Cut through the marketing jargon and stay cool this summer.
Too Many Choices
It may feel like there are simply too many choices when picking an air conditioner. Do I need central A/C, or will a window unit be enough? How many BTUs do I need? Why are A/C units rated in "Tons"? This article aims to break down the different options and make choosing easier.
The Basics
Air conditioning (A/C) captures heat from your house, and dumps it outside. To do this, compressors and fans are used. These things are often large, power-hungry and loud. As air is cooled, moisture is removed from it, which also helps us feel cool (think of a hot, dry day vs. hot, humid day).
Different Types
Here are the main types of units:
Central
Attaches to your forced-air furnace. It has an outdoor unit that houses the compressor and fan. Inside, there is a heat exchanger that sits inside the ducts near your furnace and cools the air. Cool air comes out of the air registers just like warm air does in winter. Think of it like a furnace that produces cold air.
Split
For homes that don't have a central air system. This type has an outdoor unit that feeds a number of wall-mounted air-handlers throughout the home. These air handlers cool individual rooms or areas, but do not circulate air throughout the house. Think of it like having baseboard heaters.
Window/Wall
Through the window (or sometimes through the wall) units are a single box that house the compressor, coils and fan. The box sits in a window with part of it inside, and part of it outside. These are often loud, because the noisy components are right there, not in a separate outdoor unit. Think of this like a small space heater.
Portable
Similar to the window unit, these have become very popular in recent years. Instead of sitting in the window, they sit inside on the floor and have a flexible air duct to blow the heat out of the window. They are nicer to look at, and they are portable, but they are also less efficient.
Cooling Capacity
Cooling capacity, like heating capacity, is most often measured in BTUs (British Thermal Units). With a furnace, that number indicates how many BTUs can be put into the home per hour. With air conditioning, it's how many can be removed per hour. One BTU is the amount of energy required to raise the temperature of 1 pound of water by 1 degree Fahrenheit.
Central and split units may be talked about or measured in "Tons". 1 Ton is equal to 12,000 BTUs. Because of this standard, many units are made with capacities in multiples of 12,000 BTUs. The term "Ton" actually comes from ice making in the 1800's, and it means "the amount of heat required to melt one ton of ice"!
The Higher the EER or SEER number, the more efficient the unit is.
Efficiency Rating
All new units will have a SEER (Seasonal Energy Efficiency Ratio) or an EER (Energy Efficiency Ratio). These numbers are the actual amount of heat energy removed from the house, divided by the amount of energy used to run the unit. The higher the EER or SEER, the more efficient the unit is.
Type | Window | Portable | Split | Central |
Price | $ | $ | $$ - $$$ | $$$ |
Capacity | Low | Low | High | High |
Temperature Controls | Controls On Unit | Controls On Unit | Zoned Thermostats | House Thermostat |
Efficiency (SEER/EER) | 8-12 | 8-12 | 13-22 | 13-22 |
Compressor + Fan Location | Hangs Out Window | Inside | Outside | Outside |
How Much Cooling Do I Need?
Just like sizing a furnace, that all depends on your house. The amount of insulation, number and size of windows, orientation, and sun exposure all affect how big an A/C unit will need to be in order to comfortably cool your house. The best way to know is to consult an HVAC contractor for an assessment of your house.
It's better to under-size than to over-size an A/C.
If you are unsure, it is better to under-size than to over-size. The reason for this is that a smaller unit will run continuously, and do a better job of dehumidifying, although is may provide less cooling. An oversized unit is more likely to short-cycle (turn on and run only for a short time) and can create a cool but humid house.
Heat Pumps
If you already heat with a heat pump, you automatically have A/C as well! Heat pumps may be central or split systems, and use the air conditioning process in reverse to heat the home in winter.
Key Points
There are many types of A/C units, there is no one-size-fits-all solution.
The higher the EER or SEER number, the more efficient it is.
Central and split systems are the most efficient.
A higher BTU rating means more cooling capacity.
Consult an HVAC contractor to determine what size unit you need.
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