Central Air Conditioner Sizing

Posted on: November 11, 2016
By: Alan O'Neill
Posted in: Air Conditioning

Air Conditioner Sizing For Houston Homeowners

When the weather gets hot, an air conditioner must work hard that is, run frequently and for long periods to keep a home at a comfortable temperature. Considering the energy cost of running an air conditioner, the net result of this is high utility bills.

We’re often asked the question, “What size air conditioner do I need for my home?” That’s a great question for anyone to consider when purchasing a new air conditioning system.

Sizing a Central Air Conditioner

With air conditioners, the term “size” has nothing to do with a unit’s physical dimensions but, rather, its ability to produce cooled air. Both room-size and whole-house air conditioners are manufactured in a variety of capacities, so it’s helpful to understand how they are measured.

An air conditioner that is too small can’t keep up with load requirements on a, particularly hot day. One that is too large will cycle off and on too frequently, doing a poor job of dehumidifying the air, which degrades comfort. In fact, it’s better to slightly undersize an air conditioner than to oversize it. Also, the air flow into and out of rooms must be carefully balanced to ensure efficient operation of the system.

These factors, in addition to how well a house is insulated, how it’s used by your family, your climate, and more must be taken into account when selecting and designing your system. Though you should consult a qualified air-conditioning contractor before making any purchase, you can estimate your requirements by figuring you will need about 1 ton of capacity for every 400 square feet of living space in a marginally insulated house. For example, a 2,000-square-foot house would normally require a 5-ton air-conditioning system; a newer, well-insulated house can often be served by a smaller system.

So there are a couple different ways to get a good idea of what sized central air conditioner you’ll need. Both methods are pretty close and are good for getting a ballpark estimate. Before figuring out what sized central air conditioner is right for your house, you need to know what sizes they come in (keep in mind, this article refers to your air conditioners size, not the type of air conditioner. For information on different types, see: What is a Split Air Conditioner? – Split Air Conditioner vs Packaged Air Conditioner). Central air conditioners come in a variety of sizes, and what size they are is measured in “tons.” Now contrary to what you might think, the tonnage of an AC unit is not actually based on its weight. A “ton” is a measure of an air conditioner’s ability to cool. One ton is the ability of your air conditioner to cool 12,000 BTUs (British Thermal Unit) in an hour. Likewise, a “2-ton” central air conditioner is able to cool 24,000 BTUs per hour. So now you are likely asking, what is a BTU? A BTU is the amount of energy required to heat or cool one pound of water by one degree Fahrenheit. So a 1-ton air conditioner can cool 12,000 pounds of water by one degree every hour. That’s all it means, so don’t let your HVAC contractor tell you any differently!

The central air conditioners that are installed in your home range from 1-ton to 5-ton units and increase in half-ton increments (2-ton, 2.5-ton, 3-ton, etc…). Anything over 5-tons is generally considered a commercial HVAC unit and if your house requires an AC unit of this size, then I’d recommend you install multiple units in tandem. What this means is that if your home needs 6-tons of cooling power for instance, then you’d need two, 3-ton units installed (instead of a 6-ton unit) and probably a zoning system as well. For more information on a zoning system, try: Do you need a zoning system? This seems like overkill, but the nice part about two systems running together is that if one unit completely fails, then you still have one unit running. This might not keep your house at a comfortable 68-degrees but it can at least keep your house from sky-rocketing into the 90’s in the middle of summer. In general, unless you have a mansion or zero insulation in your home then you are unlikely to need anything more than a 5-ton unit.

Why Does The Right Sized Central Air conditioner Matter So Much?

Too big – if you buy a central air conditioner that is sized too big for your house, then it will run more like that AC unit from the 80’s – it will turn on quickly, cool your house rapidly and then shut off. Too small – if your central air conditioner is sized too small, then it will run constantly and spend most of the day trying to catch up to where it should be. Again, your house will still be nice and cool (unless it’s several sizes too small). Just right – a properly sized central air conditioner will run through the required amount of cycles to keep your house comfortable, but will not run so much that it looses its efficiency. Call (713) 812-7070 us for your home service and repair needs.

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