that way), zoning can also save money on winter heating costs.