In a house without central air, a portable or window air conditioner can be the difference between a long stretch of unbearable dog days and a season of carefree comfort. These compact, self-contained cooling machines are also perfect for supplementing an overtaxed central-air system to wipe out persistent hotspots in the home on the most stifling, humid days. Don't underestimate the appeal of a spare window unit when your trusty central air goes belly up during a heat wave — your sanity may depend on it.
When comparing air conditioners, there are a number of factors to consider. Here, we'll try to arm you with the pertinent information to make an informed buying decision.
Window air conditioners are installed directly in the window frame, and typically have extendable wings to fill any horizontal gaps. Some larger models are equipped with a slide-out chassis to facilitate easy cleaning and maintenance. Take some measurements before you shop, and be sure that the model you select will properly fit the window you have in mind. Special models are available for unusual installation alternatives, such as casement windows.
Portable air conditioners require no installation; they are designed to be positioned on the floor, and can be moved easily from room to room as needed (they're not light, so if you plan to transport the unit regularly, look for a model with casters). Portable units expel heat by means of an exhaust extension that is run to an exterior window or adjacent room. Like portable dehumidifiers, they typically collect condensed moisture in a removable receptacle that must be emptied from time to time.
There are a couple of common measurements that can tell you a lot about an air conditioner's cooling capacity and energy usage. The first is the unit's BTU rating. BTUs (an abbreviation for British Thermal Units) measure the amount of heat an air conditioner can remove from the air over a given period of time; the higher the number, the greater the cooling capacity. More capable units, not surprisingly, are larger, heavier and more expensive than lesser ones.
The good news is, bigger is not always better. The key to proper cooling is to match the capacity of the air conditioner to the dimensions of the area you intend to cool. An underpowered AC unit, obviously, will struggle to cope with the heat stored in a large room. Conversely, though, a unit with too high a BTU rating may provide disappointing performance in a smaller room. This is because one of the main functions of any AC unit is to remove excess humidity from the air. An air conditioner that cools too effectively must cycle its motor on and off more frequently, giving the unit's condenser element inadequate opportunity to dehumidify the air. All this starting and stopping also exacerbates mechanical wear and tear, posing a risk to long-term reliability.
The following chart provides general guidelines for selecting an appropriate BTU rating to match the room in which you plan to use the air conditioner.
Source: United States Environmental Protection Agency
These numbers are estimates; consider other mitigating factors, and adjust as necessary to determine how much cooling power you need. For example, sunny rooms require more cooling than shady ones, and infrequently used rooms require less cooling than rooms where several people congregate for extended periods of time. If you plan to use the air conditioner in a kitchen, allow for the extra heat generated by the oven and other appliances.
The other important number to look for is an air conditioner's EER, or Energy Efficiency Rating. As you'd expect, this is a measure of the amount of energy (electricity) the unit requires to deliver a given amount of cooling; higher numbers translate to greater efficiency, i.e., lower relative energy usage. Air conditioners with high EERs come at a price, at least initially. Weigh the purchase-price increase of more efficient models against your potential savings on your monthly energy bill to decide what's best for you. Ideally, selecting an energy-efficient unit that's properly sized for the intended area will provide you with the best return on your cooling investment.
Once you've determined your specific power and efficiency requirements, it's time to consider features that enhance convenience and performance. Here are a few to look for:
Programmable thermostat. All AC units have a thermostat (the device that senses air temperature and regulates the operation of the condenser). Higher-end models may feature a programmable thermostat that allows you to specify different temperatures for different times of day. This feature can also help you save on energy costs by minimizing usage during times when cooling is less necessary (for example, when you're at work).
Timed operation. Less sophisticated than a programmable thermostat, a simple timer serves a similar function, allowing the air conditioner to activate and deactivate automatically at a specified time each day.
Energy-saver mode. Some models are equipped with a relay to turn on the fan only when the condenser is activated. This can marginally reduce energy costs (and noise, intermittently), but increased wear on the fan motor may shorten its life.
Motorized louvers. Virtually all window and portable air conditioners feature adjustable louvers to help direct the flow of cool air from the vents. Some go a step further, providing motorized louvers that cycle automatically (up and down and/or left and right) to circulate cool air more efficiently throughout the room.
Noise damping. While modern air conditioners in general are quieter than those manufactured 10 years ago, some models are specifically designed to minimize ambient fan noise.
Easy-access filter. Unobstructed flow of clean air at intake is essential to effective cooling, and necessary for maintenance of the mechanical parts. Look for models that provide easy access to the air filter for regular cleaning.
Low-profile design. A low-profile window air conditioner obscures less of the windows in which it's mounted, improving visibility and allowing more daylight into the room.
Remote control. An increasing number of air conditioners offer remote-control operation of various functions. It's a nice touch if you'd rather not stand right in the refrigerated air stream to adjust the controls... or if you just want to stay on the couch.