It's been hot and muggy in Atlanta the past few days. Now I'm not your typical Atlantan who says that lightly. I grew up in Houston and south Louisiana and spent nine years in Florida, so I know hot and muggy. Overall, summers in Atlanta are pretty nice because of our elevation (~1000 feet), but we do have our moments. Anyway, it's been hot enough that we've hit our summer design temperature, which is defined as the temperature exceeded only 1% of the time. In Atlanta, the summer design temperature is 92 degrees Fahrenheit. That's the temperature at which air conditioners ideally will provide exactly as much cooling as the house needs, when sized according to the Manual J cooling load calculation protocol developed by ACCA, the Air Conditioning Contractors of America. When outdoor conditions are at the design temperature, an air conditioner should run pretty much continuously and be able to keep the house at the ACCA recommended indoor design temperature of 75 degrees Fahrenheit.

When outdoor conditions go above the design temperature, the AC should run continuously and not quite keep the house at 75 degrees. Now, the thing is, Manual J has some built in oversizing, so even if the air conditioner were sized according to an accurately performed Manual J load calculation, it's not going to run continuously at the summer design temperature. ac unit miami flIt should run long enough to make the house comfortable, however.multi-split system air conditioner installation Last week I wrote about the 4 factors of comfort, temperature and humidity being the first two. no refrigerant in ac unitAn air conditioner actually does two jobs - it lowers the temperature and removes moisture from the air, thus taking care of the first two factors, with HVAC design anyway.

To remove the moisture, however, the air conditioner needs long runtimes. That's because the air moving over the cold evaporator coil causes the water vapor to condense. The more air moves over the coil, the more water condenses out and gets carried away. It takes about 15 minutes of runtime before you start getting serious dehumidification of the air, so oversized systems will not dehumidify well. If the AC comes on, runs 10 minutes or less, and then shuts off, the house may be cool, but in a humid climate, the indoor humidity levels will stay high, probably over 60%. I know this from personal experience. Last year we replaced the AC in our condo, and I did the Manual J load calculation. The result was that we needed a 1.6 ton air conditioner for our 1500 square feet, so we had a 2 ton system installed. Knowing that Manual J has a built-in oversizing bias, I wanted to go with the 1.5 ton system, but I chickened out. Our AC runs maybe 15 minutes max on a hot afternoon, and our relative humidity stays around 60%.

The Manual J bias is real, and it's not small. And that's with an accurate HVAC load calculation. I've seen plenty of load calculations that use incorrect inputs so the contractor can come up with a cooling load to match the AC size he wants to install. Now, back to the point of this article - how to tell if you have an oversized air conditioner. Just get your stop watch and time how long the AC runs on a hot afternoon. Ten minutes or less, and it's definitely oversized. Twenty minutes would be OK. Thirty minutes at a time or longer, and your humidity levels should be fairly low. So, how long does your air conditioner run on a hot day? See my followup article with real data on our AC runtimes:If you’re trying to cut down the expense of your energy bills, you’ve probably thought about how your daily activities around the house can add to that cost. Beyond shutting off lights and limiting your usage, what are some other ways to use less energy and spend less on electricity?

Here are some quick facts about some of the appliances that could be contributing to your bill. At eight loads of laundry a week, the average family uses both the washing machine and the dryer six hours each on average per week. Even with some of the more energy efficient models, the combined cost of doing laundry can add up to more than $115 a year for the electricity alone. If your family is in the market for a new washer and dryer, high-efficiency appliances will use less electricity and run more efficiently—especially if you’re upgrading from an older, top-load set. Dryers use more electricity of the two appliances, so you may also consider air drying clothes when possible to minimize your electricity consumption on laundry day. At eight loads of laundry a week, using the washing machine and the dryer six hours each per week on average can add up to more than $115 a year. To cut down costs even further, washing clothes in cold water will minimize the electricity used to heat the water otherwise needed—according to the EPA, 17% of the average home’s energy bill comes from the energy needed to heat water.

So, set your loads to cold water cycles and help minimize hot water usage. The average American watches 4 hours of television a day. Over the span of an entire year, that adds up to over 2 months of viewing. If you have an HDTV over 40 inches and are tuning in at the same rate as the US average, your television may be using about 341 kWh a year and costing you $41 annually. Watching 4 hours a television a day means your television may be using about 341 kWh a year, costing you $41 annually. If you’re looking into upgrading your television soon, switching to an LCD television can cut the cost of your television’s energy nearly in half; the average LCD TV runs at 150 watts, meaning you could cut your TV’s electricity cost to under $27 a year. As an appliance in your house that runs all day, every day, the average refrigerator uses an estimated 657 kWh of electricity a year, costing you upwards of $78.84 over 12 months. Compared to other large household appliances, this cost may be surprising given its heavy use, but maximizing your refrigerator’s efficiency can lower your home electricity usage and add to your energy savings.

Keeping your refrigerator set to an optimal temperature between 37 and 40 degrees can ensure your food stays cold enough without making your unit work harder than is has to. The average refrigerator uses an estimated 657 kWh of electricity a year, costing you about $78.84. While your air conditioner may vary depending on the size of your home and where you live, a 3-ton central air conditioning unit is common in many homes. The average 3-ton air conditioner operates at around 3500 watts per hour—or 3.5 kWh—while in use. If you ran your A/C unit for 9 hours a day during the 4 warmest months of the year, the electricity needed to cool your home would cost you around $460 annually. Keep your house at a comfortable temperature while keeping an eye on the energy bill by using the program features available on your thermostat. Be sure to make sure your A/C unit is set to turn off during the times of day when you don’t need it. Running a 3-ton A/C unit for 9 hours a day over 4 months uses about 3832 kWh of electricity annually and cost around $460 annually.