Apparently, I haven't written a single article this summer about oversized air conditioners. I've written several in previous summers, mostly about how HVAC contractors bypass the HVAC design process by using rules of thumb and how they mess up the Manual J load calculations when they do go the preferred route. I've never written an article focusing on the reasons to size an air conditioner properly, though, so here it is. When an air conditioner runs, it does two jobs. It lowers the temperature of the air, and it removes moisture from the air. To do the second one, the AC has to run for a while. As the air passes over the evaporator coil, it encounters a very cold surface. Ideally, when the air passes over that cold surface, the air temperature drops about 20° F. In places where we have higher relative humidity in the summer, the other important process that happens when the air hits the coil is that the temperature of the coil is below the dew point of the air. As a result, water vapor condenses on the coil.

Here's the thing, though. Water vapor condenses on coils in oversized air conditioners, too, but you have to get enough condensation on the coil for the water to start dripping down into the pan below the coil. Even then, you're not there yet. You still have to get enough water in the pan for it drain to the outside. Until the water that condenses actually makes it to the outside, you haven't really dehumidified the air. average price for an air conditioning unitBecause that water on the coil can evaporate and get back into the air in the home. low profile ac unit windowIf you make the mistake of leaving your thermostat in the Fan-On position instead of Auto (Don't do that!), the water on the coil gets back into the air even quicker.air conditioner split system installation

Oversized air conditioners don't run for a long time because they satisfy the cooling load quickly and then shut off. Properly sized air conditioners run longer, so if you want your air conditioner to dehumidify your home as well as cool it, don't let the HVAC contractor oversize it. If you live in the desert, what I just said above doesn't matter to you. There's no water vapor to condense on the coil, and if there were, your dew point is probably close to absolute zero. (Well, OK, that might be a bit of an exaggeration.) The second reason definitely should matter to you, however. The thing that wears equipment out is starting up and shutting down. The more it happens, the shorter the life of the equipment. When an air conditioner is oversized, it starts up and shuts down a lot more because it runs for only a short time to meet the thermostat setpoint. Then a few minutes later, it comes on again and runs for a short time. Over the course of a day, an oversized air conditioner can have a lot more start-ups and shut-downs than a properly sized air conditioner.

That means you'll probably be repairing it more often and replacing it sooner. People in the industry used to think you'd save money on your air conditioning bills with a properly sized air conditioner, but that thinking has changed. John Proctor wrote an article for Home Energy Magazine that shows only a small savings for the homeowner (but the utilities benefit by lower peak loads). You should save money on the upfront cost, though, because you're putting in a smaller AC. (As Proctor points out in his article, oversizing isn't always the worst problem, though. You have to be a subscriber to Home Energy Magazine to read the article, but it's well worth the read if you can get it.) For new homes, use the full HVAC design process, which starts with Manual J, the heating and cooling load calculation protocol from the Air Conditioning Contractors of America, the trade association for AC contractors. For existing homes, the best way to do it is to see how long your current air conditioner runs when it's at the design conditions.

Of course, you want to do this while your AC is still in good shape, not when it's on its last legs. If it runs for only 5 to 10 minutes before shutting off, it's definitely oversized. If your runtimes are over half an hour at design conditions, it's probably sized close to the actual cooling load. If you're building a new home, remodeling an existing home, or just getting a new air conditioner in an existing home, ask your builder, remodeler, or HVAC contractor how they're planning to size the air conditioner. If they tell you they're basing it on the size of your house, don't let them do it. There's a lot more to it than square footage of conditioned floor area, and of course, I haven't touched at all on the capabilities of different types of equipment (single speed, fixed capacity, multi-stage, mini-splits...). More on that in future articles. Oversized AC, Screwed-up Manual J, ENERGY STAR HVAC Tirade!Don't Assume It's Correct. An Oversized Cooling System Isn't Always a Bad Thing

SEER: 3 Free Tools That Show You How Much You’ll Save with a New AC So you’ve determined that you need a new air conditioner, and you’re well aware that SEER is a big deal when it comes to energy efficiency. But you have no idea how much money you’ll save by picking one SEER rated air conditioner versus another.In other words, you have no idea how much a 14 SEER saves versus a 13 SEER, or an 18 SEER versus a 16 SEER--and so forth.How much you’ll save depends on several factors. But there are a few tools you can use to get a general sense--making shopping for a new air conditioner much easier.Here are 3 free tools that will empower you to estimate the savings you’ll get when picking one SEER rated air conditioner over another. 1) Annual Energy Savings Calculator from NATE Why this tool rocks: Simple and easy-to-useLets you see how much money you’ll save by picking one SEER over another when upgrading from your old system.Allows you to input the tonnage (cooling ability) of the unitWhat it‘s lacking:

Can’t change the electric rate per kilowatt (measurement of the cost of electricity)No options for calculating based on your locationCan’t input the cost of the air conditioner to determine a break-even analysisBeginning SEER only goes as high as 12 SEER--limiting the comparisons you can make2) Energy Savings Calculator from Lennox Visually engagingUnlike the NATE tool, you can compare higher rated SEER air conditioners to each otherGives you 5, 10 and 15-year savings (this is important because air conditioners typically last about 15 years according to Consumer ReportsLets you input the electric rate per kilowatt (measurement of the cost of electricity)Lets you calculate based on locationWhat it‘s lacking: Only lets you compare with 10 SEER rated air conditioners or higher (older air conditioners may be 8 SEER)Can’t input tonnageCan’t input the cost of the air conditioner to determine a break-even analysis3) Energy Usage and Payback Calculator (Excel File) From Energy Star

More customization than other toolsGives you ability to input cost of air conditioner in order to see complete life cycle savingsLets you add number of air conditioner unitsLets you add electric rate per kilowattGives you option to calculate based on locationLets you see side by side comparisons of conventional vs Energy Star rated air conditionersWhat it‘s lacking: Not visually engaging and it’s not easy-to-use (especially if you’re not familiar with Excel)Lacks guidance for beginners. This tool can be overwhelming and not helpful if you don’t have all the information needed to make an accurate calculationIn summary Hopefully these tools will empower you to see the savings each SEER rated air conditioner will provide to make your air conditioner shopping easier.Just remember than each tool is limited in how accurate it can be because there are many factors that determine your electric bill.To get a complete view of how much money you can save by upgrading from one SEER to another, contact Cool Today online for help.