Atel Air Blog

family indoors in summerWhen it comes to getting enough cooling, proper humidity control and the best possible energy efficiency, the size of your air conditioner really does matter. The wrong size and improper installation of air conditioners are the main reasons people have trouble with them.

Sadly, you can’t just ask for the same size air conditioner you have already, as you don’t know if that was sized correctly when it was chosen and installed. Instead, there are a surprising number of factors that go into determining the correct size of air conditioner for your home.

First, though, let’s make sure you understand why it’s important to get the right sized air conditioner.

Goldilocks Was Right

This One’s Too Small…

If your air conditioner is too small, you’ll never get the cooling results you want. Even worse, your air conditioner will be running constantly, which will result in really high monthly electricity costs. Furthermore, your poor little undersized AC will wear itself out long before its time from all that extra work.

This One’s Too Big…

So bigger is better, right? Not necessarily. An air conditioner that’s too large also has serious drawbacks.

Sure, a monster AC will quickly cool your home – too quickly, in fact. The problem is it won’t run long enough to dehumidify your home properly, which means you run a serious risk of developing mold problems. And once mold gets started it can be really tricky to eradicate.

When it’s too big, an AC will do what’s called “short cycling”, which means it will turn off and on for brief periods much more often than it should. Again, this wastes energy and can cause early breakdowns.

This One is Just Right!

Just like with the bowls of porridge, the middle one works. The right size air conditioner will run at a nice steady pace for the right amount of time, the way it was designed to do.

So How Do I Ensure I Get the Right Size?

The short answer is that you’re better off hiring a reliable air conditioner technician and getting them to do the work for you. After all, that’s why they went to school and passed a Refrigeration Technician licensing exam, and why you’re paying them.

As a homeowner, the question you need to ask your potential contractor is, “what do you include in your calculations?”

In order to determine the right size of air conditioner for a home, a technician needs to do what’s called “Manual J heat load” calculations. Basically they figure out how hot your home will get, and from there they can determine how much power is needed to cool it down. These calculations actually get quite complicated.

The heat load calculations include:

  1. The overall square footage of the home.
  2. How many stories a home has.
  3. Exterior construction materials: brick, vinyl siding, wood, etc.
  4. The overall level of insulation behind the walls and under the roof.
  5. How many doors, windows, and skylights you have, and how large they are. Insulation levels count with these too: for example the number of panes of glass in the windows.
  6. How high the ceilings are.
  7. The direction the majority of windows are facing. If the home is shaded by trees, other houses, etc.
  8. The transfer of heat between rooms of the house.

There are more things to consider, but you get the idea: you can’t just buy any size of air conditioner, or “ballpark it”.

Here’s an example that should show you why. A home that has lots of south-facing windows and poor insulation will require more cooling power than the same size home that’s well insulated and shaded. The sunnier home will be subject to what’s called “passive solar gain” – otherwise known as the “parked car” effect – and will get much hotter.

That’s why that old rule of thumb “one ton of air conditioning for every 500 square feet” isn’t really reliable.

Other Things to Watch For

parts of a central air conditioner system
Image from

Once you know the right size of air conditioner you need, there are other things that determine the effectiveness and efficiency of your air conditioner. These are things that many contractors skimp on so that can offer a lower price. In the end, however, it’s the unfortunate homeowner who pays the price.

Matching the Condenser to the Air Handler/Evaporator

As you know there are two units that make up an air conditioner: the condenser is the outdoor unit and the air handler or evaporator is the indoor unit. It’s important that they match, and some contractors try and reduce the size of the indoor unit to cut costs.

If the air handler is too small, the cooling won’t be delivered into your ductwork. A buildup of ice on the evaporator can also result. You can find more details about how a central air conditioner works on


Ducting has to be designed properly for the home to deliver cool air where it’s needed most. The duct size, number of bends, and length of the run can all affect how much air gets to which location. There’s a whole science behind designing ductwork, especially when independent zones are needed.

Your ducts also need to be sealed tight to prevent your cool air from escaping into the walls of your home, instead of going to your rooms.

Trane air conditioner installed on wall bracketsSolid Ground to Stand On

It’s important that the condenser is installed on level ground, and can’t sink or become tilted over time.

In many cases a concrete pad must be created to support the condenser, but if possible we like to install the condenser on wall brackets. This ensures the condenser will stay level no matter how the ground swells, sinks, or tilts over time. It also keeps the unit out of the dirt, leaves and debris that can clog it.

Details Matter

When asking for quotes from contractors, don’t just go for the lowest bid: ask questions and find out why some are cheaper or more expensive. Are they doing the calculations? What kind of system are they recommending? Do they offer any kind of guarantees?

To get the right air conditioner, it makes sense to do the legwork when hiring the right contractor, then let them do the legwork for getting you the right size AC. In the end, if you’re going to invest in a central air system, it makes sense to get one that works well and uses as little energy as possible.

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