I remember getting a haircut once when I was a boy and how the barber bragged and bragged at how big his rooftop air conditioner was. It was long ago, and so I don’t remember much other than that – only the blast of cold air streaming out of the supply register. He probably told everyone how many tons it was. Believe it or not, I do seem to remember it cycling on and off a lot.
The lesson today: don’t listen to your barber when getting an air conditioner. Bigger (than needed) is not better. It’s actually worse, and here’s why.
Air conditioning cooling has two components. It removes both sensible heat – that is, the air temperature, the heat you measure with a thermometer – and it removes latent heat. Latent heat is the heat energy stored in the air’s water vapor. We all know it takes heat to create steam. But once vaporized, that heat doesn’t just disappear, it is held by the water vapor in the air. In fact, in places like St. Louis (where “St.” stands for “Steamy”) a big part of what an air conditioner does is remove that humidity. If an air conditioner is sized too big, it cools the sensible heat down fairly quickly. The thermostat then switches off, but more times than not, it didn’t remove much moisture from the air because not enough air circulated through the duct work and over the air conditioning coils long enough. Walking into a home with an oversized air conditioner sometimes feels like you’re walking into a cave: cool, yes, but clammy. An air conditioner actually needs to run long enough for the water vapor in the air to condense on the coils. If it’s two big, it cycles on and off before it can do that.
In fact, on a really hot day, a correctly-sized air conditioner will run almost all the time.
Unfortunately, most air conditioning units are too big for the homes. Why? There are all sorts of reasons. Larger units cost more and so make more for the HVAC company. HVAC companies want to make sure you don’t call them back out because your house is too warm, and so that fudge some oversizing in. And, so on and so forth…
If you’re in the market for a new air conditioner, request that the HVAC company do a “Manual J” and show you their calculations. A Manual J is the industry standard method of determining how much cooling you home will need (both in sensible and latent heat). If they don’t want to do that, then move on to another HVAC company. (Plus, the Manual J already includes a calculation for those really, really hot days.)
The benefits are many. Properly sized units cost less than units that are too large. The home is more comfortable. They cool more efficiently. They need less fan power so there’s an electric and equipment savings there, too.
So, no Manual J, no business. It’s as simple as that.
Nice article, David. For some reason, this is one of the least understood principles in the HVAC industry. Long run times are good. Short cycling is bad. It’s pretty simple.
Manual J load calculations are required by code, but almost no one in the industry does them. And when they do, they’re often not correct. They overestimate the temperature differences and underestimate the building envelope. I just wrote about this last week, and you can read it here: http://hub.am/9Dphag.
Keep up the good work, David. It’s going to take a lot of us constantly getting the word out about this to bring about major change.