Rule of Thumb #6 - Size Matters

When we bought our house in Arlington in 2000, our agent noted the outside air conditioner and commented, "I like to see these big air conditioners.  They really keep the house cool."  He was so very, very wrong.  His comment was the equivalent of saying, "I really like these big refrigerators.  They really keep your food cold."  Huh?  It's not the size of your refrigerator that determines how cold it is.

When it comes to central air conditioning, bigger is not better.  In fact, smaller is generally better for several reasons--particularly in a humid climate like this one.  EPA Energy Star has a fact sheet about this very topic.

Here's your simple test.  On the hottest days of the year--when it's 95 degrees or above-- note whether your air conditioner cycles on and off.  If it does, it's too big.  An air conditioner should be sized so that it runs continuously on the hottest days of the year, essentially from 3 or 4 in the afternoon until 8 at night nonstop.  (If it runs constantly all the time, even when it's only 80 and doesn't seem to cool your house, you have a different problem and need to get it looked at.)

What's the dif, you ask?  A larger air conditioner sucks up more energy, costing you dollars and adding to harmful emissions.  Air conditioners do their best job dehumidifying and reach peak efficiency after they have been running for about 10-15 minutes, so if yours is cycling on and off, it's not dehumidifying well and it's running at lower efficiency.  People with air conditioners that are too big often lower the temperature to make up for this, exacerbating the costs of running it.

Here's an analogy.  You're driving on a street with timed traffic lights. Having a too-big air conditioner is like having a muscle car and racing from one light to the next, then stopping, then racing to the next one.  A right-sized air conditioner is like having a smaller car, driving along at an even 25 mph and hitting the lights all green.  You get where you're going at the same time, but use less gas driving the smaller one at a steady speed.

So what do you do?  Well, don't go out and replace your AC unless you were planning to anyway.  But when you do, make sure you show the EPA fact sheet to your contractor and make him or her do an accurate sizing calculation.  Then don't be swayed into rounding up for safety's sake.  If anything, round down in size for comfort's sake.  (On a side note, your ductwork is probably all wrong, too, making the situation even worse.  Aargh!)

But for now, the thing to do is have your programmable thermostat raise the temperature while you are out and lower it just before you come back.  Then your AC will run longer and more efficiently for at least that one cycle, saving you money and dehumidifying better. 
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Steve Offutt, Arlington resident and Director - Arlington Initiative to Reduce Emissions (AIRE) .Transportation Projects

Comments

[Steve]

Hello David,

On the surface that might seem to be the case, but it's actually better. That's because the smaller, more efficient units are using so much less energy, they actually use less in aggregate than, say 60% of the too big units running instead.

To simplify, if you have 100 efficient air conditioners that use 1 unit of energy, then you are using 100 units of energy when they are all running.

If you have 100 ACs that use 2 units of energy and 60 of them are running, then they are using 120 units of energy. So it costs everyone more, including the customers who have to pay to build the power plants.

Thanks for your comment.

[David]

Steve, the one flaw in this plan is that if everyone followed it, everyone's AC would be running all day during the hot days. This leads to power plants running above their optimum efficiency. (Possibly also causing brownouts.) I'm not sure which inefficiency is the better one to have though -- it might very well pay off to have powerplants running overtime a few days a year and A/C units that operate efficiently all other times.

[AC lover]

I have a big air conditioner 15 seer, it is more energy efficient than my old one, cut the electric bill in half...the a/c units are built bigger to be more energy efficient....

Your point is absolutely valid, but here's a twist on the appearance of size. What appears to be a too large outdoor compressor unit might just be a very efficient, properly-sized unit. One of the ways air conditioning has become more efficient over the years is by increasing the area of heat exchanger fins, to allow more complete heat transfer. So visible size alone doesn't necessarily relate to capacity. And this is one way to identify old, inefficient units: they're tiny.