Last updated on March 16th, 2024 at 02:10 pm

There is a natural tendency for people, especially Americans, to always opt for the biggest and best air conditioner they can possibly afford. The going thought is, the bigger the unit (as long as it fits in the window), the better it will work. It will cool the room faster, and thus use less energy.

This isn’t the case.

A professional knows what the homeowner doesn’t. In this instance, the right sized air conditioner should be selected for the room it will be placed in.

Under most circumstances, the btu, or cooling power, of the unit is directly proportional to the size of the room. But here’s the trick. This is true only in perfect, or ideal, conditions. In other words, this assumes good to excellent insulation in the walls, a good quality window that will be sealed and has a good R-value. Placing a unit in less than ideal conditions forces you to revise the original estimates given on the box.

For example, a 5,000 btu air conditioner with a 9.7 EER rating may say it will effectively cool approximately 1500 square feet efficiently. This is roughly equivalent to a 15x10x10 room. That’s assuming 10 foot ceilings.

So you buy the unit for a 15×10 room in your house, it fits fine in the window, but it doesn’t seem to be working as efficiently as you’d like. Why is that the case? What could be wrong?

Simply put, the insulation or window R-value may not be as good as you think.

The easiest way to measure whether or not an air conditioner will work in your room is with the heating estimator calculator in the Home Energy Audit Kit. It makes the calculation simple – all you do is plug in the dimensions of the room and “guestimate” your insulation. This is an important step that homeowners always miss – but professional contractors don’t. Insulation is in your walls to minimize the heat loss. The better the insulation, the less heat is transferred.

So, let’s go with the assumption that our room is 15x10x10. Furthermore, let’s say that the air conditioner we want is at least 85% efficient (so its not a high efficiency energy star model) and the average temperature outside is 85 degrees F. We want it to be a cool and comfortable 76 degrees F in the room. And lets say our insulation is just average.

Using the calculator, we see that at its most efficient state, the A/C unit would have to output 6728 BTU, or about 34% more cooling power than the unit we bought can possibly provide. Therefore, the A/C unit isn’t sufficient.

Let’s say we opt for a 93% efficient model, an energy star one. Our btu need goes down by a few hundred to 6260. That’s still relatively insufficient.

As you might be able to see, there are two cuplrits here. The first is the insulation. Average insulation is dragging us down by about half in this instance. We know this because if we change our insulation grade to “excellent”, and keep everything else constant, our BTU need plummets to 3130 – which means under ideal conditions, the 5000 btu air conditioner will work fine.

The second culprit is the misleading advertising of the unit. You should always assume the manufacturer’s estimations to be under perfect conditions.

So right now you’re asking “Well how can I improve the efficiency of the air conditioner?” You can’t improve the efficiency of the air conditioner directly, but you can make the temperature gradient less drastic than the 9 degrees from 85 to 76. In fact, increasing the temperature requirement to 78 degrees F, and keeping everything else the same gives us a completely different result. Now, we need 5233 btu for an 85% efficient unit, and 4869 btu for a 93% efficient unit. These are both within range for the 5000 btu unit.

You can also circulate the cooled air with a ceiling fan. Drawing shades will help – and keeping the door to the room closed.

There are a few lessons to take awy from this. First, now you can see why changing the thermostat on your air conditioner is such a bad idea, unless you increase the temperature – the cost for running the unit increases exponentially. You also can see how important good insulation is, and why sizing a room before you go to the store is so important. Professionals always make these measurements and calculations before going to the store, and then select the best, most efficient model in a price range that will efficiently cool the room.