Central AC: How big is too big?

Before I began working in the field of energy efficiency, I had no idea that air conditioners dehumidified a home’s air. In fact, as it turns out, the majority of the benefit one feels from the use of an air conditioner is from the dehumidification it provides.

If an AC cooled only, but didn’t pull any of that excess moisture out of the air, you would certainly feel a little cooler but the air would still be thick and clammy. You wouldn’t feel nearly as comfortable as in a dehumidified house. In fact, you’d probably keep turning the thermostat lower to make the house colder to offset the lack of dehumidification.

And that is exactly what happens when your Central AC (or heat pump) is over-sized. An over-sized unit cools the home air so quickly that it doesn’t get the chance to dehumidify that air like it’s meant to.

So this brings us back to the question originally posed in the title of this blog: How big is too big? And the answer is, it all depends.

There is only one way to accurately determine the right sized unit for your home. A calculation developed for exactly this purpose, called a Manual J, which takes into consideration the size of your home, it’s location, it’s thermal boundary (insulation), how leaky it is, and more… (There are more intricate calculations that help even more to get the exact right sized unit, such as Manual D and Manual S, but these are not required by code and are even less often performed than a Manual J)

Manual J is a complicated, time-consuming calculation. Which is exactly why most contractors don’t like to do it (even though technically they are required, by building, code to do so). What they typically would rather do is sell you an over-sized unit which guarantees you won’t complain that it’s not strong enough to cool your home. They’ll also make more money selling you a more expensive air conditioner (or heat pump)

So now you are armed with a little extra knowledge. Tell your contractor, before the contact is signed, that you want a Manual J performed. Tell him also that you want to see a copy (otherwise many will say they’ll do it and then not do so).

But there’s one more option: let me do an energy audit of your home before buying new heating or cooling equipment. For one thing, an energy audit by the Green Detective, always provides you a Manual J – both for the home as it is today and for the load your home would need after all the recommended energy efficient measures are completed.

A typical home, after an audit and after improvements have been made, will have at least a 30% drop in the its requirements for heating and cooling. That means if you have a 3-ton AC now, you might need only a 2-ton after you’ve finished making your home more energy efficient. And that means, an air conditioner that’s both cheaper to buy and cheaper to run.