The most common way to cool a home is with a split system, central air conditioner. The system includes an external condenser unit and compressor which sit outside your home as well as an evaporator coil, which generally sits above your furnace within your home. If your air conditioner is integrated with your furnace system, it can take advantage of the furnace ﬁlter and any additional air purifying equipment you have added. This helps to clean the air throughout your home.
How Does Central Air Conditioning Work?
As the temperature in your house rises beyond what you have set on your thermostat, a signal is sent from your thermostat to the circuit board in your furnace or air handler. This tells the system that cold air is needed and turns on both the blower motor inside your house and the condenser, which sits outside your home. Warm air is then drawn in through your home’s ductwork and cooled as it passes over the evaporator coil. This cooled air is then returned to the home through the return air vents. Meanwhile refrigerant in your air conditioner absorbs the heat from the air as it flows through the evaporator coil. This heated refrigerant is then pumped to the condenser or outside portion of your air conditioner. The condenser blows outside air across a different set of coils, cooling the refrigerant, which is then sent back inside the home, to start the process over again. Once the indoor air temperature matches the thermostat temperature, the unit will shut off. The entire cycle will repeat once the temperature inside rises above the thermostat setting again.
When shopping for an air conditioner, you may choose between single-stage, two-stage, and variable speed compressors.
A single-stage unit can only be on or off. When on, it operates at full capacity regardless of how much cooling is required. Because air conditioning systems are sized to provide adequate cooling on the hottest day of the year, this means single-stage systems are often providing more cooling than is required. This reduces the efficiency of the system. However, air conditioners with single stage compressors are generally the most affordable.
Two-stage compressors help address this mismatch of cooling needs and system output. A unit equipped with a two-stage compressor will have two distinct levels of cooling output, typically called the low and high stages. The high stage will provide capacity to maintain comfortable temperatures in extreme weather, while the low stage will allow for more efficient operation during mild conditions. The initial cost for a two-stage air conditioner is normally higher than a single stage, however, the energy savings in the long run may offset this initial cost difference.
Variable speed compressors are the full-out extension of two-stage compressors. Rather than having specific output levels at which to operate, a variable speed unit can produce any output within a range. This range will vary from model to model and will enable the unit to match the cooling load required to maximize comfort and efficiency. These are the most expensive up front but are usually the least expensive to run on a daily basis.
Air conditioner efficiency is rated in SEER (Seasonal Energy Efficiency Ratio). This is the ratio of the cooling output of an air conditioner over a typical cooling season, divided by the energy it uses in Watt-Hours. It’s important to note, however, that the efficiency of a system and its SEER rating depends on all the components in the system and not just the condenser. The SEER rating for a condenser is primarily used for marketing and product differentiation and you should always use the rating for a complete matched system when comparing options.
Today the U.S. Department of Energy enforces minimum SEER requirements. The minimum standard SEER for new air conditioners in Colorado is 13, though most new air conditioners have a SEER that ranges from 13 to 21. The higher the number the more efficient the unit and accordingly the less it will cost to provide cooling. Many years ago, it was not uncommon to find AC units with SEER ratings of 6-10, so even the lowest available SEER rated system you buy today should be much more energy efficient.
A SEER ratio is a maximum efficiency rating, like the miles per gallon for your car. Say your car gets 28 miles per gallon on the highway. But if you’re stuck in city traffic it’s a lot less efficient. The same goes for your air conditioner. If your SEER ratio is 21, that’s the maximum efficiency and it could be lower depending upon conditions. The efficiency of your system can vary based on the size of your home, your current ductwork and other variables.
What Size Air Conditioner Do I Need?
Residential central air conditioning units come in a variety of different sizes ranging from 1-ton to 5-tons. This doesn’t have anything to do with how much the machines weigh. Instead, the unit “ton” is in relation to your air conditioner’s ability to cool. A 1-ton air conditioner will be able to remove 12,000 BTUs (British Thermal Units) in an hour. A 2-ton air conditioner can remove 24,000 BTUs per hour, and so on. So, what size of air conditioner do you need to effectively cool your home? That depends on many factors. You may get a rough estimate based upon the square footage of the area of your home to be cooled. Below is a chart containing some of the typical air conditioning sizes by square footage. If your home is larger than these examples, you may need multiple air conditioners working simultaneously. Anything over a 5-ton air conditioner is considered a commercial size and is not recommended for residential use.
SQ FT Size (in Tons)
750 to 1,000 1.5 – 2
1,050 to 1,400 2.5
1,450 to 1,700 3
1,750 to 1,900 3.5
1,950 to 2,200 4
However, many other factors should also be considered
Quality and size of ductwork
Quality and type of home insulation
Amount of direct sun exposure
Number of people who live in the home
Geographic climate and average temperatures
Quality and number of windows and exterior doors
Home construction materials (i.e., brick, wood, etc.)
Home features that modify indoor temperature (i.e., fireplace, sunroom, etc.)
Having the right sized air conditioner is a key component in cooling your space effectively. A unit that is too small has to run constantly, playing catch up all day trying to cool a space that it isn’t designed for.
Oversized units will run through their cooling cycle much faster than a correctly sized system would. This is known as short cycling. This means that the unit will pump out a short blast of cooled air and incorrectly assume it has finished cooling your space. In reality, there may be areas in your home that haven’t yet had an opportunity to receive enough cooled air, leaving you with uneven temperatures. In addition, this short cycling, on and off frequently, results in higher electric usage and an excessive amount of equipment wear and tear.
Finding an efficient, quality AC unit that is the right size for your home can be tricky. At Elbert Heating & Air, we specialize in helping customers choose the best AC units for their specific needs.